Submitted to: Journal of Clinical Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/4/2012
Publication Date: 11/1/2012
Citation: Molla, B., Byrne, M., Abley, M.J., Matthews, J., Jackson, C.R., Cray, P.J., Sreevatsan, S., Wang, P., Wondwossen, G. 2012. Epidemiology and genotypic characteristics of Methicillin-Resistant Staphylococcus aureus strains of porcine origin. Journal of Clinical Microbiology. 50(11):3687-3693. Interpretive Summary: Methicillin-resistant Staphylococcus aureus (MRSA) is implicated in human disease and, until recently, food animals had not been considered as sources of infection. Among food animals, pigs can carry MRSA and potentially infect humans including farmers, slaughterhouse workers, and veterinarians who are in frequent contact with MRSA colonized pigs. Livestock-associated MRSA (LA-MRSA) has been reported among pigs, pig farmers, and the environment in a number of European countries and North America. A study was conducted to determine the occurrence and persistence of MRSA among swine herds on-farm, in holding pens and on carcasses at the plant, and on retail pork. Our goal was to compare characteristics of MRSA recovered throughout the pork production chain. Pork samples from the same groups of swine from the farm were collected at the retail market. All samples were examined following conventional microbiological methods. Isolates were tested for antimicrobial resistance using broth microdilution and a molecular test, polymerase chain reaction (PCR), was used to confirm an isolate as MRSA. The degree by which isolates appeared the same at a molecular level was determined using Pulsed-Field Gel Electrophoresis (PFGE), Multi-Locus Sequence Typing (MLST) andStaphylococcus protein A (spa) typing. The overall prevalence of MRSA in pigs was 3% on-farm and 11% at holding pens at the plant. MRSA was detected in 2% of the carcass swabs and 4% of the retail pork samples. The majority of the isolates were similar in their genetic structure regardless of the genotypic typing method used, although PFGE did show a slightly increased, but not significantly different, ability to detect differences between the isolates. Four groups of genetically related isolates were identified using PFGE and one group contained isolates from the farm to retail meat sampling. MLST showed that sequence type 5 was the most common subtype (32/50) followed by the LA-MRSA (12/50) type which was also detected in all farm to retail samples. Nine of the 50 (18%) MRSA isolates belonged to spa type 539/t034 which were also identified as LA-MRSA based on MLST. Contamination of retail meat with LA-MRSA and others strains that have previously impacted public health are important findings. This research is useful to policy makers, physicians and veterinarians to make informed decisions regarding control of MRSA infections.
Technical Abstract: The main goal of this study was to determine the prevalence of methicillin-resistant Staphylococcus aureus (MRSA), particularly livestock-associated (LA)-MRSA in pigs and pork. Genotypic relatedness of isolates on-farm, at slaughter and retail was assessed. Paired nasal and peri-anal swab samples were collected from ten cohorts of market-age pigs (24 pigs per cohort), carcasses at slaughterhouse and pork samples were collected at retail. Staphylococci were isolated using selective enrichment method. Isolates were tested for antimicrobial resistance by broth microdilution. Duplex PCR was used to confirm MRSA using species-specific (nuc) and methicillin resistance (mecA) genes. The clonal relatedness of isolates was determined using Pulsed-Field Gel Electrophoresis (PFGE), Staphylococcus protein A (spa) typing, Multi-Locus Sequence Typing (MLST) and SCC-mec typing. MRSA was detected in five of the ten cohorts (50%) with prevalence ranging from 0% to 12.5% per cohort. Of the total pigs sampled on-farm before marketing, 3% (7/240) were MRSA positive. Higher prevalence of MRSA was detected at holding pens at the slaughterhouse (11%; 27/240). MRSA was also detected in 2% (4/235) of the carcasses and 4% (5/135) of the retail pork. While the isolates appear predominantly to be highly clonal, PFGE had a relatively higher discriminatory power (DI=0.624). Four genotypic clusters were identified by PFGE of which clonal type B was predominant across the farm to retail continuum. MLST findings revealed that sequence type (ST)-5 was the most predominant subtype (32/50). The livestock-Associated (LA)-MRSA (CC/ST398) was the second common type (12/50) and was detected at all stages of farm to retail. Nine of the 50 (18%) MRSA isolates belonged to spa type 539/t034 that were of ST398 based on MLST. The study confirms that MRSA including LA-MRSA is common in Ohio swineherds and hereby shown to persist in the farm to processing and retail continuum.