PULSED VOLTAMMETRIC DETECTION OF MICROORGANISMS

Abstract: Reliable and rapid detection of microorganisms is accomplished in an electroanalytical cell using a pulsed voltammetric detection technique employing the growth medium as the electrolyte and analyte and using simple wire electrodes fabricated from readily available materials. Organism detection occurs as a consequence of the depletion of oxygen in the growth medium/electrolyte caused by aerobic metabolism. Times-to-detection vary with inoculum strength in a predictable fashion, permitting quantification of the organism in question when results are compared to those obtained using known inocula of the same organism. The low duty cycle of the pulsed measurement enables the determination of the relative redox potential in the same cell using the same set of electrodes in order to provide information which may be characteristic of the type of organism being studied.

Description: PULSED VOLTAMMETRIC DETECTION OF MICROORGANISMS The present invention relates to a method for the detection of microorganisms. More particularly, the present invention relates to a simple, efficient and reliable electrochemical method for the detection of bacteria by measuring the decrease in polarographic oxygen current passing through an electroanalytical cell containing two dissimilar wire electrodes immersed in a liquid culture medium. The determination of whether or not a substance is contaminated with biologically active agents such as bacteria is of great importance to the medical field, the pharmaceutical industry, the public health field, the cosmetic industry, the food processing industry, and in the preparation of interplanetary space vehicles. One of the most widely used techniques for making this determination, especiaEy in medical applications, has been nutrient agar plating. In this method a microorganism is allowed to grow on an agar nutrient substrate, and the growth of the microorganism is observed, at first visually and thereafter by microscopic examination. This technique, which is most commonly used clinically, requires overnight incubation of plates before results are available. Another technique widely used for the determination of microorganisms involves supplying a microorganism in a growth medium with carbon-14 labeled glucose or the like. See Waters U.S. Patent No. 3,676,679 and Waters U.S. Patent No. 3,935,073. The microorganism metabolizes the radioactive glucose and evolves C O 2 , which is sampled and counted. While positive results can be obtained by this radiometric method in a relatively short period of time, this method requires the use of comparatively expensive and complex apparatus and involves the handling of radioactive materials. The prior art also describes a number of detection techniques based on electrochemical phenomena. Generally these techniques employ very delicate and expensive electronic equipment and are extremely difficult to use in an on-going detection program. One of these described methods involves the measurement of polarographic oxygea current in an electroanalytical cell. Cell current is a function of the dissolved oxygen content of the electrolyte, and the metabolic activity of any oxygen- consuming microorganisms present will, therefore, cause the current values to fall off. For a general discussion of this electroanalytical technique see Hitchman, Measurement of Dissolved Oxygen (1977); Fatt, Polarographic Oxygen Sensors (1976); and Norris, Methods in Microbiology (1970). Modern techniques of polarographic oxygen measurement rely almost exclusively on the so called Clark- type electrodes which employ a semi-permeable membrane to prevent the electrodes from contacting the solution; see Clark U.S. Patent No. 2,913,386. The commercially available membrane polarographic oxygen detector (MPOD) is presently used to determine dissolved oxygen in BOD studies, marine ecology, wastewater treatment and the like. The MPOD is usually constructed with an inert cathode material (gold, platinum) and a silver- silver chloride reference electrode, and uses a relatively concentrated (0.3 - 3.0M) potassium chloride electrolyte. The electrode areas are relatively large (ca. 1.0cm ) and are prevented from contacting the solution to be analyzed by a semi-permeable membrane, usually of polyethylene or polytetrafluoroethylene. The potential applied to the electrodes is normally about 0.8V, cathode negative. This potential must be applied to the MPOD several minutes prior to any use of the detector, and must remain applied throughout the duration of any measurements to be made. The MPOD is thus a "steady-state" device, in that all electrode reactions stabilize at new equilibrium values under the influence of an applied potential constant in time. The steady-state cell current detected under these circumstances is a measure of the dissolved oxygen content of the solution. Because proper electrode operation depends upon diffusion of dissolved oxygen through the membrane to reach the cathode, the solution must be stirred or agitated constantly to prevent the depletion of oxygen from the sample solution in O the immediate vicinity of the membrane from affecting the results. Some investigation of MPOD operation under non-steady state or pulsed conditions has been undertaken; see Hitchman, supra, Chapter 6. The Clark-type polarograph sensors, however, suffer from serious drawbacks which make them undesirable for the detection of microorganisms. These sensors are expensive and cumbersome to use. The relatively high eost of the electrodes precludes the use of a separate electrode for each sample. Thus in order to prevent cross contamination of samples, the electrode surfaces have to be sterilized between samples using a strong bactericide, and then rinsed completely with a sterile rinse solution so as not to kill organisms in or contaminate the contents of the next sample cell tested. The electroanalytical detection of microorganisms by measurement of oxidation-reduction potential has also been described in the prior art; see generally Norris, supra, Chapter 4. In the redox potential method a platinum electrode in combination with any commonly used reference electrode such as the calomel electrode will evidence an equilibrium potential in growth medium proportional to dissolved oxygen in the medium and to any other oxidation-reduction (electron transport) reactions taking place in the solution. Because this is an equilibrium measurement, a voltmeter with very high input impedance must be used to measure the potential existing between the electrodes so as not to displace the equilibrium as a result of current flow. Microorganisms growing in the medium use oxygen from the solution, and may possibly contribute to other redox reactions which cause the measured potential, usually greater than +100mV (cathode or platinum positive with reference to the standard calomel electrode) in sterile medium, to shift toward more negative values. Aerobic organisms are able to reduce the solution enough to yield measured potentials of -lOOmV to -200mV (vs. SCE). This is also the range of redox potentials where the voltammetric methods cease to function; the cell current due to dissolved oxygen is by this time very small, and is usually swamped by the residual cell current due to solution impurities and electrode imperfections. Facultative anaerobes, however, may reduce the solution extensively. An exhausted culture of P. mirabilis will have reduced the medium in a sealed container to a value of QMPI _ ~ O~ * around -550mV (vs. SCE) before ceasing growth. The redox potential method by itself is not well suited to the detection of bacteria because it is relatively slow and the response will depend on the type of organism being detected. From the foregoing it is clear that a need exists for a simple rapid and reliable method of detecting the presence of microorganisms in a suspect sample. Accordingly, it is an object of the present invention to provide a method for detecting the presence of microorganisms which employs apparatus which is relatively simple in both construction and operation and which uses relatively inexpensive non-radiolabeled materials. It is also an object of the present invention to provide a method for the detection of microorganisms which facilitates computer controlled automation and which can incorporate disposable components. Further objects of the invention will be apparent from a consideration of the following description. These and other objects of the invention are achieved by providing an electroanalytical method for detecting the presence of oxygen-consuming microorganisms in a sample comprising the steps of providing a mixture of said sample and a fluid culture medium capable of supporting microorganism growth in an electroanalytical cell equipped with two electrodes which are in contact with said mixture; applying a series of voltage pulses of substantially constant amplitude and duration across said electrodes; and measuring the resulting current prior to the trailing edge of each of said applied voltage pulses; the presence of oxygen-consuming microorganisms being indicated by a decrease in cell current which is a function of the dissolved oxygen content of said mixture. In a preferred embodiment the present invention also contemplates a process for the detection of microorganisms as described above and additionally comprising measuring the open-circuit voltage potential across said electrodes during the interval between successive applied voltage pulses. Determination of the dissolved oxygen content of the cell is accomplished by pulsing the electrodes briefly with a known potential (cathode negative) and measuring the resulting current through the cell prior to the trailing edge of the applied voltage pulse. The growth medium in the cell is used as both the analyte and the electrolyte for the determination. The very low duty cycle of the pulse with respect to the overall sampling interval obviates the need for constant agitation or stirring of the sample solution required by conventional steady-state methodology, and permits the same electrodes to be used to determine the relative oxidation-reduction potential in the cell through the measurement of the open-circuit potential existing between the electrodes. Bacterial detection is best accomplished by measuring the decrease in pulsed voltammetric oxygen current, while information characteristic of the type of organism present is best furnished by the relative cell potential determination. Times-to-detection for all organisms studied vary with inoculum strength in a predictable fashion, permitting accurate quantification of the organism in question when results are compared with times-to-detection obtained using known inocula of the same organism. The process of the present invention provides numerous advantages compared to traditional manual methodology and present automated systems. This process requires a cell of very simple construction, provides ample opportunity for the creation of disposables, promotes automated quality control, prevents any chance of cross-contamination, and can be configured as an instrument very sophisticated in operation, yet extremely simple to operate. Figure 1 is a front elevation view of an electroanalytical cell useful in the process of the present invention. Figure 2 is a top plan view of the electroanalytical cell of Fig. 1. Figure 3 is a sectional view of the electroanalytical cell of Fig. 1 taken along line 3-3. Figure 4 is a simplified schematic diagram of an analog conditioning circuit useful in the process of the present invention. Figure 5 is graph showing the cell current response of a fully-grown E. coli culture at various pulse widths as a function of applied cell voltage. Figure 6 is a graph showing the cell current response of a sterile cell with constant applied potential as a function of elasped time since pulse application. Figure 7 is a graph showing the cell current response as in Figure 6 with the elapsed time extended to 300 seconds. Figure 8 is a graph showing the cell current response as a function of applied potential for a sterile electroanalytical cell and for a cell containing fully-grown E. coli culture. Figure 9 is a graph showing voltammetric cell current response for a sterile cell purged with dry nitrogen; cell current is recorded as a function of elasped time during the purge. Figure 10 is a graph showing the cell current response for the sterile cell of Fig. 9 purged with dry nitrogen, then purged with room air. ' Figure II is a schematic representation of the sampling and dilution scheme used in the preparation of electroanalytical cells and pour plates for the examples. Figure 12 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism E. coli. Figure 13 is a graph ' showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism E. coli. Figure 14 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism E. cloacae. Figure 15 is a graph showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism E. cloacae. Figure 16 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism P. mirabilis. Figure 17 is a graph showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism P. mirabilis. Figure 18 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism P. aeruginosa. -BUR O y. WI Figure 19 is a graph showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism P. aeruginosa. Figure 20 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism S. aureus. Figure 21 is a graph showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism S. aureus. Figure 22 is a graph showing normalized voltammetric cell current response as a function of incubation time for varying inoculum strengths of the organism S. bovis. Figure 23 is a graph showing normalized cell potential response as a function of incubation time for varying inoculum strengths of the organism S. bovis. Figure 24 is a graph showing normalized cell current response as a function of incubation time for the organism E. coli with four decades of initial inoculum concentration. Figure 25 is a graph showing the logarithm of the initial inoculum dilution ratio as a function of time-to-detection at a 60% detection threshold for the data shown in Fig. 24. Figure 26 is a graph showing normalized voltammetric cell current response as a function of incubation times for varying inoculum strengths of the organism E. coli using cells fitted with gold cathodes. Figure 27 is a graph showing normalized voltammetric cell current response as a function of incubation times for varying inoculum strengths of the organism P. mirabilis using cells fitted with gold cathodes. Figure 28 is a graph showing normalized voltammetric cell current response as a function of incubation times for varying inoculum strengths of the organism P. aeruginosa using cells fitted with gold cathodes. Figure 29 is a top plan view of another electroanalytical cell useful in the practice of the present invention. Figure 30 is a sectional view of the electroanalytical cell of Figure 29 taken along line 30-30. Figure 31 demonstrates the effect of a data-correcting algorithm upon raw current data from an uninoculated well. Figure 32 is a graph showing normalized voltammetric cell current response as a function of nculation time for the organism E. coli employing a cell having plated silver wire electrode. Figures 33 and 34 are graphs showing normalized voltammetric cell current response as a function if incubation time for the organisms E. cloacae and P. aeruginosa employing a cell having plated silver wire electrodes. The present invention detects the presence of bacteria in a suspect sample primarily by measuring the decrease in voltammetric oxygen current passing through an electroanalytical cell containing the sample and a fluid growth medium. Viable organisms capable of utilizing dissolved oxygen during metabolism will cause the detected oxygen current to decrease with continued incubation, signifying detection. Sterile inocula will evidence no such current decrease. Additional means is provided to measure the open- circuit voltage of the analytical cell in order to obtain information as to the type of bacteria (primarily aerobic or facultative anaerobic) present in the cell. Organisms that consume little or no oxygen from the growth medium, yet which have the ability to alter the solution redox potential, may be detected by noting the change in the solution redox potential with incubation, as furnished by the open-circuit cell potential measurement. The method of the present invention can be used to detect the presence of any aerobic or facultative organisms which consume oxygen from a liquid medium during metabolism. Specific examples of such organisms include bacteria such as E. coli, E. cloacae, P. mirabilis, P. aeruginosa, S. aureus, S. bovis, K. peneumoniae, S. albus, K. oxytoca, E. aerogenes, E. agglomerans, C. freundii, P. morganii, P. stuartii, S. marcescens, Group B. Beta strep, Grp. D. Strep, and yeasts such as C. albicans. In the process of the present invention a small portion of a suspect sample is first introduced into an electroanalytical cell containing a liquid growth medium. The growth medium also serves as the primary electrolyte in the cell. Any medium which will support the growth of oxygen - consuming microorganisms may be utilized. Typical growth media generally contain water, a carbon source, a nitrogen source, calcium, magnesium, potassium, phosphate, sulfate, and trace amounts of other minor elements. The carbon source may be a carbohydrate, amino acid, mono- or dicarboxylic acid or salt thereof, polyhydroxy alcohol, hydroxy acid or other metabolizable carbon compound. Usually the carbon source will comprise at least one sugar such as glucose, sucrose, fructose, xylose, maltose, lactose etc. Amino acids such as lysine, glycine, alanine, tryrosine, threonine, histidine, leucine, etc. also frequently comprise part of the culture media carbon source. The nitrogen source may be nitrate, nitrite, ammonia, urea or any other assimilable organic or inorganic nitrogen source. An amino acid might serve as both a carbon and a nitrogen source. Sufficient nitrogen should be present to facilitate cell growth. A variety of calcium, potassium and magnesium salts may be employed in the growth medium including chlorides, sulfates, phosphates and the like. Similarly, phosphate and sulfate ions can be supplied as a variety of salts. As such materials are conventional in fermentation media, the selection of specific materials as well as their proportions is within the skill of the art. The so called minor elements which are present in trace amounts are commonly understood to include manganese, iron, zinc, cobalt and possibly others. Due to the fact that most biologically active species cannot function in strongly acidic or strongly alkaline media, suitable buffers such as potassium or ammonium phosphates may be employed, if desired, to maintain the pH of the growth medium near neutrality. Examples of well known growth media which may be used in the present invention are peptone broth, tryptic soy broth, nutrient broth or thioglycolate broth. Tryptic soy broth-based medium (6B Medium, Johnston Laboratories Inc., Cockeysville, Md.) has been found to work well. The amount of growth medium provided in the electroanalytical cell is not overly critical. 5.0cc of 6B medium has proven very effective. The analytical cell useful in the process of the present invention may be of any convient size and shape. The cell can be formed from any materials normally used in the manufacture of electroanalytical cells such as glass, plastic and the like. Any material which does not affect the growth the microorganisms or the measurement of electrochemical phenomenon in the cell can be employed. In the preferred form, the electronalytieal cell useful in the process of the present invention comprise a plastic container of the general configuration shown in Figs. 1-3. CeE volume may vary according to the cell design and is not critical. A cell of the type shown in Figs. 1-3 has been effectively used at a capacity of about 10-15 mis. In the preferred manner of operation a number of these cells can be utilized in the form of an array to permit testing of multiple samples.- The electroanalytical cell is also equipped with two electrodes in electrical contact with the growth medium. The working electrode (cathode) is normally a noble metal, for example, gold, silver or platinum. When only voltametric measurements are to be taken gold, platium or silver are preferred for the cathode. When potentiometric (redox) measurements are also taken gold should not be used as the cathode materiaL The reference electrode is preferably pure silver (99.95% or better) electrσlyzed in place •using a basic electrolyte to deposit Ag„O on the silver. Silver chloride may be electrolytically deposited from HC1 solution to form the alternative Ag/AgCl reference electrode, but this electrode has been found less stable in this application. In actual practice clean, unprepared silver wire will quickly become covered with a mixture of Ag„O and AgCl due to Cl~ ion in the medium and to the very high pH around the anode when pulsed. Although pure silver is known to be bactericidal, no evidence of such toxicϊty has been noted using oxidized silver reference electrodes. Silver cathodes have also been used without rendering the process inoperative. The electrodes may be used in any convenient form. Preferred are wires of the above materials although other forms such as printed circuit traces can be used. Most preferred are U-shaped staples inserted through the bottom of the cell as best seen in Fig. 3. The electrodes, however, can be of other conventional forms including spaced apart vertically disposed hairpin shaped electrodes. The electrode wire diameter is not critical. Wires as small as 0.010" may be used, provided their frangibility and low sensitivity can be tolerated. Wires approaching 0.040" probably represent a practical upper limit since these materials are quite expensive. Preferred electrode diameters are in the range of from about .015 to .050", with about 0.020" to 0.040" being most preferred. Electrode lengths are likewise noncritical. In practice, lengths of from about 0.5cm to 2.0cm and preferably about 1.0 to 1.5 cm are suitable. The wires may be separated by about 0.5 to 2.5cm and preferably about 1.0 to 2.0 cm. It will be apparent to those skilled in the art that solid precious metal wires can be replaced with less expensive wire electroplated with the precious metals of choice. The electrode pair may be covered with a porous gel, preferably a nutrient gel such as tryptic soy agar (TSA). Other gel materials which may be used include gelatin, dextran gel, carrageenan gel and the like. Best results are obtained when the gel just covers the electrodes. The main benefit of the gel is to reduce measurement baseline drift sometimes ' caused by the introduction of biological samples (urine, etc.), presumably by preventing the migration of large charged molecules to the electrodes. The quantity of gel is not critical; l.Oec of TSA has served to just cover the electrodes in the type of cell shown in Figs. 1-3. It may be appreciated that , some ionic conduction is necessary in the gel; hence its equivalent conductance when saturated with growth medium/electrolyte should approach that of the medium alone. The electrode pair may also be isolated from the effects of large charged molecules by positioning a layer of porous material such as ordinary filter paper over the electrodes. While this will not prevent contact of the electrodes with the analyte or even with the microorganisms in the sample, it will limit migration of large charged molecules to the electrode region. In another embodiment of the present invention the reference electrode can be isolated from the analyte mixture by the use of a salt bridge or other conventional means. In this manner it is possible to employ a single reference electrode in conjunction with a plurality of working electrodes in separate analytical half cells. After the cell has been inoculated with a sample to be tested a series of voltage pulses of substantially constant amplitude and duration are applied across the electrodes. The voltage amplitude of the pulses can vary from about -0.35 v. to about -0.90v. Preferred is an applied voltage pulse of about -0.70v. The pulse duration should be at least about 600 milliseconds. The upper limit of the pulse duration is not critical. As a practical matter, times much over about 3 seconds result in a reduced current signal but may be used. Preferably the pulse duration can be from about 800 to 2000ms. with about 1200 ms. being most preferred. The pulse interval is not critical and should be short enough to follow the biological changes but long enough to allow the cell to approach equilibrium conditions for redox potential measurements in the pulse intervals. Times of from about 5 min. to 20 min. are suitable. A pulse interval of about 10 minutes is preferred. During the testing period the analytical cell and its contents should be held at a constant temperature, preferably 37° C _+ 0.2° C. It is understood, however, that not all biologically active agents exhibit maximum growth within the cited temperature range. If it is of interest to determine whether or not a specific microorganism which grows better at some other temperature is present, then the temperature at which the organism in question exhibits maximum growth should be employed. Current readings from the cell are taken prior to the trailing edge of each of the applied voltage pulses. The pulsed, periodic nature of the measurements involved obviates the need for constant agitation or stirring of the sample as would be required for conventional steady-state polarographic oxygen determination. The requirement of extremely high input impedance for the potential measuring circuitry is similarly relaxed, since the analytical cell is connected to the external electronics only long enough for appropriate potential and current readings to be taken. The simplicity of the analytical cell coupled with the complexities of sample selection and interrogation suggest that the technique of the present invention be practiced in a fully automated fashion. A microcomputer system can be used to control all aspects of experimental measurement and to analyze, tabulate, plot, and store on floppy disk the information gathered during each experiment. The construction of a typical analytical cell useful in the process of the present invention is shown in Figs. 1-3. A semi-flexible plastic container (1) receives the two wire electrodes (2,3) in the form of U-shaped "staples" inserted through the bottom of the container. The working electrode (2) is analytical-grade platinum, 0.035" diameter. Reference electrode (3) is 0.040" diameter Ag/AgO. The electrode pair is covered with a nutrient gel _Q PI WIPO (4) such as tryptic soy agar (TSA). A quantity of liquid growth medium (5) is also present in the cell to serve as the primary electrolyte and to facilitate the growth of microorganisms. The performance of a single experimental data-gathering cycle may be understood by considering a single two-electrode cell as part of the signal conversion circuitry, as presented in simplified form in Figure 4. A data-gathering cycle begins with the selection of this particular cell. After a short delay to allow the cell selection relays to settle, a potential reading is taken with relay 11 open via operational amplifiers OA1 and OA2. Because the cell redox potentials on platinum are bipolar with respect to the reference electrode (+150mv to -550mv) with incubation of a facultative anaerobe, provision is made to offset the potential reading using OA2 to present a unipolar, positive signal of adjustable gain to the computer A/D converter. Immediately following the potential reading, relay 11 closes, and a positive- going pulse derived from the computer D/A converter is applied to OA3. The inverted, unity-gain output of OA3 causes current to flow through the selected cell in proportion to the dissolved oxygen content. The cell current is sensed by OA4, connected as a current-to-voltage converter. The output of OA4 is sampled by a second input of the A/D converter card prior to the termination of the voltage pulse from OA3. All relays are switched off and allowed to settle prior to the selection of the next cell to be tested, at which point the process begins again for the newly selected cell. After all cells have been tested, all relays are deselected while the programmed time interval between readings, usually ten minutes, is allowed to elapse. The voltage signal input resistor (R. , OA1 in Figure 4) has the value of 2.2Megohms. Although this is by no means an electrometric input resistance as is normally employed to measure electroanalytical cell potentials, it must be remembered that the cell is loaded with this resistance for only a few hundred milliseconds before the voltage reading is stored, and that the following voltage pulse drives the cell far from equilibrium in any case. The 2.2M resistor provides a good compromise between cell loading and noise pickup in the cell environment. The cell current and potential values measured respectively during and between the successive applied pulses can be compared to the initial values to determine when the threshold of detection has been reached. This process is facilitated by normalization of the collected values as described more fully in Example 2. When the current level has fallen to a predetermined percentage of the initial value, e.g., 60-80%, detection is found to have occurred. It is also possible to make determinations by comparing the collected data to that obtained in a separate reference well. The method of the present invention can be used in any application where the detection of microorganisms in a sample is desired. This method finds particular utility in the detection of bacteria in biological fluids such as blood, urine and the like. As will be readily appreciated by one skilled in the art, bacterial testing can include screening, identification, and antibiotic susceptibility testing. Other areas of utility include the detection of microorganism contamination in food, pharmaceutical and cosmetic products. The following examples are included for illustrative purposes only and are not intended to limit the scope of the invention. EXAMPLE 1 This example demonstrates ' the selection of optimum values of the voltage pulse amplitude and duration. In order to separate the effect of pulse width from that of pulse amplitude in a semi-quantitative fashion, a test was conducted to * determine the cell current at various pulse widths for applied voltages ranging from 0.00V to -1.00V in steps of 0.05V in a cell containing a spent culture. As in all examples, the platinum electrode received the pulse, while the reference electrode was held at virtual ground. The current-to-voltage conversion gain was held constant. A single sample cell of an array consisting of 8 cells as shown in Figure 1 was prepared using l.Occ of molten TSA (90° C) transferred to the cell via sterile syringe. 5.0cc of a fully-grown E. coli culture in 6B medium (oxygen depleted) was similarly transferred to the cell after the agar had solidified. The cell array was placed in a warm-air incubator at 37° C and "BUR OM allowed to equilibrate for 40 minutes, at which time readings were begun under computer control. A voltage scan of the cell was obtained and plotted for each chosen pulse width. The interval between pulses was approximately 10 seconds, depending somewhat upon the selected pulse width. Cell current responses for selected pulse widths are presented in Figure 5. It may be noted that the desired cell response is achieved with pulse widths greater than about 640 milliseconds. In practice, pulses of about 1200ms. duration have beed used with good success. Because cell current begins to fall rapidly from the instant the pulse is applied, there is little to be gained from the use of a pulse width in excess of that necessary to produce the desired response. This effect may be noted graphically in Figures 6 and 7, which depict the results obtained when a single cell containing l.Occ TSA and 5.0cc 6B medium is subjected to a continuous applied voltage of -0.70V, with current readings obtained every 0.5 seconds beginning 0.5 seconds after voltage application (Fig. 6). In Figure 8, one point is plotted for each 20 collected, stored and tabulated. The cell current has decreased to less than half its value at 0.5 seconds (217) after only 4.0 seconds (106); after 5 minutes, cell current has fallen to 16.6% of the initial measurement value (36), yet steady-state conditions have not been achieved. Once the desired cell response of low residual current when containing microbiologically reduced medium has been achieved, further increases in pulse width merely decrease the desired current signal and displace the cell further from thermodynamic equilibrium (Fig. 5). Conversely, pulses shorter than about 600ms provide unsatisfactory results. Figure 8 illustrates a voltage scan overlay for two cells, one containing l.Occ of a fully-grown culture of E. coli in 6B, the other containing sterile 6B medium, both incubated at 37° C for 4 hours prior to measurement. A pulse width of about 1300ms was employed. The cell responses are observed to separate at about -0.35V, reach maximum divergence near -0.75V, and converge again about -0.90V. The detection of microorganisms, then, requires a pulse potential near -0.75V for best efficiency under these conditions. Consideration of the results presented in Figures 5 and 6, together with similar data obtained during the early phases of experimentation led to the selection of -0.70V as the pulse amplitude, with a width of about 1200ms OMPI ... for use in the remainder of the examples. It is also observed that several minutes are required for the cell to return to conditions approaching equilibrium following the application of a voltage pulse. Ideally, the time between pulses should be very large compared to the applied pulse width. Ten minutes was selected as the sampling interval, since it is a short period of time microbiologically speaking, yet it provides for a pulse duty cycle of only 0.2% while at the same time permitting the acquisition of semi-continuous data for rapidly growing organisms. Even at this low duty cycle, measured cell potential values are depressed somewhat from their equilibrium values. The measured potentials do, however, adequately represent relative redox potential changes in the • growth medium. Finally, in order to insure that dissolved oxygen content is in fact the major parameter determined by the pulse polarographic technique under the chosen experimental conditions, a single cell of the 8-cell array was prepared as usual, containing l.Occ TSA and lO.Occ 6B medium. The cell was incubated at 37° C for 30 minutes, then read every 60 seconds for 10 minutes. Dry nitrogen gas (Matheson, High-Purity) was then bubbled through the ceE by means of a disposable glass capillary pipette. Readings were continued until the cell current dropped to a reasonably constant value. In a separate experiment conducted in the same cell, nitrogen was again bubbled through the cell, and readings begun just prior to reaching the purge plateau. The gas supply tubing was quickly transferred to a small aquarium pump with flow constrictor, and readings continued for twenty additional minutes. Data from these tests is plotted in Figs. 9 and 10. Cell response is shown to be clearly related to the oxygen content of the solution; response also appears to be totally reversible in the case of this sterile cell. In actual use with bacteria, however, cell reversibility becomes a strong function of the amount of time the cell is exposed to a fully-grown culture. EXAMPLE 2 In all the following examples, electrochemical measurements, unless otherwise noted, were carried out in an 8-cell array using platinum (0.035") and silver/silver oxide (0.040") wire electrodes of 1.0cm length separated by 1.0cm. Prior to each run, the cells were carefully rinsed with deionized water and vigorously shaken to dislodge the larger droplets. l.Occ TSA (Trypic Soy Agar, 40.0g/l, BBL, Cockeysville, MD) at about 90° C was then transferred to each cell using a sterile 3.0cc syringe with 18ga. needle. The array was then covered, and the agar allowed to solidify. 5.0cc sterile 6B medium (Johnston Laboratories, Inc., Cockeysville, MD) was then added to each cell, together with O.lcc of a 4.5g/20ml sterile glucose solution and/or O.lcc of a 1.5g/20ml sterile glycine solution as desired. The prepared cells were covered and set aside while the inoculum dilutions and pour plate dilutions were prepared. Fresh cultures of the organisms to be studied were prepared in 6B medium the day before each test, and allowed to incubate at room temperature until needed. Previously prepared sterile 20cc vials fitted with rubber septa and aluminum closures containing 9.0cc TSB (27.5g/l,BBL) were used for all inoculum dilutions. Similar lOOcc vials containing 99.9cc 1/2- strength TSB were used to prepare pour plate dilutions. The test was initiated by preparing a sterile l.Occ syringe containing about 0.5cc of the overnight culture of the desired, organism. This culture was added dropwise to a 9.0cc dilution vial with agitation until visual turbidity was achieved. After complete mixing, LOcc was removed from this vial and used to inoculate a second (X 0.1) vial containing 9.0cc, achieving 1:10 dilution. LOcc from this vial was used to inoculate a third (X 0.01) vial, which was in turn used to inoculate a fourth (X 0.001). Additionally, O.lcc was removed from the X 1.0 vial and used to inoculate one 99.9cc vial to obtain 1:1000 dilution for pour plate preparation. Similarly, O.lcc was withdrawn from the X 0.01 vial and used to inoculate a second 99.9cc vial to obtain 1:100,000 dilution. A fifth vial containing 9.0cc TSB was used as the source of sterile, control inocula. LOcc of the X 1.0 vial was then transferred to cell (1) of the array. Cells (2) and (3) received LOcc each from the X 0.1 dilution vial; cells (4) and (5) were inoculated with l.Occ from the X 0.01 vial, while cells (6) and (7) each received an inoculum from the X 0.001 vial. Cell (8) was inoculated with l.Occ sterile TSB to serve as the control. The inoculated cells were placed in a warm-air incubator held at 37° C +_ 0.2, connected to the analog conditioning electronics, and the measurements begun. No preinoculation incubation of the array was employed. Cell current and cell potential readings were obtained at 10- inute intervals using a pulse amplitude of -0.70V and a pulse duration of 1200 milliseconds. Duplicate pour plates were prepared containing 10-15ml TSA (40g/l) using l.Occ and O.lcc from each of the 99.9cc dilution vials to yield pairs A K β * of plates at 1:10 , 1:10 , 1:10 , and 1:10 dilutions. The plates were allowed to harden at room temperature prior to 24-hour incubation. Details of the sampling and dilution scheme are set out in Figure 11. All values of current and potential recorded by computer in these examples range from 0 to 255 as a consequence of unipolar 8-bit conversion of the input signals. These raw data values are stored in the appropriate memory array during the experiment. All data manipulation is performed after the experiment is terminated. Raw data and experimental specifies are stored on a floppy disk for future retrieval. Cell current I(T,N) as a function of time (T) and sample number (N) is normalized at a chosen time interval Tl for sample N by dividing cell currents observed at all times T for sample N by the cell current value observed at time T=T1, and then multiplying by 100.0, e.g.: X(T,N) = ^^1 100.0 I(T1,N) The same normalization time (Tl) is used for all samples. The normalized current values X(T,N), now ranging nominally from 0 to 100, are then scaled for plotting through division by a scale factor, herein 2.0, so that all data together with the machine-generated coordinate time axis will fit on the 80-character CRT/printer line. Cell currents for each sample are thus easily presented as a percentage of the normalized current value, usually taken as the current value observed after 30 minutes experiment time. Cell potential results V(T,N) are normalized by simple Y-axis translation and uniform scaling. A constant is first derived from the voltage -^0 observed at the normalizing time interval Tl: C = V(T1,N) - 20 which is in turn used to translate all observed voltage values for a given sample N: Z(T,N) = V(T,N) - C The translated values Z(T,N) are then scaled to page width by dividing by the scale factor, herein 3.5, and then printed. Potential normalization is * usually performed at 60-100 minutes after the experiment has begun. Cell potential readings require about 60 minutes longer, on average, to stabilize than do the current readings. For the purposes of the following examples, detection of the organism is said to occur when the pulse voltammetric cell current has fallen to 80% of its value at the normalization time interval. Potential measurements are considered positive when a relative normalized value of 20 is attained. AH values obtained for cell (1) were used as high-inoculum markers only, and do not appear in the results. This example demonstrates the detection and quantification of E. coli. A fresh overnight culture of E. coli in 6B medium was used as the inoculum source. The sampling and dilution scheme of Figure 11 was employed to prepare sample cells and pour plates, The sample cell medium was enriched with O.lcc (of the) 4.5g/20ml glucose stock solution. Incubation of all vials, sample cells and plates was at 37° C. Cell readings were continued for 8 hours. Pulsed voltammetric current responses for three decade dilutions of the organism plus control are shown in Figure 12. The related cell potential results are presented in Figure 13. The Y-axis indicates potential increasing in the negative direction. The short plateau evidenced at relative potential values of 30-40 probably indicates a change in the metabolism of the organism triggered by the reduced oxygen tension in the medium. Both cell parameters give times-to-detection which vary in a predictable manner with inoculum strength. Pour plate results indicate that 1.0 x 10 cfu/ml E. coli were present in the freshly inoculated X 0.1 cell. Times-to-detection for the duplicate cell current and potential measurements are presented in Table 1. Table 1 Times-to-Detection in Minutes for Cell Current and Cell Potential for the Organism E. coli Initial Inoculum Cell Current CeE Potential in Cell A B A B 1.0 x 10 5 cfu ml ' 120 120 160 180 1.0 x 10 4 " 150 160 220 230 1.0 x 10 3 « 200 200 260 270 EXAMPLE 3 This example demonstrates the detection and quantification of E. cloacae. A fresh culture of E. cloacae was incubated overnight at room temperature to serve as the inoculum source. The sampling and dilution scheme presented in Figure E was used to prepare duplicate sample cells and pour plates. The ceH medium was enriched with O.lcc glucose stock solution. All incubations were performed at 37° C. The inoculated cell array was covered with clean aluminum foil, placed in the incubator, and connected to the analog conditioning electronics moments before the start of the test. The test was continued for 520 minutes (8-2/3 hours). The ceU current response for each of the three decade dilutions of E. cloacae plus control is presented in Figure 14. CeE currents are seen to rise relatively rapidly from their attained minimum values probably as a consequence of electrode-active etaboEc products synthesized by the organism in its latter stages of growth under reduced conditions. The related ceE potential data is shown in Figure 15. A short plateau in redox -BUR O - W1 potential values is again noted at relative normalized values between 30 and 40, again attributed to an organism metaboEc pathway change. Duplicate pour plate counts were used to determine the initial inoculum level in the 5 X 0.1 cells to be 3.7 x 10 cfu/ml. The incubation times required to detect the organism are listed in Table 2. Table 2 . Times-to-Detection in Minutes for CeE Current and CeE Potential - for the Organism E. cloacae Initial Inoculum CeE Current CeE Potential in Cell A B A B 3.7 x 10 5 cfu/ml 150 150 230 200 3.7 x 10 4 n 210 210 260 240 3.7 x 10 3 " 270 270 300 310 EXAMPLE 4 This example demonstrates the detection and quantification of P. mirabilis. A fresh overnight culture of P. mirabilis in 6B medium was used as the inoculum source. Sample cells and pour plates were prepared as per the sampling and dilution scheme presented in Figure E. 5.0cc 6B medium was enriched with O.lcc sterile glucose stock solution in each of the ceEs. All incubations were performed at 37° C. The electroanalytical measurement was begun immediately foEowing ceE inoculations without pre- inoculation incubation, and was continued for 540 minutes (9 hrs). Pulsed voltammetric ceE current responses for the three decade dEutions of the organism are presented in Figure. 16. CeE responses appear normal as oxygen is consumed and ceE current faEs to the residual level, then rapid vertical transitions appear lasting only 20-30 minutes. CeE current values seem to stabilize foEowing these events, but do not return to pre- transition levels. CeE potential responses are shown in Figure 17. Again, a slight plateau is observed at normalized relative potentials between 30 and 40 units. Instead of the rapidly increasing negative potentials noted for E. coli and E. cloacae, P. mirabilis potentials faE sharply after a slight increase foEowing the plateau. The time intervals noted for this potential decrease correlate weE with the observed vertical transitions in ceE current previously noted. Since P. mirabilis is a facultative organism known to efficiently reduce growth media, and has been used as a standard organism for medium reduction measurements, these anomalous results in the long-incubation regime are best explained by the formation of electrode-active metabolic by-products, most probably sulfi de-containing molecules (H„S, CHgSCHg, CHgSCE CHg etc.) which certainly would perturb the electrode system. The silver/silver oxide electrode is noticeably blackened by exposure to P. mirabilis for extended periods. The electrodes do not seem to be permanently damaged by such exposure, and may be returned to their initial condition by careful washing and wiping of the electrode surfaces. The discoloration can be removed only by mechanical poEshing. The X0.1 cells (2) and (3) contained 2.0 x 10 cfu/ml of P. mirabilis at inoculation as determined by dupEcate pour plate counts. Times-to-detection for ceE current and ceE potential are listed in Table 3. Table 3 Times-to-Detection in Minutes for CeE Current and CeE Potential for the Organism P. mirabilis Initial Inoculum CeE Current CeE Potential in CeE A B A B 2.2 x 10 5 cfu/ml 190 170 250 230 2.0 x 10 220 230 270 280 2.0 x 10 v 290 300 340 330 EXAMPLE 5 This example demonstrates the detection and quantification of P. aeruginosa. A freshly inoculated vial of 6B medium was ineubated overnight at room temperature for use as the source of inocula. The sampling and dilution scheme iEustrated in Figure E was employed to prepare sample cells and pour plates. The sample ceE medium was enriched with O.lcc sterile 1.5g/20ml glycine stock solution in each ceE. AH cells and plates were incubated at 37° C. Electroanalytical ceE readings were begun immediately after inoculation and were continued for 490 minutes (8 1/6 hours). CeE current response of the - organism with decade dilution and of the control ceE containing sterile medium are shown in Figure 18. Normal ceE current behavior is observed. The related ceE potential responses are presented in Figure 19. Because P. aeruginosa is a relatively slow growing obligate aerobe, ceE potential response at each inoculum level changes more slowly and reaches a limiting value of considerably less amplitude than do the facultative anaerobes. Pour plate counts in duplicate were used to 4 determine the initial inoculum level in the X0.1 cells as 7.7 x 10 cfu/ml. Times-to-detection for the detection methods are presented in Table 4. Table 4 Times-to-Detection in Minutes for CeE Current and CeE Potential for the Organism P. aeruginosa Initial Inoculum CeE Current CeE Potential in CeE A B A B 7.7 x 10 4 cfu/ml 190 160 220 170 7.7 x 10 3 n 220 250 240 310 7.7 x 10 2 " 300 310 390 390 EXAMPLE 6 This example demonstrates the detection and quantification of S. aureus. A freshly inoculated vial of 6B medium was incubated overnight at room temperature to serve as the source of aE inocula. The sampling and dUution scheme of Figure E was again employed to prepare sample ceEs and pour plates used in the test. The growth medium in each ceE was enriched with O.lcc sterile glucose stock solution. All incubations were carried out at 37° C in a warm-air incubator. CeE current and potential readings were recorded every 10 minutes under computer control. The experiment was continued for 480 minutes (8 hours). CeE current response for the organism at decade inoculum levels is shown in Figure 20. Normal ceE current behavior is obtained. The related ceE potential variations are presented in Figure 21. Note that very little potential change occurs with continued growth of S. aureus; threshold detection is barely achieved. Duplicate 24-hour pour plate counts were used to determine the inoculum 4 level in the XO.l cells to be 8.2 x 10 cfu/ml. Times-to-detection for ceE current and ceE potential methods are given in Table 5. Table 5 Times-to-Detection in Minutes for CeE Current and CeE Potential for the Organism S. aureus Initial Inoculum CeE Current CeE Potential in CeE A B A B 8.2 x 10 5 cfu/ml 150 130 290 200 8.2 x 10 4 " 180 180 250 260 8.2 x 10 3 " 250 260 320 340 EXAMPLE 7 This example demonstrates the detection and quantifiction of S, bovis. A fresh overnight culture of S. bovis in 6B medium was used as the inoculum source. Sample cells and pour plates were prepared with reference to Figure E. Each ceE also received O.lcc glucose stock solution as enrichment. AE incubations were carried out at 37° C. The measurements were continued for 450 minutes (7 1/2 hours). CeU current measurements are presented in Figure 22. Normal current response is observed prior to the current minimum at each inoculum level. Values recorded after each minimum rise more rapidly then usual. CeE potential responses are shown in Figure 23. S. bovis causes little change in the potential observed as growth progresses, save for the smaE singularity usuaEy observed in conjunction with the current minima in Figure 22. Duplicate 24-hour pour plate counts indicated 6.5 x 5 10 cfu/ml to be present in the XO.l cells initiaEy. Times-to-detection for the detection methods are presented in Table 6. Table 6 Times-to-Detection in Minutes for CeE Current and CeE Potential for the Organism S. bovis Initial Inoculum CeE Current CeE Potential in CeE A B A B 6.5 x IO 5 cfu/ml 200 180 280 2<40 6.5 x 10 4 " 220 220 290 280 6.5 x 10 3 » 280 290 330 340 EXAMPLE 8 This example demonstrates the extended quantification of E. coli using pulsed voltammetric detection. A fresh overnight culture of E. coli in 6B medium was used as the inoculum source. Dilution vials and pour plates were prepared with reference to Figure 11, except that dilution vials were prepared g out to a dilution ratio of 1:10 . The growth medium in each of the cells was enriched with O.lcc sterile glucose stock solution. The first seven cells in the 8-ceE array each received l.Occ from the appropriate dilution vial as inoculum. CeE (8) received LOcc sterile TSB inoculum as the control. Because the ceEs had previously been mechanicaUy polished and reconditioned, the current sensitivity for this experiment was reduced somewhat to insure that aE recorded values would remain within the dynamic range of the A/D converter. Incubation and testing were carried out at 37° C. No pre- inoculation incubation period was used. The normalized ceE current responses of cells (3) through (8) are shown in Figure 24. CeE (1) was used as a high-inoculum marker only, since the ceE assembly requires at least 30 minutes to attain temperature equilibrium. CeE (2) results were anomalous with respect to detection time, probably due to slight contamination of the ceE waEs or electrode surfaces during reconditioning, and are not shown. Over the four decades of inoculum level considered, time-to-detection is seen to vary linearly with the logarithm of inoculum strength. Note that the detection threshold has been reduced to 60% of the value observed at the time of normalization; this provides more dependable time-to-detection values in prolonged tests where the slight downward baseline drift with time can generate detection times slightly shorter than the correct values. * Note that the 60% current level occurs near the point of maximum slope of the growth curves. Experience with the system has shown that best quantification when using this technique is obtained when the detection time is taken to be the time at which maximum slope of the growth curve is evidenced. Figure 25 iHustrates the good 5 1 quantification achieved for inocula of 1.5 x 10 cfu to 1.8 x 10 cfu in the present example. Repeated trials with the system has shown that inocula 5 greater than about 5 x 10 cfu require slightly longer to detect than predicted. This is partly due to the lag induced by the time required for the ceE array to reach temperature equilibrium. The remainder of the problem is most likely caused by the finite timie required for the organism in question to adapt to the new environment imposed by dilution and sampling. The short time interval between recorded data points (10 minutes) makes even slight deviations from expected behavior noticeable. EXAMPLE 9 The results for the organisms studies in Examples 2-7 using the 8- ceE array (platinum cathodes) are summarized in Table 7. Data from parallel radiometric tests (BACTEC) are included for comparison. CeE current duplicate results are shown to differ by a maximum of 30 minutes at any inoculum level for aE organisms. CeE potential detection is less reliable for quantification, differing in duplicates by as much as 90 minutes in one ease (S. aureus). Pulsed voltammetric detection of * the test organisms compares weE with detection based upon the BACTEC system; E. coli and E. cloacae are detected with approximately equal facility by both methods. P. mirabEis and most notably, P. aeruginosa are. detected significantly faster using the ceE current measurement. Detection of S. aureus by the ceE current method is about 40 minutes faster then BACTEC, while S. bovis detection is accomplished about 1 hour sooner by the BACTEC system. CeE potential detection of organism growth compared to either the ceE current determination or to the BACTEC system leaves much to be desired. Results can be quite unreliable for organisms such as S. aureus (Fig. 21) and S. bovis (Fig. 23) which produce little change in solution redox potential with growth. Thresholds for detection are approached slowly and barely exceeded by such organisms as compared to the Enterobacteriacae, thus promoting a test of widely varying sensitivity as a function of the organism being detected. The widely differing redox potential patterns do, however, provide good clues as to the type of organisms simultaneously detected by other means. Table 7 Summary of Results Times-to-Detection for aE Organisms by AH Methods TIMES-TO-DETECTIQN, MINUTES Tested Inoculum Level BACTEC CeE Current CeE 3 Potent Organism in CeE or Vial A B A B E. coli 1.0 x cfu/ml 120 120 120 160 180 1.0 X cfu/ml 180 150 160 220 230 1.0 X cfu/ml 240 200 200 260 270 E. cloacae cfu/ml 120 150 150 230 200 cfu/ml 180 210 210 260 240 cfu/ml 240 270 270 300 310 P. mirabilis 2.0 X cfu/ml 240 190 170 250 230 2.0 X 10 3 cfu/ml 300 220 230 270 280 2.0 X IO * - cfu/ml 420 290 300 340 330 P. aeruginosa cfu/ml 480 190 160 220 170 cfu/ml 600 220 250 240 310 cfu/ml 660 300 310 390 390 S. aureus 8.2 X 10* cfu/ml 180 150 130 290 200 8.2 X cfu/ml 240 180 180 250 260 8.2 X cfu/ml 300 250 260 320 340 S. bovis 6.5 X cfu/ml 120 200 180 280 240 6.5 X cfu/ml 180 220 220 290 280 6.5 X cfu/ml 240 280 290 330 340 EXAMPLE 10 This example demonstrates the use of electrodes of other materials and sizes then those used in the foregoing examples. A 6-ceE array constructed with 0.020" gold wire as cathode and 0.020" silver wire as anode in each ceE was used to test the response of the system toward pulse polarographic detection only. The gold cathode prevented collection of potential data, but gold is somewhat less expensive then platinum to use in cases where detection alone wiE suffice. The gold and silver wires, arranged in paraEel fashion in the bottom of each ceE, were each 1.4cm long and were separated by 1.6cm. The observed cell currents were appreciably lower than those observed when using the 8-ceE array with larger diameter wires and platinum cathode. No attempt was made to measure true current sensitivity. The ceEs were prepared with either L5.cc or 2.0ce TSA and 5.0cc enriched 6B medium, and were tested with various organisms under the same conditions as for the previously noted experiments. No electrode pretreatment was employed. The pulse amplitude (-0.70V) and the pulse duration (1200ms) were the same as in the previous examples. Duplicate pour plates were also prepared for these experiments. CeE current data normaEzation was carried out as previously described. Fresh overnight cultures of E. coli, P. mirabilis, and P. aeruginosa in 6B medium were used as inoculum sources. AE sample and pour plate preparations were preformed with reference to Figure E. CeE current readings were obtained at 10- minute intervals. No pre-inoeulatiόn incubation was employed. Results obtained for separate tests using E. coli, P. mirabilis and P. aeruginosa are shown in Figures 26, 27 and 28, respectively. The organism inoculum level for the most concentrated ceE in each experiment is listed below: 5 E. coli 1.5 x 10 cfu/ml P. mirabilis 1.2 x 10 5 cfu/ml 5 P. aeruginosa 1.8 x 10 cfu/ml The other tracings in each Figure are for decade dilutions and a sterile control ceE. In aH cases, detection of the organism was readily accomplished, as indicated by a drop in recorded ceE current to 80% of the normaEzation value. Times-to-detection for all dilutions in each of the figures are seen to depend upon the inoculum level in a predictable manner, varying essentiaEy as the logarithm of inoculum concentration. The slight scatter noted in some of the data is due to stray pickup by the analog ' BU E tT O PI conditioning electronics; a capacitor was added across the voltage-to-current conversion operational amplifier to aEeviate this problem. Downward baseline drift noted for the control samples is most likely due to the lack of any preconditioning of the cells. Any increase in times-to-detection noted at a given inoculum level of a specific organism when using the 6-ceE array is probably a function of the thickness of TSA over the electrodes, particularly in the case where 2.0cc was used. Once the electrodes are covered, detection times increase as the level of TSA in the ceE increases as a consequence of the increased diffusion path. 2.0cc TSA was used in the experiment run with P. aeruginosa. The E. coE and P. mirabilis experiments both used 1.5cc TSA. EXAMPLE E This example demonstrates a clinical trial of the pulsed voltammetric detection technique of the present inventor for the detection of significant bacteriuria. The test was conducted in conjunction with Sinai Hospital of Baltimore. Over the 33-day period of the study, 389 urine samples were coEected from Sinai and tested at Johnston Laboratories using the pulsed voltammetric technique in paraEel with the BACTEC radiometric system. TSA pour plates were employed to cheek actual organism counts. The test was carried out as foEows. Urine specimens sampled following Sinai collection and planting were picked up from the hospital at approximately E:00AM each day. Prior to sample arrival at JLI, the 16- ceE assembly to be used with the Pulsed Voltammetric instrumentation was fiEed with scalding hot water and aEowed to stand for at least ten minutes. The assembly was then rinsed twice with sterile deionized water, then shaken vigorously to dislodge any large water droplets. LOcc sterile Tryptic Soy Agar (40.0g/l; BBL or DIFCO) at about 95° C was then added by sterile syringe to each cavity of the assembly. The assembly was then covered with a double thickness of aluminum foil sterilized with isopropanol, and the agar aEowed to solidify. 5.0cc of sterile TSB (27.5g/l; BBL or DIFCO) containing Dextrose (2.5g/l; MALLINCKRODT or J.T. BAKER) was then added to each cavity via syringe, and the cover replaced. BUR£ Upon arrival at JLI, urine samples were cataloged as to JLI daily and consecutive sequence numbers, Sinai reference number, and gross physical characteristics. LOcc of each urine specimen was inoculated via syringe into one ceE of the assembly. Similarly, l.Occ was used to inoculate a septum-fitted vial of nominal 50cc capacity containing 5.0cc of JLI 4A Urine Screening Medium (JLI B/N 037901U-1.5uCi/vial) for use in the paraEel BACTEC study. O.lcc of each specimen was inoculated into previously refrigerated, septum-fitted vials containing 99.9cc 1/2-strength TSB to obtain 1:1000 sample dilution for the pour plate studies. Plates were prepared at 3 4 1:10 and 1:10 dEutions for each sample using 10-15 ml TSA (40.0g/l) and l.Occ and O.lcc from each dilution vial, respectively. The inoculated ceE assembly was placed in a 37° C warm air incubator without agitation and pulsed voltammetric testing begun under computer control. Test values were recorded for aE samples at 10-minute intervals. Data normalization was based upon sample data values recorded after the first 10 minutes for tabulation; aE data values recorded for each sample were ultimately expressed as a percentage of the 10-minute value. A sample was considered to be positive when the normalized data value for that sample at any given time interval after normaEzation fell below 70 or rose above 140. The latter criterion was employed to permit detection of some highly positive (ca. 10 cfu/ml) samples which produced data minima slightly greater than 70, yet which interfered with normal operation sufficiently to produce data maxima over 140. Each day of testing concluded with the generation of a computer printout which included plots of relative ceE potential readings, and a table of normaEzed ceE current readings for each sample, aE as a function of incubation time. The normalized, tabulated results were used to determine sample result classifications. Of the 389 tested samples, 45 were omitted from the study usually for experimental reasons (Incubator Failure, 12; CeE Reconditioning Failure, 14; Contaminated Petri Dishes, 12; JLI/Hospital Data Discrepancy, 7). Contaminated samples numbered 30, and were similarly omitted from further consideration. Of the remaining 314 samples, 84 were considered significant clinically. Table 8 lists the organisms identified by Sinai Hospital found to be present in the significant samples. The number of samples containing each organism is also noted, as is the percentage of the total containing that organism. Non-integer sample numbers are due to samples containing more than one organism. TABLE 8 Organisms Contributing to Significant Samples Organism No. of Samples Percentage E. coli 34.5 41.07 P. aeruginosa 10.5 12.50 K. pneumoniae 9.0 10.71 P. mirabilis 6.0 7.14 Yeast, unspecified 3.5 4.17 S. aureus 3.0 3.57 C. albicans 2.5 2.98 Grp. B, Beta Strep 2.5 2.98 Grp. D Strep 2.0 2.38 . S. albus 2.0 2.38 K. oxytoca 1.5 1.79 E. aerogenes 1.0 1.19 E. agglomerans 1.0 1.19 E. cloacae 1.0 1.19 C. freundii 1.0 1.19 P. morganii 1.0 1.19 Gram-Negative Rod 1.0 1.19 P. stuartii 0.5 0.60 S. marcescens 0.5 0.60 84.0 100.01 " BU E _ OMPI ^NAT Results of the test for the 314 samples considered for data analysis are set out in Table 9. TABLE 9 True Positives 80 (25.48%) True Negatives 215 (68.47%) False Positives 15 ( 4.78%) False Negatives 4 ( 1.27%) 314 100.0% Of the 84 samples considered cEnicaEy significant, the pulsed voltammetric method detected 80 (95.24%). Of the four samples missed by the pulsed voltammetric technique, two were missed by BACTEC as weE. One of these is known to be from a patient receiving antibiotic therapy; the other contained S. Aureus. The remaining two false negative samples contained P. Aeruginosa and an unspecified yeast. The BACTEC system detected 76 (90.48%) of the 84 samples considered elinieaEy significant. The pulsed voltammetric detection technique properly identified 95.24% of urines considered significant in the study, yielding a total sample false negative rate of 1.27%, with a false positive rate of 4.78%. The BACTEC system detected 90.48% of significant urines, with a total sample false negative rate of 2.55% and a 1.27%, with a false positive rate of 4.78%. The BACTEC system detected 90.48% of significant urines, with a total sample false negative rate of 2.55% and a false positive rate of 1.27%. BACTEC required 3 hours to achieve the reported level of performance; the pulsed voltammetric technique required 4 hours. Another ceE assembly was constructed to permit the use of disposable plated wire electrodes. A single ceE of the 16-ceE assembly is presented in Figures 29 and 30. Oxygen-free hard copper wire plated with 50 microinches silver was used as the electrode material. The identical electrodes (6) pass through the Plexiglas (TM) lid (7) and are supported by a polycarbonate wire support (8). Contact with each electrode is made by capturing the bent upper end of each electrode wire between a bottom beryllium copper plate (9) and a like upper contact (10). The contact assembly is held in place by screws (E) and Nylon spacers (12). Connection to the electronics is by means of flexible insulated wires (13) soldered to the upper contacts. All working parts of the electrode assembly are thus attached to tray top (7). The top is fabricated to fit a 16-weE plastic tray (14) which contains the growth medium and inoculum. The contact assemblies are actuaEy disposed as shown in the top view, arranged as two rows of eight sets each. The electrode wires (6) are bent in "hairpin" fashion so as to permit the ends of the wires to be clear of the solution (15). This is to prevent any spurious responses due to the high electric field which could be generated at a sharp wire end, and also to insure that the copper base metal does not come into contact with the solution at the exposed end. This ceE assembly was employed for Examples 12-14. AE experimental details regarding organism preparation, growth, planting, incubation, and the Eke are the same as described above, unless otherwise noted. No agar or other substance is used to partiaEy isolate the electrodes. Silver plated copper wire was supplied hard-drawn by Sigmund Cohn, NY, on spools. The spooled wire was first straightened by reverse-bending and tensioning, and then cut into pieces about 2.5 inches in length. These were bent in an "L" shape to accommodate the tray lid and contact assembly, and were ultrasonicaEy cleaned in methanol for 15 minutes. The wires were then air dried between Kim wipes in a fume hood. Rubber gloves were worn to transfer the dried wires to clean, dry polypropylene bottles for storage. Sterile rubber gloves were also worn when assembling the wires into the tray top and contact assemblies. The bottom "hairpin" bend was made last, with the wire held in the contact assembly and aligned by the wire support. FinaEy, excess wire was trimmed from the exposed end so as to leave about 1/4" clear of the solution. Computerized data coEection was carried out in the same fashion as described above except for the inclusion of a raw data correction algorithm that was added to correct an exponential decline in ceE current noted for the majority of electrode pairs studies. The reason(s) for the decline in ceE current with or without inoculum in the first 60 minutes or so of an experimental run is not known. One possible explanation is that the solution wets the electrode above the solution proper, and that this wet area enjoys far better gas-liquid exchange than does the bulk of the electrode submerged in the growth medium solution. The observed current would thus faE as the wet area evaporated until equEibrium was achieved. Another explanation might involve the equilibrium of the plated wire surface proper (Ag/Ag„O) at either the cathode or the anode. The correction algorithm employed adjusts the raw data prior to the normalization process described earEer. The purpose of the algorithm is to "add back" an inverse exponential to compensate for the observed decline in electrode current with time. The algorithm has the functional form: C(T,N) = I(T,N + I(T,N) l-2.8e -0.3T In practice, the values of C(T,N) are stored in a separate array. The usual normalization can then be applied to either the raw or the corrected data. AH subsequent data reduction and plotting, including normalization, takes place as outlined earlier. The correcting effect of the algorithm may be seen in Figure 31. EXAMPLE 12 A fresh overnight culture of E. coli in 6B medium was used as the inoculum source. The sampling and dilution scheme presented in Figure E was employed to prepare sample ceEs and pour plates. TSB with dextrose (30.0g/l) was used as the medium for detection as described above. AE incubations were performed at 37 degrees C. CeE readings were continued for 6 hours. The normalized pulsed voltammetric current response for three decade dilutions of the organism plus control is presented in Figure 32. Times-to- detection at the 80% threshold are seen to vary in a predictable manner as a function of inoculum strength. Duplicate pour plate results indicate that 1.4 x 10E5 cfu/ml E. coli were present in the XO.l weE. Detection times and inoculum levels for aE wells are given below: Inoculum Level Detection Time 1.4 x 10E5 150 minutes 1.4 x 10E4 210 minutes 1.4 x 10E3 300 minutes EXAMPLE 13 A fresh overnight eulture of E. cloacae in 6B medium was used as the source for aE inocula. The sampling and dEution scheme presented in Figure E was used to prepare sample cells and pour plates. Tryptic soy broth with dextrose (30.0g/l) was used as the growth medium for detection. AE incubations were performed at 37 degrees C. CeE readings were continued for 7 hours. The ceE current response for each of the three decade dEutions of E. cloacae plus control is shown in Figure 33. Detection time is observed to vary predicatably with the level of inoculum in each weE. Pour plates prepared at the time of inoculation showed the XO.l weE to contain 2.2 x 10E5 cfu/ml of the organism. Inoculum levels and associated detection times for aE weEs are given below: Inoculum Level Detection Time 2.2 x 10E5 170 minutes 2.2 x 10E4 250 minutes 2.2 x 10E3 320 minutes EXAMPLE 14 A fresh overnight culture of P. aeruginosa in 6B medium was used as the inoculum source. Once again, the sampling and dilution scheme presented in Figure E was used to prepare sample weEs and pour plates for the experiment. TSB with dextrose (30.0g/l) was used as the growth medium and electrolyte for the study. AE incubations were performed at 37 degrees C. CeE readings were taken under computer control for 8 hours. The ceE current response normalized at the 20-minute interval for the three decade dEutions of the organism is presented in Figure 34. Detection time at the 80% threshold is seen to vary in a predictable manner with the level of initial inoculum. Duplicate pour plate results indicate that 1.4 x 10E5 cfu/ml P. aeruginosa was present in the XO.l weE at the time of inoculation. Each inoculum level and like time-to-detection is presented below: Inoculum Level Detection Time 1.4 x 10E5 210 minutes 1.4 x 10E4 320 minutes 1.4 x 10E3 420 minutes The technique of the present invention is thus shown to provide competent detection of significant bacteriuria, with clinicaEy acceptable levels of false negative and false positive results. The rapidity and sensitivity of the method compare favorably with paraEel results obtained using the BACTEC system. WhEe certain specific embodiments of the invention has been described with particulars herein, it should be recognized that various modifications thereof wiE occur to those skiEed in the art. Therefore, the scope of the invention is to be limited solely by the scope of the claims appended hereto.

Claims: CLAIMS: 1. An electroanalytical method for detecting the presence of oxygen - consuming microorganisms in a sample comprising the steps of: a) providing a mixture of said sample and a fluid culture medium capable of supporting microorgaism growth in an electroanalytical ceE equipped with two electrodes which are in contact with said mixture; b) applying a series of voltage pulses of substantiaEy constant amplitude and duration across said electrodes; and c) measuring the resulting current through said ceE prior to the trailing edge of each of said applied voltage pulses; the presence of oxygen-consuming microorganisms being indicated by a decrease in ceE current which is a function of the dissolved oxygen content of said mixture. 2. The method of claim 1 additionaEy comprising the measurement of the open-ceE oxidation-reduction potential across said electrodes during the interval between successive applied voltage pulses. 3. The method of claim 1 or 2 wherein said voltage pulses have an amplitude of from about -0.35v. to about -0.90v. 4. The method of claim 3 wherein said voltage pulses have a duration of at least about 600 milliseconds. 5. The method of claim 1 or 2 wherein said voltages pulses have a duration of at least about 600 miUiseconds. 6. The method of claim 5 wherein said voltages pulses have a duration of about 1200 miEiseconds. 7. The method of claim 1 or 2 wherein said voltage pulses are separated by an interval of about 5 to 20 minutes. 8. The method of claim 1 wherein the cathode in said electroanalytical ceE is made from a noble metal. 9. The method of claim 8 wherein said cathode is made from platinum, gold or silver. 10. The method of claim 2 wherein said cathode in said electroanalytical ceE is platinum. 11. The method of claim 1 or 2 wherein the reference electrode in said electroanalytical ceE is silver /silver oxide or silver/silver chloride. 12. The method of claims 1 or 2 wherein said electrodes are covered with a conductive porous gel. 13. The method of claim 12 wherein said gel is a nurient gel, 14. The method of claim 13 wherein said nutrient gel is tryptic soy agar. 15. The method of claims 1 or 2 wherein said electroanalytical ceU and its contents are maintained in a constant temperature environment during said measuring. 16. The method of claim 15 wherein said ceE is maintained at about 37° C. 17. The method of claims 1 or 2 wherein said fluid culture medium comprises tryptic soy broth.

Inventors: AHNELL J
Applicants: JOHNSTON LAB INC
Application year: 1980
Main classification: C12Q1
IPC codes: C12M1, C12Q1, G01N27, G01N33