Method 1103.1: Escherichia coli (E. coli)

in Water by Membrane Filtration Using

membrane-Thermotolerant Escherichia

coli Agar (mTEC) 

March 2010 
U.S. Environmental Protection Agency

Office of Water (4303T)

1200 Pennsylvania Avenue, NW

Washington, DC 20460

EPA-821-R-10-002
Acknowledgments

The following laboratories are gratefully acknowledged for their participation in the validation study for 

this method in disinfected wastewater effluents which was conducted in 2004:

Volunteer Research Laboratories

• EPA Office of Research and Development, National Risk Management Research Lab: Mark C.

Meckes 

• U.S. Army Corps of Engineers, Washington Aqueduct: Elizabeth A. Turner, Michael L.

Chicoine, and Lisa Neal

Volunteer Verification Laboratory

• Orange County Sanitation District, Environmental Sciences Laboratory: Charles McGee, Michael

von Winckelmann, Kim Patton, Linda Kirchner, James Campbell, Arturo Diaz, and Lisa McMath

Volunteer Participant Laboratories

• American Interplex: John Overbey, Steve Bradford, and Jessica Young

• County Sanitation Districts of Los Angeles County, Joint Water Pollution Control Plant

(JWPCP): Kathy Walker, Michele Padilla, and Albert Soof 

• East Bay Municipal Utility District: Bill Ellgas and Daniel Mills 

• Environmental Associates (EA): Susan Boutros and Madelyn Glase

• Hampton Roads Sanitation District (HRSD): Anna Rule, Paula Hogg, and Bob Maunz

• Hoosier Microbiological Laboratories (HML): Don Hendrickson, Katy Bilger, Keri Nixon, and

Lindsey Shelton

• Massachusetts Water Resources Authority (MWRA): Steve Rhode and Mariya Gofshteyn

• San Francisco Water: Phil Caskey, Paul McGregor, and Bonnie Bompart

• University of Iowa Hygienic Laboratory: Nancy Hall and Cathy Lord

• Wisconsin State Laboratory of Hygiene (WSLH): Sharon Kluender, Linda Peterson, and Jeremy

Olstadt
iii

Disclaimer

Neither the United States Government nor any of its employees, contractors, or their employees make any

warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use of

or the results of such use of any information, apparatus, product, or process discussed in this report, or

represents that its use by such party would not infringe on privately owned rights. Mention of trade

names or commercial products does not constitute endorsement or recommendation for use. 

Questions concerning this method or its application should be addressed to:

Robin K. Oshiro

Engineering and Analysis Division (4303T)

U.S. EPA Office of Water, Office of Science and Technology

1200 Pennsylvania Avenue, NW

Washington, DC 20460

oshiro.robin@epa.gov or OSTCWAMethods@epa.gov
iv

Table of Contents

1.0 Scope and Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.0 Summary of Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3.0 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4.0 Interferences and Contamination ................................................. 2

5.0 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

6.0 Equipment and Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

7.0 Reagents and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

8.0 Sample Collection, Preservation, and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9.0 Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10.0 Calibration and Standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

11.0 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

12.0 Verification Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

13.0 Data Analysis and Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

14.0 Sample Spiking Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

15.0 Method Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

16.0 Pollution Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

17.0 Waste Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

18.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
v

List of Appendices

Appendices A and B are taken from Microbiological Methods for Monitoring the Environment: Water

and Wastes (Reference 18.5).

Appendix A: Part II (General Operations), Section A (Sample Collection, Preservation, and Storage)

Appendix B: Part II (General Operations), Sections C.3.5 (Counting Colonies) and C.3.6 

(Calculation of Results)
1 March 2010

Method 1103.1 Escherichia coli (E. coli) in Water 

by Membrane Filtration Using 

membrane-Thermotolerant Escherichia coli Agar (mTEC)

March 2010

1.0 Scope and Application

1.1 Method 1103.1 describes a membrane filter (MF) procedure for the detection and enumeration of

Escherichia coli bacteria in ambient water. E. coli is a common inhabitant of the intestinal tract of

warm-blooded animals, and its presence in water samples is an indication of fecal pollution and

the possible presence of enteric pathogens.

1.2 The E. coli test is recommended as a measure of ambient recreational fresh water quality. 

Epidemiological studies have led to the development of criteria which can be used to promulgate

recreational water standards based on established relationships between health effects and water

quality. The significance of finding E. coli in recreational fresh water samples is the direct

relationship between the density of E. coli and the risk of gastrointestinal illness associated with

swimming in the water (Reference 18.1).

1.3 For method application please refer to Title 40 Code of Federal Regulations Part 136 (40 CFR Part

136).

1.4 Method 1103.1 and 1603 were submitted to interlaboratory validation in wastewater matrices and

study results compared. The estimated percent of total colonies that would have resulted in false

positives for disinfected, secondary, and combined disinfected/secondary wastewater were

significantly lower for Method 1603 (0.6%, 1.4%, and 1.0%, respectively) compared to Method

1103.1 (6.2%, 11.2%, and 10.3%, respectively). The estimated percent of total colonies that

would have resulted in false negatives for disinfected and combined disinfected/secondary

wastewater were also significantly lower for Method 1603 (2.5%, and 2.2%, respectively)

compared to Method 1103.1 (12.3% and 6.0%, respectively). The estimated percent of total

colonies that would have resulted in false negatives were not significantly different for secondary

wastewater samples for Methods 1603 and 1103.1 (2.6% and 2.0%, respectively). 

Based on the high Method 1103.1 false positive and false negative levels when compared to

Method 1603, Method 1103.1 is not approved for the analysis of disinfected wastewater. 

Laboratories wishing to test for E. coli in wastewater using a membrane filtration method are

referred to EPA Method 1603. A summary of Method 1103.1 false positive and negative results is

provided in Section 15.3.2. Detailed Method 1103.1 and 1603 study results are provided in the

validation study reports (References 18.2 and 18.3, respectively). 

2.0 Summary of Method

2.1 The MF method provides a direct count of bacteria in water based on the development of colonies

on the surface of the membrane filter (Reference 18.4). A water sample is filtered through the

membrane which retains the bacteria. After filtration, the membrane is placed on a selective and

differential medium, mTEC, incubated at 35°C ± 0.5°C for 2 ± 0.5 hours to resuscitate injured or

stressed bacteria, and then incubated at 44.5°C ± 0.2°C for 22 ± 2 hours. Following incubation,

the filter is transferred to a filter pad saturated with urea substrate. After 15 minutes, yellow,

yellow-green, or yellow-brown colonies are counted with the aid of a fluorescent lamp and a

magnifying lens. 
Method 1103.1 

March 2010 2

3.0 Definitions

3.1 In Method 1103.1, E. coli are those bacteria which produce colonies that remain yellow, yellowgreen, or yellow-brown on a filter pad saturated with urea substrate broth after primary culturing

on mTEC medium.

4.0 Interferences and Contamination

4.1 Water samples containing colloidal or suspended particulate material can clog the membrane

filter and prevent filtration, or cause spreading of bacterial colonies which could interfere with

enumeration and identification of target colonies.

5.0 Safety

5.1 The analyst must know and observe the normal safety procedures required in a microbiology

laboratory while preparing, using, and disposing of cultures, reagents, and materials and while

operating sterilization equipment.

5.2 Mouth-pipetting is prohibited.

5.3 This method does not address all of the safety issues associated with its use. It is the

responsibility of the laboratory to establish appropriate safety and health practices prior to use of

this method. A reference file of material safety data sheets (MSDSs) should be available to all

personnel involved in Method 1103.1 analyses.

6.0 Equipment and Supplies

6.1 Glass lens with magnification of 2-5X, or stereoscopic microscope

6.2 Lamp, with a cool, white fluorescent tube

6.3 Hand tally or electronic counting device

6.4 Pipet container, stainless steel, aluminum or borosilicate glass, for glass pipets

6.5 Pipets, sterile, T.D. bacteriological or Mohr, glass or plastic, of appropriate volume

6.6 Sterile graduated cylinders, 100-1000 mL, covered with aluminum foil or kraft paper

6.7 Sterile membrane filtration units (filter base and funnel), glass, plastic, or stainless steel, wrapped

with aluminum foil or kraft paper

6.8 Ultraviolet unit for sanitization of the filter funnel between filtrations (optional)

6.9 Line vacuum, electric vacuum pump, or aspirator for use as a vacuum source (In an emergency or

in the field, a hand pump or a syringe equipped with a check valve to prevent the return flow of

air, can be used)

6.10 Flask, filter, vacuum, usually 1 L, with appropriate tubing

6.11 A filter manifold to hold a number of filter bases (optional)

6.12 Flask for safety trap placed between the filter flask and the vacuum source

6.13 Forceps, straight or curved, with smooth tips to handle filters without damage

6.14 Ethanol, methanol, or isopropanol in a small, wide-mouth container, for flame-sterilizing forceps

6.15 Burner, Bunsen, or Fisher type, or electric incinerator unit for sterilizing loops and needles
Method 1103.1 

3 March 2010

6.16 Thermometer, checked against a National Institute of Standards and Technology (NIST) certified

thermometer, or one that meets the requirements of NIST Monograph SP 250-23

6.17 Petri dishes, sterile, plastic, 9 × 50 mm with tight-fitting lids; and 15 × 100 mm with loose fitting

lids 

6.18 Bottles, milk dilution, borosilicate glass, screw-cap with neoprene liners, 125 mL volume

6.19 Flasks, borosilicate glass, screw-cap, 250-2000 mL volume

6.20 Membrane filters, sterile, white, grid marked, 47 mm diameter, with 0.45 μm pore size

6.21 Absorbent pads, sterile, 47 mm diameter

6.22 Platinum wire inoculation loops, at least 3 mm diameter in suitable holders; or sterile plastic

loops

6.23 Sterile disposable applicator sticks

6.24 Incubator maintained at 35°C ± 0.5°C

6.25 Waterbath maintained at 44.5°C ± 0.2°C

6.26 Waterbath maintained at 50°C for tempering agar

6.27 Test tubes, 20 × 150 mm, borosilicate glass or plastic

6.28 Test tubes, 10 × 75 mm, borosilicate glass (durham tubes)

6.29 Caps, aluminum or autoclavable plastic, for 20 mm diameter test tubes

6.30 Test tubes screw-cap, borosilicate glass, 16 × 125 mm or other appropriate size

6.31 Filter Paper

6.32 Whirl-Pak® bags 

6.33 Autoclave or steam sterilizer capable of achieving 121°C [15 lb pressure per square inch (PSI)]

for 15 minutes

7.0 Reagents and Standards

7.1 Purity of Reagents: Reagent-grade chemicals shall be used in all tests. Unless otherwise

indicated, reagents shall conform to the specifications of the Committee of Analytical Reagents of

the American Chemical Society (Reference 18.5). The agar used in preparation of the culture

media must be of microbiological grade.

7.2 Whenever possible, use commercial culture media as a means of quality control.

7.3 Purity of Water: Reagent-grade water conforming to specifications in: Standard Methods for the

Examination of Water and Wastewater (latest edition approved by EPA in 40 CFR Part 136 or

141, as applicable), Section 9020 (Reference 18.6).
Method 1103.1 

March 2010 4

7.4 Phosphate buffered saline

7.4.1 Composition:

Monosodium phosphate (NaH2PO4) 0.58 g

Disodium phosphate (Na2HPO4) 2.5 g

Sodium chloride 8.5 g

Reagent-grade water 1.0 L

7.4.2 Dissolve the ingredients in 1 L of reagent-grade water, and dispense in appropriate

amounts for dilutions in screw-cap bottles or culture tubes, and/or into containers for use

as rinse water. Autoclave at 121°C (15 PSI) for 15 minutes. Final pH should be 7.4 ±

0.2.

Note: The initial and ongoing precision and recovery (IPR and OPR) performance criteria

established for Method 1103.1 were determined using spiked PBS samples (Section 9.3,

Table 1). Laboratories must use PBS when performing IPR and OPR sample analyses. 

However, phosphate-buffered dilution water (Section 7.6) may be substituted for PBS as

a sample diluent and filtration rinse buffer. 

7.5 Phosphate buffered dilution water (Reference 18.7) 

7.5.1 Composition of stock phosphate buffer solution:

Monopotassium phosphate (KH2PO4) 34.0 g

Reagent-grade water 500.0 mL

Preparation: Dissolve KH2PO4 in 500 mL reagent-grade water. Adjust the pH of the

solution to 7.2 with 1 N NaOH, and bring the volume to 1 L with reagent-grade water. 

Sterilize by filtration or autoclave at 121°C (15 PSI) for 15 minutes.

7.5.2 Preparation of stock magnesium chloride solution: Add 38 g anhydrous MgCl2 or 81.1 g

MgCl2C6H2O to 1 L reagent-grade water. Sterilize by filtration or autoclave at 121°C (15

PSI) for 15 minutes. 

7.5.3 After sterilization, store the stock solutions in the refrigerator until used. Handle

aseptically. If evidence of mold or other contamination appears, the affected stock

solution should be discarded and a fresh solution should be prepared.

7.5.4 Working phosphate buffered dilution water: Mix 1.25 mL of the stock phosphate buffer

and 5 mL of the MgCl2 stock per liter of reagent-grade water. Dispense in appropriate

amounts for dilutions in screw-cap bottles or culture tubes, and/or into containers for use

as rinse water. Autoclave at 121°C (15 PSI) for 15 minutes. Final pH should be 7.0 ±

0.2.
Method 1103.1 

5 March 2010

7.6 mTEC agar

7.6.1 Composition:

Protease peptone #3 5.0 g

Yeast extract 3.0 g

Lactose 10.0 g

Sodium chloride 7.5 g

Dipotassium phosphate (K2HPO4) 3.3 g

Monopotassium phosphate (KH2PO4) 1.0 g

Sodium lauryl sulfate 0.2 g

Sodium desoxycholate 0.1 g

Bromcresol purple 0.08 g

Bromphenol red 0.08 g

Agar 15.0 g

Reagent-grade water 1.0 L

7.6.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve. 

Autoclave at 121°C (15 PSI) for 15 minutes, and cool in a 50°C waterbath. Pour the

medium into each 9 × 50 mm culture dish to a 4-5 mm depth (approximately 4-6 mL),

and allow to solidify. Final pH should be 7.3 ± 0.2. Store in a refrigerator.

7.7 Urea substrate medium

7.7.1 Composition:

Urea 2.0 g

Phenol red 0.01 g

Reagent-grade water 100.0 mL

7.7.2 Add reagents to 100 mL reagent-grade water and mix thoroughly to dissolve. Adjust to

pH 5.0 ± 0.2 with 1 N HCl. The substrate solution should be a straw-yellow color at this

pH (See Photo 1). The substrate solution should be stored at 6-8°C for no more than one

week.
Method 1103.1 

March 2010 6

Photo 1. Urea substrate medium. After adjusting the pH of the medium to 5 ± 0.2, the urea substrate

medium should be straw-yellow in color.

7.8 Tryptic soy agar (TSA)

7.8.1 Composition:

Pancreatic digest of casein 15.0 g

Enzymatic digest of soybean meal 5.0 g

Sodium chloride 5.0 g

Agar 15.0 g

Reagent-grade water 1.0 L

7.8.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve

completely. Autoclave at 121°C (15 PSI) for 15 minutes and cool in a 50°C waterbath. 

Pour the medium into each 15 × 100 mm culture dish to a 4-5 mm depth, and allow to

solidify. Final pH should be 7.3 ± 0.2.
Method 1103.1 

7 March 2010

7.9 Lauryl Tryptose Broth (LTB)

7.9.1 Composition

Tryptose 20.0 g

Lactose 5.0 g

Dipotassium phosphate (K2HPO4) 2.75 g

Monopotassium phosphate (KH2PO4) 2.75 g

Sodium chloride 5.0 g

Sodium lauryl sulfate 0.1 g

7.8.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and warm to dissolve

completely. Dispense into fermentation tubes (20 × 150 mm tubes containing inverted 10

× 75 mm tubes). Autoclave at 121°C (15 PSI) for 15 minutes. Final pH should 6.8 ± 0.2.

7.10 Nutrient agar (NA)

7.10.1 Composition:

Peptone 5.0 g

Beef extract 3.0 g

Agar 15.0 g

Reagent-grade water 1.0 L

7.10.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to boiling to

dissolve completely. Dispense in screw-cap tubes, and autoclave at 121°C (15 PSI) for

15 minutes. Remove the tubes and slant. Final pH should be 6.8 ± 0.2.

7.11 Tryptic/trypticase soy broth (TSB)

7.11.1 Composition:

Pancreatic digest of casein 17.0 g

Enzymatic/papaic digest of soybean meal 3.0 g

Sodium chloride 5.0 g

Dextrose 2.5 g

Dipotassium phosphate (K2HPO4) 2.5 g

Reagent-grade water 1.0 L

7.11.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve

completely. Dispense in screw-cap tubes, and autoclave at 121°C (15 PSI) for 15

minutes. Final pH should be 7.3 ± 0.2.
Method 1103.1 

March 2010 8

7.12 Simmons citrate agar slants

7.12.1 Composition:

Magnesium sulfate (MgSO4) 0.2 g

Monoammonium phosphate (NH4H2PO4) 1.0 g

Dipotassium phosphate (K2HPO4) 1.0 g

Sodium citrate (Citric acid) 2.0 g

Sodium chloride 5.0 g

Bromthymol blue 0.08 g

Agar 15.0 g

Reagent-grade water 1.0 L

7.12.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to boiling to

dissolve completely. Dispense into screw-cap tubes, and autoclave at 121°C (15 PSI) for

15 minutes. After sterilization, slant until solid. Final pH should be 6.9 ± 0.2.

7.13 Tryptone water

7.13.1 Composition:

Tryptone 10.0 g

Sodium chloride 5.0 g

Reagent-grade water 1.0 L

7.13.2 Add reagents to 1 L of reagent grade water and mix thoroughly to dissolve. Dispense in

5-mL volumes into tubes, and autoclave at 121°C (15 PSI) for 15 minutes. Final pH

should be 7.3 ± 0.2.

7.14 EC broth

7.14.1 Composition:

Tryptose or trypticase peptone 20.0 g

Lactose 5.0 g

Bile salts No. 3 1.5 g

Dipotassium phosphate (K2HPO4) 4.0 g

Monopotassium phosphate (KH2PO4) 1.5 g

Sodium chloride 5.0 g

Reagent-grade water 1.0 L

7.14.2 Add reagents to 1 L of reagent-grade water, mix thoroughly, and heat to dissolve

completely. Dispense into fermentation tubes (20 × 150 mm tubes containing inverted 10

× 75 mm tubes). Autoclave at 121°C (15 PSI) for 15 minutes. Final pH should be 6.9 ±

0.2.

Note: Do not use tubes if the inverted tubes (durham tubes) are not completely filled with

medium after sterilization.

7.15 Oxidase reagent

7.15.1 Composition:

N, N, N',N'-tetramethyl-p-phenylenediamine dihydrochloride, 1% aqueous solution (1 g

per 100 mL sterile reagent-grade water)

Note: Prepared oxidase test slides are commercially available.
Method 1103.1 

9 March 2010

7.16 Kovacs indole reagent

7.16.1 Composition:

D-dimethylaminobenzaldehyde 10.0 g

Amyl or Isoamyl alcohol 150.0 mL

Concentrated (12 M) hydrochloric acid 50.0 mL

7.16.2 Dissolve $-dimethylaminobenzaldehyde in alcohol, slowly add hydrochloric acid, and

mix.

7.17 Control cultures 

7.17.1 Positive control and/or spiking organism (either of the following are acceptable)

• Stock cultures of Escherichia coli (E. coli) ATCC #11775

• E. coli ATCC #11775 BioBalls (BTF Pty, Sydney, Australia) 

7.17.2 Negative control organism (either of the following are acceptable)

• Stock cultures of Enterococcus faecalis (E. faecalis) ATCC #19433

• E. faecalis ATCC #19433 BioBalls (BTF Pty, Sydney, Australia) 

OR

• Stock cultures of Enterobacter aerogenes (E. aerogenes) ATCC #13048
Method 1103.1 

March 2010 10

8.0 Sample Collection, Handling, and Storage

8.1 Sampling procedures are briefly described below. Detailed sampling methods can be found in

Microbiological Methods for Monitoring the Environment: Water and Wastes, Part II, Section A

(see Appendix A). Adherence to sample handling procedures and holding time limits is critical to

the production of valid data. Samples should not be analyzed if these conditions are not met.

8.1.1 Sampling Techniques

Samples are collected by hand or with a sampling device if the sampling site has difficult

access such as a dock, bridge, or bank adjacent to a surface water. Composite samples

should not be collected, since such samples do not display the range of values found in

individual samples. The sampling depth for surface water samples should be 6-12 inches

below the water surface. Sample containers should be positioned such that the mouth of

the container is pointed away from the sampler or sample point. After removal of the

container from the water, a small portion of the sample should be discarded to allow for

proper mixing before analyses.

 

8.1.2 Storage Temperature and Handling Conditions

Ice or refrigerate water samples at a temperature of <10°C during transit to the

laboratory. Do not freeze the samples. Use insulated containers to assure proper

maintenance of storage temperature. Take care that sample bottles are not totally

immersed in water during transit or storage.

8.1.3 Holding Time Limitations

Sample analysis should begin immediately, preferably within 2 hours of collection. The

maximum transport time to the laboratory is 6 hours, and samples should be processed

within 2 hours of receipt at the laboratory.

9.0 Quality Control

9.1 Each laboratory that uses Method 1103.1 is required to operate a formal quality assurance (QA)

program that addresses and documents instrument and equipment maintenance and performance,

reagent quality and performance, analyst training and certification, and records storage and

retrieval. General requirements and recommendations for QA and quality control (QC)

procedures for microbiological laboratories are provided in Reference 18.7.

9.2 The minimum analytical QC requirements for the analysis of samples using Method 1103.1

include routine analysis of positive and negative controls (Section 9.5), filter sterility checks

(Section 9.7), method blanks (Section 9.8), and media sterility checks (Section 9.10). Additional

analytical QC for the analysis of samples using Method 1103.1 include an initial demonstration of

laboratory capability through performance of the initial precision and recovery (IPR) analyses

(Section 9.3), ongoing demonstration of laboratory capability through performance of the

ongoing precision and recovery (OPR) analysis (Section 9.4). For the IPR and OPR analyses, it

is necessary to spike PBS samples with either laboratory-prepared spiking suspensions or

BioBalls as described in Section 14. 
Method 1103.1 

11 March 2010

9.3 Initial precision and recovery (IPR)—The IPR analyses are used to demonstrate acceptable

method performance (recovery and precision) by each laboratory before the method is used for

monitoring field samples. EPA recommends but does not require that IPR analyses be performed

by each analyst. IPR samples should be accompanied by an acceptable method blank (Section

9.8) and appropriate media sterility checks (Section 9.10). The IPR analyses are performed as

follows:

9.3.1 Prepare four, 100-mL samples of PBS and spike each sample with E. coli ATCC #11775

according to the spiking procedure in Section 14. Spiking with laboratory-prepared

suspensions is described in Section 14.2 and spiking with BioBalls is described in

Section 14.3. Filter and process each IPR sample according to the procedures in Section

11 and calculate the number of E. coli per 100 mL according to Section 13.

9.3.2 Calculate the percent recovery (R) for each IPR sample using the appropriate equation in

Section 14.2.4.3 or 14.3.2 for samples spiked with laboratory-prepared spiking

suspensions or BioBalls, respectively.

9.3.3 Using the percent recoveries of the four analyses, calculate the mean percent recovery

and the relative standard deviation (RSD) of the recoveries. The RSD is the standard

deviation divided by the mean, multiplied by 100.

9.3.4 Compare the mean recovery and RSD with the corresponding IPR criteria in Table 1,

below. If the mean and RSD for recovery of E. coli meet acceptance criteria, system

performance is acceptable and analysis of field samples may begin. If the mean or the

RSD fall outside of the required range for recovery, system performance is unacceptable. 

In this event, identify the problem by evaluating each step of the analytical process,

media, reagents, and controls, correct the problem and repeat IPR analyses.

Table 1. Initial and Ongoing Precision and Recovery (IPR and OPR) Acceptance Criteria

Performance test Lab-prepared spike

acceptance criteria

BioBallTM spike

acceptance criteria

Initial precision and recovery (IPR)

• Mean percent recovery 

• Precision (as maximum relative standard deviation)

76% - 124% 68% - 96%

41% 25%

Ongoing precision and recovery (OPR) as percent recovery 54% - 146% 58% - 106%

9.4 Ongoing precision and recovery (OPR)—To demonstrate ongoing control of the analytical

system, the laboratory should routinely process and analyze spiked PBS samples. The laboratory

should analyze one OPR sample after every 20 field and matrix spike samples or one per week

that samples are analyzed, whichever occurs more frequently. OPR samples should be

accompanied by an acceptable method blank (Section 9.8) and appropriate media sterility checks

(Section 9.10). The OPR analysis is performed as follows:

9.4.1 Spike a 100-mL PBS sample with E. coli ATCC #11775 according to the spiking

procedure in Section 14. Spiking with laboratory-prepared suspensions is described in

Section 14.2 and spiking with BioBalls is described in Section 14.3. Filter and process
Method 1103.1 

March 2010 12

each OPR sample according to the procedures in Section 11 and calculate the number of

E. coli per 100 mL according to Section 13.

9.4.2 Calculate the percent recovery (R) for the OPR sample using the appropriate equation in

Section 14.2.2 or 14.3.4 for samples spiked with BioBalls or laboratory-prepared spiking

suspensions, respectively.

9.4.3 Compare the OPR results (percent recovery) with the corresponding OPR recovery

criteria in Table 1, above. If the OPR result meets the acceptance criteria for recovery,

method performance is acceptable and analysis of field samples may continue. If the

OPR result falls outside of the acceptance criteria, system performance is unacceptable. 

In this event, identify the problem by evaluating each step of the analytical process,

media, reagents, and controls, correct the problem and repeat the OPR analysis. 

9.4.4 As part of the laboratory QA program, method recovery results for OPR and IPR samples

should be charted and updated records maintained in order to monitor ongoing method

performance. The laboratory should also develop a statement of accuracy for Method

1103.1 by calculating the average percent recovery (R) and the standard deviation of the

percent recovery (sr

). Express the accuracy as a recovery interval from R - 2sr to R + 2sr.

9.5 Culture Controls

9.5.1 Negative controls—The laboratory should analyze negative controls to ensure that the

mTEC agar and urea substrate are performing properly. Negative controls should be

analyzed whenever a new batch of media or reagents is used. On an ongoing basis, the

laboratory should perform a negative control every day that samples are analyzed.

9.5.1.1 Negative controls are conducted by filtering a dilute suspension of viable E.

faecalis (e.g., ATCC #19433) and analyzing as described in Section 11. 

Viability of the negative controls should be demonstrated using a

non-selective media (e.g., nutrient agar or tryptic soy agar).

9.5.1.2 If the negative control fails to exhibit the appropriate response, check and/or

replace the associated media or reagents, and/or the negative control and

reanalyze the appropriate negative control. 

9.5.2 Positive controls—The laboratory should analyze positive controls to ensure that the

mTEC agar and urea substrate are performing properly. Positive controls should be

analyzed whenever a new batch of media or reagents is used. On an ongoing basis, the

laboratory should perform a positive control every day that samples are analyzed. An

OPR sample (Section 9.4) may take the place of a positive control.

9.5.2.1 Positive controls are conducted by filtering a dilute suspension of viable E.

coli (e.g., ATCC #11775) and analyzing as described in Section 11. 

9.5.2.2 If the positive control fails to exhibit the appropriate response, check and/or

replace the associated media or reagents, and/or the positive control and

reanalyze the appropriate positive control. 
Method 1103.1 

13 March 2010

9.5.3 Controls for verification media—All verification media should be tested with

appropriate positive and negative controls whenever a new batch of media and/or

reagents are used. On an ongoing basis, the laboratory should perform positive and

negative controls on the verification media with each batch of samples submitted to

verification. Examples of appropriate controls for verification media are provided in

Table 2.

Table 2. Verification Controls

Medium Positive Control Negative Control

Cytochrome oxidase reagent P. aeruginosa E. coli

Kovacs indole reagent E. coli E. aerogenes

Simmons citrate agar E. aerogenes S. flexneri

EC broth (44.5°C ± 0.2°C) E. coli E. aerogenes

9.6 Colony verification—The laboratory should verify 10 typical colonies (positive) and 10 atypical

colonies (negative) per month or 1 typical colony and 1 atypical colony from 10% of all positive

samples, whichever is greater. Verification procedures are provided in Section 12.

9.7 Filter sterility check—Place at least one membrane filter on a TSA plate, and incubate for 24 ± 2

hours at 35°C ± 0.5°C. Absence of growth indicates sterility of the filter. On an ongoing basis,

the laboratory should perform a filter sterility check every day that samples are analyzed.

9.8 Method blank—Filter a 50-mL volume of sterile PBS or phosphate-buffered dilution water,

place the filter on an mTEC agar plate and process according to Section 11. Absence of growth

indicates freedom of contamination from the target organism. On an ongoing basis, the

laboratory should perform a method blank every day that samples are analyzed.

9.9 Filtration blank—Filter a 50-mL volume of sterile PBS or phosphate-buffered dilution water

before beginning sample filtrations. Place the filter on a TSA plate, and incubate for 24 ± 2 hours

at 35°C ± 0.5°C. Absence of growth indicates sterility of the buffer and filtration assembly.

9.10 Media sterility check—The laboratory should test media sterility by incubating one unit (tube or

plate) from each batch of medium (TSA, mTEC, urea substrate, and verification media) as

appropriate and observing for growth. Absence of growth indicates media sterility. On an

ongoing basis, the laboratory should perform a media sterility check every day that samples are

analyzed.

9.11 Analyst colony counting variability—Laboratories with two or more analysts should compare

each analyst’s colony counts from one positive field sample per month. Colony counts should be

within 10% between analysts. Laboratories with a single analyst should have that analyst

perform duplicate colony counts of a single membrane filter each month. Duplicate colony

counts should be within 5% for a single analyst. If no positive field samples are available, a OPR

sample may be substituted for these determinations.
Method 1103.1 

March 2010 14

10.0 Calibration and Standardization

10.1 Check temperatures in incubators twice daily with a minimum of 4 hours between each reading to

ensure operation within stated limits.

10.2 Check thermometers at least annually against a NIST certified thermometer or one that meets the

requirements of NIST monograph SP 250-23. Check mercury columns for breaks.

10.3 Refrigerators used to store media and reagents should be monitored daily to ensure proper

temperature control.

11.0 Procedure

11.1 Prepare mTEC agar and urea substrate as directed in Sections 7.6 and 7.7, respectively.

11.2 Mark the petri dish and report form with the sample identification and volume.

11.3 Place a sterile membrane filter on the filter base, grid side up, and attach the funnel to the base so

that the membrane filter is held between the funnel and the base.

11.4 Shake the sample bottle vigorously at least 25 times to distribute the bacteria uniformly, and

measure the desired volume of sample or dilution into the funnel.

11.5 Select sample volumes based on previous knowledge of E. coli concentration, to produce 20-80

E. coli colonies on the membranes. It is recommended that a minimum of three dilutions be

analyzed to ensure that a countable plate (20-80 E. coli colonies) is obtained.

11.6 Smaller sample volumes or sample dilutions can be used to minimize the interference of turbidity

or for high bacterial densities. Multiple volumes of the same sample or sample dilutions should

be filtered. 

Note: When analyzing smaller sample volumes (e.g., < 20 mL), 20-30 mL of PBS or phosphatebuffered dilution water should be added to the funnel or an aliquot of sample should be dispensed

into a dilution blank prior to filtration. This will allow even distribution of the sample on the

membrane.

11.7 Filter the sample, and rinse the sides of the funnel at least twice with 20-30 mL of sterile buffer. 

Turn off the vacuum, and remove the funnel from the filter base.

11.8 Use sterile forceps to aseptically remove the membrane filter from the filter base, and roll it onto

the mTEC agar to avoid the formation of bubbles between the membrane and the agar surface. 

Reseat the membrane if bubbles occur. Run the forceps around the edge of the filter outside the

area of filtration, close to the edge of the dish, to be sure that the filter is properly seated on the

agar. Close the dish, invert, and incubate 35°C ± 0.5°C for 2 ± 0.5 hours. 

11.9 After a 2 ± 0.5 hour incubation at 35°C ± 0.5°C, transfer the plates to a Whirl-Pak® bag, seal the

bag, and submerge in a 44.5°C ± 0.2°C waterbath for 22 ± 2 hours. 

Do not overfill the Whirl-Pak® bag because this will prevent proper sealing allowing liquid to

enter the bag and possibly contaminating the plates. 

11.10 After 22 ± 2 hours, remove the plate from the waterbath. Place an absorbent pad in the lid of the

same petri dish, and saturate the pad with urea substrate medium. Aseptically transfer the

membrane from mTEC agar to the absorbent pad saturated with urea substrate medium, and allow

to sit at room temperature for 15-20 minutes. (See Photo 2)
Method 1103.1 

15 March 2010

Photo 2. Escherichia coli produces yellow, yellow-green, or yellow-brown colonies on mTEC agar. 

11.11 After incubation on the urea substrate at room temperature, count and record colonies on those

membrane filters containing 20-80 yellow, yellow-green, or yellow-brown colonies. Use

magnification for counting and a small fluorescent lamp to give maximum visibility of colonies.

(See Photo 3)

Photo 3. E. coli colonies remain yellow, yellow-green, or yellow-brown when the filter is placed on

the urea substrate medium, while non-target colonies turn pink or purple.

12.0 Verification Procedure

12.1 Yellow, yellow-green, or yellow-brown colonies from the urease test are considered “typical” E.

coli. Verification of typical and atypical colonies may be required in evidence gathering and is

also recommended as a means of quality control. The verification procedure follows.

12.2 Using a sterile inoculating loop or needle, transfer growth from the centers of at least 10 wellisolated typical and 10 well-isolated atypical colonies to nutrient agar plates or slants and to

tryptic/trypticase soy broth. Incubate the agar and broth cultures for 24 ± 2 hours at 35°C ±

0.5°C.
Method 1103.1 

March 2010 16

12.3 After incubation, transfer growth from the nutrient agar slant and perform cytochrome oxidase

test. If the area where the bacteria were applied turns deep purple within 15 seconds, the test is

positive. 

Note: Use only platinum, plastic or wooden applicators to perform the oxidase test. Do not use

iron or other reactive wire because it may cause false positive reactions.

12.4 Transfer growth from the tryptic/trypticase soy broth tube to Simmons citrate agar, tryptone

water, and an EC broth fermentation tube.

12.4.1 Incubate the Simmons citrate agar for 4 days at 35°C ± 2.0°C in an aerobic atmosphere. 

A positive reaction is indicated by growth with an intense blue color on the slant. E. coli

is citrate negative, and thus for this organism on this medium there should be either no

growth or trace growth with no change in agar color (i.e., medium remains dark green).

12.4.2 Incubate the EC broth at 44.5°C ± 0.2°C in a waterbath for 24 ± 2 hours. The water level

must be above the level of the EC broth in the tube. A positive test is indicated by

turbidity and production of gas as seen in the inner durham tube.

12.4.3 Incubate the tryptone water for 18-24 hours at 35°C ± 2.0°C with loosened caps. After

the incubation period, add 0.5 mL of Kovacs indole reagent and shake the tube gently. 

Allow the tubes to stand for 5-10 minutes at room temperature. A positive test for indole

is indicated by a deep red color which develops in the alcohol layer on top of the broth. 

12.5 E. coli are oxidase-negative, citrate-negative, EC growth and gas-positive, and indole-positive.

12.6 Alternately, commercially available multi-test identification systems may be used to verify

colonies. Inoculate the colonies into an identification system for Enterobacteriaceae that

includes lactose fermentation, F-nitrophenyl-$-D-galactopyranoside (ONPG), and cytochrome

oxidase test reactions.

13.0 Data Analysis and Calculations

Use the following general rules to calculate the E. coli count per 100 ml of sample:

13.1 If possible, select a membrane filter with 20-80 yellow, yellow-green, or yellow-brown colonies

on the urea substrate, and calculate the number of E. coli per 100 mL according to the following

general formula:

E. coli / 100 mL =

Number of E. coli colonies

× 100

Volume of sample filtered (mL)

13.2 See general counting rules in Reference 18.7 (see Appendix B).

13.3 Report results as E. coli CFU per 100 mL of sample. 
Method 1103.1 

17 March 2010

14.0 Sample Spiking Procedure

14.1 Method 1103.1 QC requirements (Section 9) include the preparation and analysis of spiked

reference (PBS) samples in order to monitor initial and ongoing method performance. For the

IPR (Section 9.3) and OPR (Section 9.4) tests it is necessary to spike samples with either

laboratory-prepared spiking suspensions (Section 14.2) or BioBalls (Section 14.3) as described

below. 

14.2 Laboratory-Prepared Spiking Suspensions

14.2.1 Preparation

14.2.1.1 Stock Culture. Prepare a stock culture by inoculating a TSA slant (or other

non-selective media) with Escherichia coli ATCC #11775 and incubating at

35°C ± 3°C for 20 ± 4 hours. This stock culture may be stored in the dark at

room temperature for up to 30 days.

14.2.1.2 Undiluted Spiking Suspension. Prepare a 1% solution of lauryl tryptose

broth (LTB) by combining 99 mL of sterile PBS and 1 mL of sterile singlestrength LTB in a sterile screw-cap bottle or re-sealable dilution water

container. Inoculate the 1% LTB using a small amount of growth from the

stock culture. Disperse the inoculum by vigorously shaking the broth culture

and incubate at 35°C ± 3°C for 20 ± 4 hours. This culture is referred to as

the undiluted spiking suspension and should contain approximately 

1.0 × 107

 - 1.0 × 108 E. coli colony forming units (CFU) per mL of culture.

14.2.1.3 Mix the undiluted spiking suspension thoroughly by shaking the bottle a

minimum of 25 times and prepare a series of dilutions (4 total) in the

following manner:

14.2.1.3.1 Dilution "A"—Aseptically transfer 1.0 mL of the undiluted

spiking suspension to 99 mL of sterile PBS and mix

thoroughly by shaking the bottle a minimum of 25 times. 

This is spiking suspension dilution "A" and 1 mL contains

10-2 mL of the original undiluted spiking suspension.

14.2.1.3.2 Dilution "B"—Aseptically transfer 1.0 mL of dilution "A" to

99 mL of sterile PBS and mix thoroughly by shaking the

bottle a minimum of 25 times. This is spiking suspension

dilution "B" and 1 mL contains 10-4 mL of the original

undiluted spiking suspension.

14.2.1.3.3 Dilution "C"—Aseptically transfer 11.0 mL of dilution "B"

to 99 mL of sterile PBS and mix thoroughly by shaking the

bottle a minimum of 25 times. This is spiking suspension

dilution "C" and 1 mL contains 10-5 mL of the original

undiluted spiking suspension.
Method 1103.1 

March 2010 18

14.2.1.3.4 Dilution "D"—Aseptically transfer 11.0 mL of dilution "C"

to 99 mL of sterile PBS and mix thoroughly by shaking the

bottle a minimum of 25 times. This is spiking suspension

dilution "D" and 1 mL contains 10-6 mL of the original

undiluted spiking suspension. 

14.2.2 Sample Spiking

14.2.2.1 Add 0.3 mL of the spiking suspension dilution "D" to 100 mL of PBS and

mix thoroughly by shaking the bottle a minimum of 25 times. The volume of

undiluted spiking suspension added to each 100 mL sample is 3.0 x 10-7 mL

[(0.3 mL x 10-6 mL) per 100 mL of sample] which is referred to as Vspiked per 100

mL sample in Section 14.2.4.2 below. Filter the spiked sample and analyze the

filter according to the procedures in Section 11. 

14.2.3 Enumeration of Spiking Suspension

14.2.3.1 Prepare trypticase soy agar (TSA) spread plates, in triplicate, for spiking

suspension dilutions "B", "C", and "D".

Note: Agar plates must be dry prior to use. To ensure that the agar surface is

dry, plates should be made several days in advance and stored inverted at

room temperature or dried using a laminar-flow hood.

14.2.3.2 Mix dilution "B" by shaking the bottle a minimum of 25 times. Pipet 0.1 mL

of dilution "B" onto the surface of each TSA plate in triplicate.

14.2.3.3 Mix dilution "C" by shaking the bottle a minimum of 25 times. Pipet 0.1 mL

of dilution "C" onto the surface of each TSA plate in triplicate.

14.2.3.4 Mix dilution "D" by shaking the bottle a minimum of 25 times. Pipet 0.1 mL

of dilution "D" onto the surface of each TSA plate in triplicate.

14.2.3.5 Use a sterile bent glass rod or spreader to distribute the inoculum over the

surface of plates by rotating the dish by hand or on a turntable. Note: Ensure

that the inoculum is evenly distributed over the entire surface of the plate.

14.2.3.6 Allow the inoculum to absorb into the medium of each plate completely. 

Invert plates and incubate at 35°C ± 0.5°C for 20 ± 4 hours. 

14.2.3.7 Count and record number of colonies per plate. Refer to Section 14.2.4 for

calculation of E. coli concentration in the undiluted spiking suspension. The

number of E. coli (CFU / mL) in the undiluted spiking suspension will be

calculated using all TSA plates yielding counts within the countable range of

30 to 300 CFU per plate.
Method 1103.1 

19 March 2010

14.2.4 Recovery calculations for samples spiked with laboratory-prepared spiking

suspensions

14.2.4.1 Calculate the concentration of E. coli (CFU / mL) in the undiluted spiking

suspension (Section 14.2.1.2) according to the following equation. Example

calculations are provided in Table 3, below.

E. coli undiluted spike = (CFU1 + CFU2 + ...+ CFUn) / (V1 + V2 + ... + Vn) 

Where,

E. coli undiluted spike = E. coli (CFU / mL) in undiluted spiking suspension

CFU = Number of colony forming units from TSA plates yielding counts within the

countable range of 30 to 300 CFU per plate

V = Volume of undiluted sample on each TSA plate yielding counts within the

countable range of 30 to 300 CFU per plate

n = Number of plates with counts within the countable range (30 to 300 CFU /

plate)

Note: The example calculated numbers provided in the tables below have been rounded

at the end of each step. If your laboratory recalculates the examples using a spreadsheet

and rounds only after the final calculation, the percent recoveries may be slightly

different.

Table 3. Example Calculations of E. coli Spiking Suspension Concentration

Examples

CFU / plate (triplicate analyses) from 

TSA plates in Section 14.2.3.7 E. coli CFU / mL in undiluted 

spiking suspension

(EC undiluted spike)* 10-5 mL plates 10-6 mL plates 10-7 mL plates 

Example 1 TNTC, TNTC,

TNTC 94, 106, 89 10, 0, 4

(94+106+89) / (10-6+10-6+10-6) = 

289 / (3.0 x 10-6) = 96,333,333 = 

9.6 x 107

 CFU / mL

Example 2 269, 289, 304 24, 30, 28 0, 2, 0

(269+289+30) / (10-5+10-5+10-6) = 

588 / (2.1 x 10-5) = 28,000,000 = 

2.8 x 107

 CFU / mL

*EC undiluted spike is calculated using all plates yielding counts within the ideal range of 30 to 300 CFU per plate
Method 1103.1 

March 2010 20

14.2.4.2 Calculate true concentration (CFU / 100 mL) of spiked E. coli (T spiked E. coli)

according to the following equation. Example calculations are provided in

Table 4, below. 

T spiked E. coli = (E. coli undiluted spike) x (V spiked per 100 mL sample)

Where,

T spiked E. coli = Number of spiked E. coli (CFU / 100 mL)

E. coli undiluted spike = E. coli (CFU / mL) in undiluted spiking suspension

V spiked per 100 mL sample = mL of undiluted spiking suspension per 100 mL sample

Table 4. Example Calculations of Spiked E. coli 

E. coli undiluted spike V spiked per 100 mL sample

TSpiked E. coli

 9.6 x 107 CFU / mL 3.0 X 10-7 mL per 100 mL of

sample

(9.6 x 107

 CFU / mL) x (3.0 x 10-7 mL / 100 mL) =

28.8 CFU / 100 mL

2.8 x 107 CFU / mL 3.0 X 10-7 mL per 100 mL of

sample

(2.8 x 107

 CFU / mL) x (3.0 x 10-7 mL / 100 mL) = 

8.4 CFU / 100 mL

14.2.4.3 Calculate percent recovery (R) of spiked E. coli (CFU / 100 mL) according

to the following equation. Example calculations are provided in Table 5,

below. 

R = 100 x (Ns - Nu)

T

Where,

R = Percent recovery

Ns = E. coli (CFU / 100 mL) in the spiked sample (Section 13)

Nu = E. coli (CFU / 100 mL) in the unspiked sample (Section 13)

T = True spiked E. coli (CFU / 100 mL) in spiked sample (Section 14.2.4.2)
Method 1103.1 

21 March 2010

Table 5. Example Percent Recovery Calculations

Ns (CFU / 100 mL) Nu (CFU / 100 mL) T (CFU / 100 mL) Percent recovery (R)

42 <1 28.8 100 x (42 - 1) / 28.8

= 142%

34 10 28.8 100 x (34 - 10) / 28.8

= 83%

16 <1 8.4 100 x (16 - 1) / 8.4

= 179%

10 <1 8.4 100 x (10 - 1) / 8.4

= 107%

14.3 BioBallTM spiking procedure

14.3.1 Aseptically add 1 BioBallTM to 100 mL (or appropriate volume) of sample and mix by

vigorously shaking the sample bottle a minimum of 25 times. Analyze the spiked sample

according to the procedures in Section 11.

14.3.2 Recovery calculations for samples spiked with BioBalls—Calculate percent recovery (R)

of spiked E. coli (CFU / 100 mL) according to the following equation. Example

calculations are provided in Table 6, below. 

R = 100 x (Ns - Nu)

T

Where,

R = Percent recovery

Ns = E. coli (CFU / 100 mL) in the spiked sample (Section 13)

Nu = E. coli (CFU / 100 mL) in the unspiked sample (Section 13)

T = True spiked E. coli (CFU / 100 mL) in spiked sample based on the

lot mean value provided by manufacturer
Method 1103.1 

March 2010 22

Table 6. Example Percent Recovery Calculations

Ns (CFU / 100

mL)

Nu (CFU / 100

mL) T (CFU / 100 mL) Percent recovery (R)

24 <1 32 100 × (24 - 1) / 32 = 72%

36 10 32 100 × (36 - 10) / 32 = 81%

15.0 Method Performance

15.1 Performance Characteristics

15.1.1 Precision – The degree of agreement of repeated measurements of the same parameter

expressed quantitatively as the standard deviation or as the 95% confidence limits of the

mean computed from the results of a series of controlled determinations. The mTEC

method precision was found to be fairly representative of what would be expected from

counts with a Poisson distribution (Reference 18.3).

15.1.2 Bias – The persistent positive or negative deviation of the average value of the method

from the assumed or accepted true value. The bias of the mTEC method has been

reported to be -2% of the true value (Reference 18.3).

15.1.3 Specificity – The ability of a method to select and or distinguish the target bacteria under

test from other bacteria in the same water sample. The specificity characteristic of a

method is usually reported as the percent of false positive and false negative results. The

false positive rate reported for mTEC medium averaged 9% for marine and fresh water

samples. Less than 1% of the E. coli colonies observed gave a false negative reaction

(Reference 18.3).

15.1.4 Upper Counting Limit (UCL) – That colony count above which there is an unacceptable

counting error. The error may be due to overcrowding or antibiosis. The UCL for 

E. coli on mTEC medium has been reported as 80 colonies per filter (Reference 18.3).

15.2 Collaborative Study Data for Ambient Waters

15.2.1 A collaborative study was conducted among eleven volunteer laboratories, each with two

analysts who independently tested local fresh and marine recreational waters and sewage

treatment plant effluent samples, in duplicate. The data were reported to the

Environmental Monitoring and Support Laboratory - Cincinnati, U.S. Environmental

Protection Agency, for statistical calculations.

15.2.2 The results of the study are shown in Figure 1 where SO equals the pooled standard

deviation among replicate counts from a single analyst for three groupings (counts less

than 30, counts from 30 to 50, and counts greater than 50) and SB equals the pooled

standard deviation between means of duplicates from analysts in the same laboratory for

the same groupings. The precision estimates from this study did not show any difference

among the water types analyzed.
Method 1103.1 

23 March 2010

15.2.3 By linear regression, the precision of the method can be generalized as:

SO = 0.028 count/100 mL + 6.11 (dilution factor) and 

SB = 0.233 count/100 mL + 0.82 (dilution factor)

Where dilution factor =

100

Volume of Original Sample Filtered

15.2.4 Because of the instability of microbial populations in water samples, each laboratory

analyzed its own sample series and no full measure of recovery or bias was possible. 

However, all laboratories analyzed a single surrogate sample prepared from a freezedried culture of E. coli. The mean count (0) and the overall standard deviation of the

counts (ST) (which includes the variability among laboratories for this standardized E.

coli sample) were 31.6 colonies/membrane and 7.61 colonies/membrane, respectively.
Method 1103.1 

March 2010 24
Method 1103.1 

25 March 2010

15.3 Interlaboratory Validation Study Results for Wastewater

15.3.1 Ten volunteer participant laboratories, an E. coli verification laboratory, and two research

laboratories participated in the U.S. Environmental Protection Agency's (EPA's)

interlaboratory validation study of EPA Method 1103.1 for the analysis of E. coli in

disinfected wastewater. The purposes of the Study were to characterize method

performance across multiple laboratories and disinfected wastewater matrices and to

develop quantitative quality control (QC) acceptance criteria. The false positive and

negative assessments are provided in Tables 7 and 8, respectively. Detailed results of the

study are provided in the validation study report (Reference 18.2). 

15.3.2 Method 1103.1 is not approved for the analysis of E. coli in disinfected wastewater

because of the high false positive and false negative levels observed during the validation

study compared to Method 1603. Laboratories wishing to test for E. coli in wastewater

using a membrane filtration method are referred to EPA Method 1603. 

Table 7. False Positive Assessment of Unspiked Disinfected and Unspiked Secondary

Wastewater Effluents

Matrix

Total colonies False positive (FP) assessment

Typical Atypical

Typical

colonies

submitted

No. FP

colonies

FP confirmation

rate (%) a

Estimated % of total

colonies that would have

been a FP b

Disinfected 272 725 70 16 22.9% 6.2%

Secondary 1190 347 104 15 14.4% 11.2%

Disinfected + Secondary 1462 1072 174 31 17.8% 10.3%

a

 False positive confirmation rate = number of false positive colonies / number of typical colonies submitted 

b

 Percent of total colonies estimated to be false positives = [(total typical colonies × FP confirmation rate) / (total

number of typical and atypical colonies observed)] × 100; e.g., [(1190 ×(15/104))/(1190+347)] × 100 = 11.2%
Method 1103.1 

March 2010 26

Table 8. False Negative Assessment of Unspiked Disinfected and Unspiked Secondary

Wastewater Effluents

Matrix 

Total colonies False negative (FN) assessment

Typical Atypical

Atypical

colonies

submitted

No. FN

colonies

FN confirmation

rate (%) a

Estimated % of total

colonies that would have

been a FN b

Disinfected 272 725 89 15 16.9% 12.3%

Secondary 1190 347 45 4 8.9% 2.0%

Disinfected + Secondary 1462 1072 134 19 14.2% 6.0%

a False negative confirmation rate = number of false negative colonies / number of atypical colonies submitted 

b Percent of total colonies estimated to be false negatives = [(total atypical colonies × FN confirmation rate) / (total 

number of typical and atypical colonies observed)] × 100; e.g., [(347×(4/45))/(1190+347)] × 100 = 2.0%

16.0 Pollution Prevention

16.1 The solutions and reagents used in this method pose little threat to the environment when

recycled and managed properly.

16.2 Solutions and reagents should be prepared in volumes consistent with laboratory use to minimize

the volume of expired materials to be disposed.

17.0 Waste Management

17.1 It is the laboratory's responsibility to comply with all federal, state, and local regulations

governing waste management, particularly the biohazard and hazardous waste identification rules

and land disposal restrictions, and to protect the air, water, and land by minimizing and

controlling all releases from fume hoods and bench operations. Compliance with all sewage

discharge permits and regulations is also required.

17.2 Samples, reference materials, and equipment known or suspected to have viable E. coli attached

or contained must be sterilized prior to disposal.

17.3 For further information on waste management, consult “The Waste Management Manual for

Laboratory Personnel” and “Less Is Better: Laboratory Chemical Management for Waste

Reduction,” both available from the American Chemical Society's Department of Government

Relations and Science Policy, 1155 16th Street NW, Washington, DC 20036.
Method 1103.1 

27 March 2010

18.0 References

18.1 Dufour, A.P. Health Effects Criteria for Fresh Recreational Waters, EPA-600/1-84-004.

Research Triangle Park, NC: U.S. Environmental Protection Agency, 1984.

18.2 USEPA. 2004. Results of the Interlaboratory Validation of EPA Method 1103.1 (mTEC) for E.

coli in Wastewater Effluent. EPA-821-R-04-020. December 2004.

18.3 USEPA. 2004. Results of the Interlaboratory Validation of EPA Method 1603 (modified mTEC)

for E. coli in Wastewater Effluent. EPA-821-R-04-020. December 2004.

18.4 Dufour, A.P., E.R. Strickland, V.J. Cabelli. 1981. Membrane filter method for enumerating 

Escherichia coli. Appl. Environ. Microbiol. 41:1152-1158.

18.5 ACS. 2000. Reagent Chemicals, American Chemical Society Specifications. American Chemical

Society, New York. For suggestions of the testing of reagents not listed by the American

Chemical Society, see AnalaR Standards for Laboratory Chemicals, BDH, Poole, Dorset, UK

and the United States Pharmacopeia.

18.6 APHA. 1998. Standard Methods for the Examination of Water and Wastewater. 20th Edition.

American Public Health Association, Washington D.C.

18.7 Bordner, R., J.A. Winter, and P.V. Scarpino (eds.). Microbiological Methods for Monitoring the

Environment, Water and Wastes, EPA-600/8-78-017. Cincinnati, OH: U.S. Environmental

Protection Agency, 1978.

Appendix A:

Part II (General Operations), Section A (Sample Collection,

Preservation, and Storage)
Sample Collection1 

1.0 Sample Containers 

1.1 Sample Bottles: bottles must be resistant to sterilizing conditions and the solvent action 

of water. Wide-mouth borosilicate glass bottles with screw-cap or ground-glass stopper 

or heat-resistant plastic bottles may be used if they can be sterilized without producing 

toxic materials (see examples A and C in Figure 1). Screw-caps must not produce 

bacteriostatic or nutritive compounds upon sterilization. 

Figure 1. Suggested sample containers. 

1.2 Selection and Cleaning of Bottles: Samples bottles should be at least 125 mL volume 

for adequate sampling and for good mixing. Bottles of 250 mL, 500 mL, and 1000 mL 

volume are often used for multiple analyses. Discard bottles which have chips, cracks, 

and etched surfaces. Bottle closures must be water-tight. Before use, thoroughly cleanse 

bottles and closures with detergent and hot water, followed by a hot water rinse to 

remove all trace of detergent. Then rinse them three times with laboratory-pure water. 

1.3 Dechlorinating Agent: The agent must be placed in the bottle when water and 

wastewater samples containing residual chlorine are anticipated. Add sodium thiosulfate 

to the bottle before sterilization at a concentration of 0.1 mL of a 10% solution for each 

125 mL sample volume. This concentration will neutralize approximately 15 mg/L of 

residue chlorine. 

1.4 Chelating Agent: A chelating agent should be added to sample bottles used to collect 

samples suspected of containing >0.01 mg/L concentrations of heavy metals such as 

copper, nickel or zinc, etc. Add 0.3 mL of a 15% solution of ethylenediaminetetraacetic 

acid (EDTA) tetrasodium salt, for each 125 mL sample volume prior to sterilization. 

1

The text is taken from Part II, Section A, of the EPA publication "Microbiological Methods for 

Monitoring the Environment" EPA-600/8-78-017, December 1978. 

1 
1.5 Wrapping Bottles: Protect the tops and necks of glass stoppered bottles from 

contamination by covering them before sterilization with aluminum foil or kraft paper. 

1.6 Sterilization of Bottles: Autoclave glass or heat-resistant plastic bottles at 121°C for 15 

minutes. Alternatively, dry glassware may be sterilized in a hot oven at 170°C for not 

less than two hours. Ethylene oxide gas sterilization is acceptable for plastic containers 

that are not heat-resistant. Sample bottles sterilized by gas should be stored overnight 

before being used to allow the last traces of gas to dissipate. 

1.7 Plastic Bags: The commercially available bags (Whirl-pak) (see example B in Figure 1) 

are a practical substitute for plastic or glass samples bottles in sampling soil, sediment, or 

biosolids. The bags are sealed in manufacture and opened only at time of sampling. The 

manufacturer states that such bags are sterilized. 

2.0 Sampling Techniques 

Samples are collected by hand or with a sampling device if the sampling site has difficult access 

such as a bridge or bank adjacent to a surface water. 

2.1 Chlorinated Samples: When samples such as treated waters, chlorinated wastewaters or 

recreational waters are collected, the sample bottle must contain a dechlorinating agent (see 

section 1.3 above). 

2.2 Composite Sampling: In no case should a composite sample be collected for bacteriologic 

examination. Data from individual samples show a range of values. A composite sample 

will not display this range. Individual results will give information about industrial process 

variations in flow and composition. Also, one or more portions that make up a composite 

sample may contain toxic or nutritive materials and cause erroneous results. 

2.3 Surface Sampling by Hand: A grab sample is obtained using a sample bottle prepared as 

described in (1) above. Identify the sampling site on the bottle label and on a field log sheet. 

Remove the bottle covering and closure and protect from contamination. Grasp the bottle at 

the base with one hand and plunge the bottle mouth down into the water to avoid introducing 

surface scum (Figure 2). Position the mouth of the bottle into the current away from the 

hand of the collector and, if applicable, away from the side of the sampling platform. The 

sampling depth should be 15-30 cm (6-12 inches) below the water surface. If the water body 

is static, an artificial current can be created, by moving the bottle horizontally in the 

direction it is pointed and away from the sampler. Tip the bottle slightly upwards to allow 

air to exit and the bottle to fill. After removal of the bottle from the stream, pour out a small 

portion of the sample to allow an air space of 2.5-5 cm (1-2 inches) above each sample for 

proper mixing of the sample before analyses. Tightly stopper the bottle and place on ice (do 

not freeze) for transport to the laboratory. 

2
Figure 2. Grab sampling technique for surface waters. 

3.0 Selection of Sampling Sites and Frequency 

These will be described for streams, rivers, estuarine, marine, and recreational waters as well as 

domestic and industrial wastewaters. 

3.1 Stream Sampling: The objectives of the initial survey dictate the location, frequency and 

number of samples to be collected. 

3.1.1 Selection of Sampling Sites: A typical stream sampling program includes sampling 

locations upstream of the area of concern, upstream and downstream of waste 

discharges, upstream and downstream from tributary entrances to the river and 

upstream of the mouth of the tributary. For more complex situations, where several 

waste discharges are involved, sampling includes sites upstream and downstream from 

the combined discharge area and samples taken directly from each industrial or 

municipal waste discharge. Using available bacteriological, chemical and discharge 

rate data, the contribution of each pollution source can be determined. 

3.1.2 Small Streams: Small streams should be sampled at background stations upstream of 

the pollution sources and at stations downstream from pollution sources. Additional 

sampling sites should be located downstream to delineate the zones of pollution. 

Avoid sampling areas where stagnation may occur (e.g., backwater of a tributary) and 

areas located near the inside bank of a curve in the stream which may not be 

representative of the main channel. 

3
3.1.3 Large Streams and Rivers: Large streams are usually not well mixed laterally for long 

distances downstream from the pollution sources. Sampling sites below point source 

pollution should be established to provide desired downstream travel time and 

dispersal as determined by flow rate measurements. Particular care must be taken to 

establish the proper sampling points. Occasionally, depth samples are necessary to 

determine vertical mixing patterns. 

3.2 Estuarine and Marine Sampling: Sampling estuarine and marine waters requires the 

consideration of other factors in addition to those usually recognized in fresh water 

sampling. They include tidal cycles, current patterns, bottom currents and counter-currents, 

stratification, seasonal fluctuations, dispersion of discharges and multi-depth samplings. 

The frequency of sampling varies with the objectives. When a sampling program is started, 

it may be necessary to sample every hour around the clock to establish pollution loads and 

dispersion patterns. The sewage discharges may occur continuously or intermittently. 

When the sampling strategy for a survey is planned, data may be available from previous 

hydrological studies done by the Coast Guard, Corps of Engineers, National Oceanic and 

Atmospheric Administration (NOAA), U.S. Geological Survey, or university and private 

research investigations. In a survey, float studies and dye studies are often carried out to 

determine surface and undercurrents. Initially depth samples are taken on the bottom and at 

five feet increments between surface and bottom. A random grid pattern for selecting 

sampling sites is established statistically. 

3.2.1 Estuarine Sampling: When a survey is made on an estuary, samples are often taken 

from a boat, usually making an end to end traverse of the estuary. Another method 

involves taking samples throughout a tidal cycle, every hour or two hours from a 

bridge or from an anchored boat at a number of fixed points. 

In a large bay or estuary where many square miles of area are involved, a grid or 

series of stations may be necessary. Two sets of samples are usually taken from an 

area on a given day, one at ebb or flood slack water, and the other three hours earlier, 

or later, at the half tidal interval. Sampling is scheduled so that the mid-sampling time 

of each run coincides with the calculated occurrence of the tidal condition. 

In location sampling sites, one must consider points at which tributary waters enter the 

main stream or estuary, location of shellfish beds and bathing beaches. The sampling 

stations can be adjusted as data accumulate. For example, if a series of stations half 

mile apart consistently show similar values, some of these stations may be dropped 

and other stations added in areas where data shows more variability. 

Considerable stratification can occur between the salt water from the sea and the fresh 

water supplied by a river. It is essential when starting a survey of an unknown estuary 

to find out whether there is any marked stratification. This can be done by chloride 

determinations at different locations and depths. It is possible for stratification to 

occur in one part of an estuary and not in another. 

4
On a flood tide, the more dense salt water pushing up into the less dense fresh river 

water will cause an overlapping with the fresh water flowing on top. A phenomenon 

called a salt water wedge can form. As a result, stratification occurs. If the discharge 

of pollution is in the salt water layer, the contamination will be concentrated near the 

bottom at the flood tide. The flow or velocity of the fresh water will influence the 

degree of stratification which occurs. If one is sampling only at the surface, it is 

possible that the data will not show the polluted underflowing water which was 

contaminated at the point below the fresh water river. Therefore, where stratification 

is suspected, samples at different depths will be needed to measure vertical 

distribution. 

3.2.2 Marine Sampling: In ocean studies, the environmental conditions are most diverse 

along the coast where shore, atmosphere and the surf are strong influences. The 

shallow coastal waters are particularly susceptible to daily fluctuations in temperature 

and seasonal changes. 

Sampling during the entire tidal cycle or during a half cycle may be required. Many 

ocean studies such as sampling over the continental shelf involve huge areas and no 

two areas of water are the same. 

Selection of sampling sites and depths are most critical in marine waters. In winter, 

cooling of coastal waters can result in water layers which approach 0°C. In summer, 

the shallow waters warm much faster than the deeper waters. Despite the higher 

temperature, oxygen concentrations are higher in shallow than in deeper waters due to 

greater water movement, surf action and photosynthetic activity from macrophytes 

and the plankton. 

Moving from the shallow waters to the intermediate depths, one observes a 

moderation of these shallow water characteristics. In the deeper waters, there is a 

marked stabilization of conditions. Water temperatures are lower and more stable. 

There is limited turbulence, little penetration of light, sparse vegetation and the ocean 

floor is covered with a layer of silts and sediments. 

3.3 Recreational Waters (Bathing Beaches): Sampling sites at bathing beaches or other 

recreational areas should include upstream or peripheral areas and locations adjacent to 

natural drains that would discharge stormwater, or run-off areas draining septic wastes from 

restaurants, boat marinas, or garbage collection areas. Samples of bathing beach water 

should be collected at locations and times of heaviest use. Daily sampling, preferably in the 

afternoon, is the optimum frequency during the season. Weekends and holidays which are 

periods of highest use must be included in the sampling program. Samples of estuarine 

bathing waters should be obtained at high tide, ebb tide and low tide in order to determine 

the cyclic water quality and deterioration that must be monitored during the swimming 

season. 

5
3.4 Domestic and Industrial Waste Discharges: It is often necessary to sample secondary and 

tertiary wastes from municipal waste treatment plants and various industrial waste treatment 

operations. In situations where the plant treatment efficiency varies considerably, grab 

samples are collected around the clock at selected intervals for a three to five day period. If 

it is known that the process displays little variation, fewer samples are needed. In no case 

should a composite sample be collected for bacteriological examination. The National 

Pollution Discharge Elimination System (NPDES) has established wastewater treatment 

plant effluent limits for all dischargers. These are often based on maximum and mean 

values. A sufficient number of samples must be collected to satisfy the permit and/or to 

provide statistically sound data and give a fair representation of the bacteriological quality of 

the discharge. 

6
Appendix B:

Part II (General Operations), Sections C.3.5 (Counting Colonies)

and C.3.6 (Calculation of Results)
Counting Colonies1 

1.0 Counting Colonies 

Colonies should be counted using a fluorescent lamp with a magnifying lens. The flourescent lamp 

should be nearly perpendicular to the membrane filter. Count colonies individually, even if they 

are in contact with each other. The technician must learn to recognize the difference between two 

or more colonies which have grown into contact with each other and single, irregularly shaped 

colonies which sometimes develop on membrane filters. The latter colonies are usually associated 

with a fiber or particulate material and the colonies conform to the shape and size of the fiber or 

particulates. Colonies which have grown together almost invariably show a very fine line of 

contact. 

2.0 Calculation of Results 

2.1 Select the membrane filter with the number of colonies in the acceptable range and calculate 

count per 100 mL according to the general formula: 

Count per 100 mL = (No. of colonies counted/Volume of sample filtered, in mL) x 100 

2.2 Counts Within the Acceptable Limits 

The acceptable range of colonies that are countable on a membrane is a function of the 

method. Different methods may have varying acceptable count ranges. All examples in this 

appemdix assume that the acceptable range of counts is between 20-80 colonies per 

membrane. 

For example, assume that filtration of volumes of 50, 15, 5, 1.5, and 0.5 mL produced 

colony counts of 200, 110, 40, 10, and 5, respectively. 

An analyst would not actually count the colonies on all filters. By inspection the analyst 

would select the membrane filter with the acceptable range of target colonies, as defined by 

the method, and then limit the actual counting to such membranes. 

After selecting the best membrane filter for counting, the analyst counts colonies and applies 

the general formula as in section 2.1 above to calculate the count/100 mL. 

2.3 More Than One Acceptable Count 

2.3.1 If there are acceptable counts on replicate plates, carry counts independently to final 

reporting units, then calculate the arithmetic mean of these counts to obtain the final 

reporting value. 

1

The text is largely taken from Part II, Section C, of the EPA publication "Microbiological 

Methods for Monitoring the Environment" EPA-600/8-78-017, December 1978. Some examples were 

kindly provided by Kristen Brenner, US EPA. 

1 
Example, if the counts are 24 and 36 for replicate plates of 100 mL each, then the arithmetic 

mean is calculated as follows: 

(24 CFU/100 mL + 36 CFU/100 mL) 

= 30 CFU/ 100 mL

2 

2.3.2 If there is more than one dilution having an acceptable range of counts, independently 

carry counts to final reporting units, then average for final reported value. 

For example, if volumes of 100, 10, 1 and 0.1 mL produced colony counts of Too 

Numerous To Count (TNTC), 75, 30, and 1, respectively, then two volumes, 10 mL and 1 

mL, produced colonies in the acceptable counting range. 

Independently carry each MF count to a count per 100 mL: 

75

 × 100 = 750 CFU /100 mL

10 

and 

30

 × 100 = 3000 CFU /100 mL 

1 

Calculate the arithmetic mean as in section 2.3.1 above: 

(750 CFU/100 mL + 3000 CFU/100 mL) 

= 1875 CFU/100 mL

2 

Report this as 1875 CFU/100 mL. 

2.4 If all MF counts are below the lower acceptable count limit, select the most nearly 

acceptable count. 

2.4.1 For example, sample volumes of 100, 10 and 1 mL produced colony counts of 17, 1 

and 0, respectively. 

Here, no colony count falls within recommended limits. Calculate on the basis of the 

most nearly acceptable plate count, 17, and report as 17 CFU/100 mL. 

Note that in this case, because no calculations were done (i.e. this is the count for 100 

mL), the count is reported as 17 CFU/100 mL rather than an “estimated count of 

17 CFU/100 mL” 

2 
2.4.2 As a second example, assume a count in which sample volumes of 10 and 1 mL 

produced colony counts of 18 and 0, respectively. 

Here, no colony count falls within recommended limits. Calculate on the basis of the 

most nearly acceptable plate count, 18, and calculate as in section 2.3.2 above. 

18

 × 100 = 180 CFU /100 mL

10 

Report this as an estimated count of 180 CFU/100 mL. 

2.5 If counts from all membranes are zero, calculate using count from largest filtration volume. 

For example, sample volumes of 25, 10, and 2 mL produced colony counts of 0, 0, and 0, 

respectively, and no actual calculation is possible, even as an estimated report. Calculate the 

number of colonies per 100 mL that would have been reported if there had been one colony 

on the filter representing the largest filtration volume. In this example, the largest volume 

filtered was 25 mL and thus the calculation would be: 

1

 × 100 = 4 CFU /100 mL 

25 

Report this as < (less than) 4 CFU/100 mL. 

2.6 If all membrane counts are above the upper acceptable limit, calculate count using the 

smallest volume filtered. 

For example, assume that the volumes 1, 0.3, and 0.01 mL produced colony counts of 

TNTC, 150, and 110 colonies, respectively. Since all colony counts are above the 

acceptable limit, use the colony count from the smallest sample volume filtered and estimate 

the count as: 

110

 × 100 = 1,100,000 CFU /100 mL 

0.01 

Report this as estimated count 1.1 × 106 CFU/100 mL 

2.7 If typical colonies are too numerous to count (TNTC), use upper limit count with smallest 

filtration volume. 

3
For example, assume that the volumes 1, 0.3, and 0.01 mL all produced too many typical 

colonies, and that the laboratory bench record indicated TNTC. 

Use the upper acceptable count for the method (80 colonies in this example) as the basis of 

calculation with the smallest filtration volume and estimate the count as: 

80

 × 100 = 800,000 CFU /100 mL 

0.01 

Report this as > (greater than) 8 ×105 CFU/100 mL 

2.8 If colonies are both above and below the upper and lower acceptable limits (i.e., no counts 

are within the acceptable limits), select the most nearly acceptable count. 

2.8.1 For example, sample volumes of 100, 10 and 1 mL produced colony counts of 84, 8 

and 0, respectively. 

Here, no colony count falls within recommended limits. Calculate on the basis of the 

most nearly acceptable plate count, 84, and report as 84 CFU/100 mL. 

Note that in this case, because no calculations were done (i.e. this is the count for 100 

mL), the count is reported as 84 CFU/100 mL rather than an “estimated count of 

84 CFU/100 mL” 

2.8.2 As a second example, assume a count in which sample volumes of 100, 10 and 1 mL 

produced colony counts of 98, 18, and 0, respectively. 

Here, no colony count falls within recommended limits. Calculate on the basis of the 

most nearly acceptable plate count, 18, and calculate as in section 2.3.2 above. 

18

 × 100 = 180 CFU /100 mL

10 

Report this as estimated count 180 CFU/100 mL. 

2.9 If there is no result because of a confluent growth, > 200 atypical colonies (TNTC), lab 

accident, etc., report as No Data and specify the reason. 

4