Sources of Methicillin-Resistant Staphylococcus aureus (MRSA) and Other Methicillin-Resistant Staphylococci: Implications for our Food Supply?

The white paper summarized all historical data on MRSA and its relationship with animal and non-animal related infections; evaluate hospital acquired infections; evaluate worldwide understanding of MRSA infections and their sources. The white paper also identified data gaps and discussed how these gaps influence the understanding of MRSA and proposed tasks needed to close the gaps.

Update added October 2012

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Objectives

The objectives of this white paper were to: summarize all historical data on MRSA and its relationship with animal and non-animal related infections; evaluate hospital nosocomial MRSA infections; evaluate worldwide understanding of MRSA infections and their sources; and identify the data gaps and discuss how these gaps influence the understanding of MRSA and propose tasks needed to close the gaps.

Conclusions

S. aureus is commonly found in humans with approximately 50% of the population colonized in the nasal passages or on the skin.  A much smaller percentage, about 1.5% of people, are colonized with MRSA.  While many people harboring S. aureus are asymptomatic, they may pass these bacteria to others directly or contaminate food, clothing, towels, and other surfaces.  Carriage of MRSA increases risk for serious infections that are difficult and more expensive to treat.  Methicillin resistance also occurs in other staphylococci, including S. intermedius and S. pseudintermedius that colonize and infect pets and other animals.

Enterotoxigenic staphylococci (including MRSA) that grow in foods can cause foodborne intoxication.  Occasionally these staphylococci, when ingested, produce enterotoxins in the intestines causing enterocolitis in people whose normal flora has been depleted. In hospitals, S. aureus, including MRSA, cause a large proportion of invasive infections when they enter the body through surgical wounds, catheters, or other medical devices or procedures.  In the community, staphylococci primarily cause pneumonia and skin and soft tissue infections.  Some MRSA strains are highly virulent and MRSA infections cause significant morbidity and mortality.

Methicillin resistance was originally reported in hospitals and hospital acquired-MRSA strains are usually resistant to many other antimicrobials besides penicillin-related compounds.  Community acquired-MRSA strains evolved outside of health care facilities and are usually sensitive to most other antibiotics. Many of these strains have spread globally.  Methicillin resistance has also emerged in other environments where antibiotics are used including veterinary hospitals (methicillin-resistant S. pseudintermedius and methicillin-resistant S. intermedius group (MSRIG)) and livestock operations (LA-MRSA).  Methicillin resistance genes are carried on a mobile genetic element that can be transferred to other staphylococcal species.

Sources of methicillin resistant staphylococci for human infections include colonized or infected people, companion animals, and livestock and objects and surfaces contaminated by them. Staphylococci have been detected in the air indicating that aerosolization of staphylococci occurs and is potentially a transmission pathway in healthcare facilities and farms with large numbers of colonized animals.  Since MRSA has been detected in retail foods and on animal carcasses at slaughter, food may also be a source of infection to food handlers or of foodborne intoxication to consumers.  Surveys to date indicate that the prevalence of MRSA in meat is low and the concentration of bacteria in food samples is also low.  In some cases MRSA contamination of foods appears to result from MRSA present in dairy cows or in animals before slaughter and in other cases, from human food handlers.

Although MRSA and MSRIG contamination of foods is not currently a significant problem, these bacteria continue to evolve and spread in the environment.  MRSA was originally isolated in a UK hospital in 1961, community-associated MRSA strains appeared in the early 1990s as did methicillin-resistant S. intermedius (pseudintermedius) in companion animals, and livestock associated MRSA strains were first described in 2003. Ongoing monitoring of methicillin-resistant staphylococci in foods and the environment would be prudent.

Deliverable

This paper effectively summarized thedata on MRSA and its relationship with animal and non-animal related infections.  It also identified knowledge gaps which can be used to direct future research.