#08-219

Complete

Date Full Report Received

02/10/2010

Date Abstract Report Received

02/10/2010

Investigation

Institution:
Primary Investigator:

Methicillin-resistant Staphylococcus aureus (MRSA) is a critically important cause of disease in people. It has been a leading cause of infections of people in hospitals, and subsequently emerged as a common cause of infection in people in the general population. As MRSA has moved from a hospital-based problem and into the community, concerns about the role of animals in community-associated disease have emerged. A strong link between human MRSA infections and contact with pigs has been made in Europe, and concerns have arisen about the potential for food to act as a vehicle for MRSA transmission. Preliminary studies from Europe and North America identified MRSA in retail pork, yet the relevance of this finding has been unclear. There are theoretical concerns about the potential for people to become colonized (carriers) of MRSA following contact with contaminated food if good food handling practices are not used. Improper handling and storage of food could also lead to MRSA food poisoning, if MRSA contaminates food and is able to grow and produce enterotoxins. It is important to investigate MRSA contamination of retail pork, both for evaluation of human health risks and to provide important information for the pork industry that will be needed to address and/or allay public and regulatory concerns. One important aspect is development of methods of detect MRSA in meat and determine the level of contamination that is present. Preliminary studies have used different methods, none of which have been validated. Further, these studies have relied on methods that could likely detect very low levels of MRSA. Determination of the level of MRSA contamination in pork and detection thresholds for different methods may be very important aspects for understanding potential risks and for future studies evaluating food as a source of MRSA infection, sources of food contamination and measures to reduce MRSA contamination.

This study evaluated different methods to detect MRSA in meat, and to determine the level of contamination. Results indicated that enrichment culture methods that are commonly used are able to detect very low levels of MRSA in meat, as low at 10 bacteria per gram. Methods to determine the actual level of contamination are possible and practical, and while they cannot detect concentrations of MRSA quite as low as enrichment culture, they can be used to provide a better understanding of contamination.

Optimal methods for detection of MRSA and for determination of the level of contamination were chosen based on the first component of the study and used to test ground pork and pork chops. MRSA was isolated from 8/127 (6.3%) ground pork samples and 14/89 (14%) pork chops, for an overall rate of 9%. Seven of 22 positive samples were only positive using the enrichment culture method, mean the level of contamination was likely less than 20 bacteria per gram of meat. Of the samples where enough MRSA was present to determine a number, only 20 bacteria per gram were present in 9/15 (60%), while the remaining 6 samples had 30, 90, 100, 110, 340 and 3590 CFU/g.

MRSA strains that were isolated were typed and all were classified as the USA100 clone, a human-origin MRSA strain that is the most common strain found in human carriers and people with hospital-acquired infections in North America. This study confirmed previous reports of the presence of MRSA in retail pork, however it is clear that contamination is quite low-level. The relevance of contamination of this level of MRSA is currently unclear. While it should not be dismissed, care must be taken not to over-react to the presence of this organism is meat because of the low level of contamination that is present. Further, it was interesting that all MRSA isolates were a human-origin strain. This strain has been found in pigs in Canada, so it is not surprising that it would be present in some samples, however the most common strain, sequence type 398 (ST398, the strain one that has generated all of the concern regarding pigs) was not present in any sample. This raises questions about the source of MRSA contamination and whether humans or processing environments could be possible sources of contamination, or whether ST398 is less able to survive in the slaughterhouse and processing environments. The presence of USA100 in meat indicates that greater investigation of possible sources of contamination are needed, because if contamination occurs more commonly during meat processing, efforts may need to be focused on post-slaughter measures as opposed to on-farm measures.
Contact information: J Scott Weese, jsweese@uoguelph.ca