United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

Veterinary Services 

Centers for Epidemiology and Animal Health March 2011 

_________________________________________________________________________________________________________________________ 

Methicillin-resistant 

Staphylococcus aureus in Bulktank Milk on U.S. Dairy 

Operations, 2007 

Background 

Methicillin-resistant Staphylococcus aureus (MRSA) first 

emerged in hospitals during the 1970s and has recently 

become a worldwide public health problem. It remains a 

major cause of hospital-acquired infections in humans. 

In addition, community-acquired MRSA has also become 

a major concern.1

 Currently, increasing evidence points 

to domestic animals—including food animals—as 

reservoirs and shedders of MRSA, and transmission 

between host species may be possible.2

 Over the past 

decade, a growing number of MRSA isolates have been 

reported in companion and food animals and in their 

human associates, including pet owners, farmers, and 

veterinary personnel. MRSA has been detected in dogs, 

cats, horses, pigs, sheep, goats, rabbits, chickens, 

exotic species, and milk from cows with mastitis.3 4

 Recent studies have shown genetic similarity 

between MRSA isolates from food animals—including 

dairy cows— to those in humans, suggesting 

transmission between the species.5 6 MRSA infections in 

dairy cattle have been ascribed to human-to-animal 

transfer, but the directionality of transmission is not 

always known.6 7 

Although S. aureus is a common mastitis pathogen 

and among the leading causes of foodborne bacterial 

infections,8

 MRSA appears to be relatively rare in foods 

originating from animals, and there is little evidence to 

suggest that MRSA is common in milk. Two studies 

detected MRSA in less than 1 percent of meat, milk, and 

cheese samples.3 8 Pasteurization significantly reduces 

the risk of MRSA transmission via dairy products, and 

most reported instances of foodborne MRSA outbreaks 

have occurred through contamination by infected food 

handlers, rather than the food itself.9 10

An analysis of 2,778 S. aureus isolates from milk 

samples submitted to diagnostic laboratories in Michigan 

from 1994 to 2000 showed no increased resistance of 

mastitis isolates to antimicrobials (including oxacillin) 

commonly used in dairy cattle.11 Among 357 S. aureus

isolates recovered from milk in North Carolina, 

antimicrobial resistance was uncommon, and resistance 

to oxacillin was not detected.12 

Dairy 2007 study 

In 2007, the U.S. Department of Agriculture’s 

(USDA) National Animal Health Monitoring System 

(NAHMS) conducted the Dairy 2007 study. The study 

was conducted in 17 of the Nation’s major dairy States*

representing 79.5 percent of U.S. dairy operations and 

82.5 percent of U.S. dairy cows. 

One objective of the study was to evaluate the 

occurrence of MRSA in bulk-tank milk from dairy 

operations. Operations with 30 or more cows that had 

completed the initial study questionnaire were eligible for 

bulk-tank milk sampling and testing for MRSA. 

Sample collection and testing 

To estimate the prevalence of mastitis pathogens 

and the occurrence of MRSA, a single bulk-tank-milk 

sample was collected from each of 542 participating 

operations. Samples were shipped overnight on ice to 

Quality Milk Production Services (QMPS) bacteriology 

laboratory for evaluation using routine testing methods.13 

14 After the samples were cultured for the presence of 

mastitis pathogens, S. aureus-positive samples (n=218) 

were stored at -20 ̊C for 4 weeks to 4 months, until 

further processing. 

To detect MRSA in bulk-tank milk, phenotypic and 

genotypic methods were used. Phenotypic detection was 

based on plating of stored milk samples on a selective 

indicator media, CHROMagarTM MRSA.15 In a parallel 

assay, the thawed milk samples were plated on blood 

agar to obtain multiple staphylococcal-like colonies. 

Suspensions of these colonies, which might have 

contained mixtures of S. aureus strains and coagulase 

negative staphylococci, were used for subsequent 

genotypic detection of MRSA based on polymerase 

chain reaction using S. aureus and mecA specific primer 

sets.16 17 The nuc gene is specific for S. aureus while the 

mecA gene is responsible for conferring methicillin 

resistance. When mecA was identified in a colony 

suspension, efforts were made to identify individual 

mecA positive bacteria on the blood agar plates. 

Results 

Of the 218 milk samples that were originally 

S. aureus-positive, 190 were culture positive on blood 

agar after storage at -20 ̊C; however, MRSA was not 

 

*

States/Regions: 

 West: California, Idaho, New Mexico, Texas, and Washington 

 East: Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, 

New York, Ohio, Pennsylvania, Vermont, Virginia, and Wisconsin 

Technical Brief
United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 

detected in these samples tested on CHROMagar. MecA 

was detected in nine colony suspensions from blood 

agar, twice in the absence of nuc and seven times in 

combination with nuc-positive. Detection of mecA in 

these samples can be due to the presence of methicillinresistant Staphylococcus spp. other than S. aureus. In 

one study, methicillin resistance was found to be more 

common in nonaureus staphylococci from milk samples 

than in S. aureus.

18 Through analysis of individual 

colonies from nine bulk-tank milk samples, methicillinresistant Staphylococcus spp. (but not MRSA) were 

obtained from five samples. From the remaining four 

samples, no methicillin-resistant colonies were 

obtained.19

Conclusion 

MRSA could not be detected in a nationally 

representative sample of bulk-tank milk from the NAHMS 

Dairy 2007 study using phenotypic and genotypic 

methods, suggesting that bulk-tank milk in the United 

States is not a common source of MRSA. However, in 

an extremely small number of bulk-tank samples (less 

than .02 percent), methicillin resistance was identified in 

Staphylococcus spp. other than S. aureus. 

References 

1. Otter JA, French GL. 2008. The emergence of communityassociated methicillin-resistant Staphylococcus aureus at a London 

teaching hospital, 2000-2006. Clin Microbiol Infect 14(7):670-676. 

2. Loo I, Huijsdens X, Tiermersma E, Neeling A, van de SandeBruinsma N, Beaujean D, Voss A, Klutmans J. 2007. Emergence of 

methicillin-resistant Staphylococcus aureus of animal origin in humans. 

Emerg Infect Dis 13:1834-1839. 

3. Lee JH. 2003. Methicillin (oxacillin)-resistant Staphylococcus aureus 

strains isolated from major food animals and their potential 

transmission to humans. Appl Env Micro 69:6489-6494. 

4. Leonard FC, Markey BK. 2006. Methicillin-resistant Staphylococcus 

aureus in animals: a review. Vet J 175:17-36. 

5. Moon JS, Lee AR, Kang HM, Lee ES, Kim MN, Paik YM, Park YH, 

Joo YS, Koo HC. 2007. Phenotypic and genetic antibiogram of 

methicillin-resistant Staphylococci isolated from bovine mastitis in 

Korea. J Dairy Sci 90:1176-1185. 

6. Juhasz-Kaszanyitzky E, Janosi S, Somogyi P, Dan A, Van der 

Graaf-van Bloois L, Van Duijkeren E, Wagenarr JA. 2007. MRSA 

transmission between cows and humans. Emerg Inf Dis 13:630-632. 

7. Devriese LA, Hommez J. 1975. Epidemiology of methicillin-resistant 

Staphylococcus aureus in dairy herds. Res Vet Sci 19:23-27. 

8. Normanno G, Corrente M, La Salandra G, et al. 2007. Methicillinresistant Staphylococcus aureus (MRSA) in foods of animal origin 

produced in Italy. Int J Food Micro 117(2):219-222. 

9. Jones T, Kellum M, Porter S, Bell M, Schaffner W. 2002. An 

outbreak of community-acquired foodborne illness caused by 

methicillin-resistant Staphylococcus aureus. Emerg Inf Dis 8:82-84. 

10. Kluytmans J, W. van Leeuwen W, W. Goessens W, R. Hollis R, 

Messer S, Herwaldt L, Bruining H, Heck M, Rost J, van Leeuwen N, 

Van Belkum A, Verbrugh H. 1995. Food-intitiated outbreak of 

methicillin resistant Staphylococcus aureus analyzed by pheno- and 

genotyping. J Clin Microbio 33:1121-1128. 

11. Erskine RJ, Walker RD, Bolin CA, Bartlett PC, White DG. 2002. 

Trends in antibacterial susceptibility of mastitis pathogens during a 

seven-year period. J Dairy Sci 85:1111–1118. 

12. Anderson KL, Lyman RL, Bodeis-Jones SM, White DG. 2006. 

Genetic diversity and antimicrobial susceptibility profiles among 

mastitis-causing Staphylococcus aureus isolated from bovine milk 

samples. Am J Vet Res 67:1185-1191. 

13. Hogan JS, R.N. González RN, R.J. Harmon RJ, S.C. Nickerson 

SC, Oliver SP, Pankey JW, Smith KL. 1999. Laboratory handbook on 

bovine mastitis, revised edition. Madison, WI: The National Mastitis 

Council. 

14. Zadoks RN, González RN, Boor KJ, Schukken YH. 2004. Mastitiscausing streptococci are important contributors to bacterial counts in 

raw bulk tank milk. J Food Prot 67:2644-2650. 

15. Han Z, Lautenbach E, Fishman N, Nachamkin I. 2007. Evaluation 

of mannitol salt agar, CHROMagar Staph aureus and CHROMagar 

MRSA for detection of meticillin-resistant Staphylococcus aureus from 

nasal swab specimens. J Med Microbiol 56(Pt 1):43-6. 

16. Brakstad OG, Aasbakk K, Maeland JA. 1992. Detection of 

Staphylococcus aureus by polymerase chain reaction amplification of 

the nuc gene. J Clin Micro 30:1654-1660. 

17. Martineau F, Picard FJ, Lansac N, Menard C, Roy PH, Ouellette M, 

Bergeron MG. 2000. Correlation between the resistance genotype 

determined by multiplex PCR assays and the antibiotic susceptibility 

patterns of Staphylococcus aureus and Staphylococcus epidermidis. 

Antimicrob Agents Chemother 44(2):231-238. 

18. Makovec JA, Ruegg PL. 2003. Antimicrobial resistance of bacteria 

isolated from dairy cow milk samples submitted for bacterial culture: 

8,905 samples (1994–2001). J Am Vet Med Assoc 222:1582-1589. 

19. Virgin JE, Van Slyke TM, Lombard JE, Zadoks RN. 2009. Short 

Communication: Methicillin-resistant Staphylococcus aureus (MRSA) 

Detection in Bulk Tank Milk. J Dairy Sci. 92:4988-4991. 

____________________ 

For more information, contact: 

USDA–APHIS–VS–CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMS@aphis.usda.gov 

http://nahms.aphis.usda.gov 

#N576.0311 

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