New methods to identify MRSA in pigs

We are only at the beginning of understanding livestock-associated methicillin resistant Staphylococcus aureus (LA-MRSA) concludes Danish PhD student Mette Theilgaard who recently completed a three-year investigation of the strain.

Based at the National Food Institute, Technical University of Denmark, Ms. Theilgaard’s work is highlighted on the Institute’s website under the headline “New methods to identify MRSA in pigs”. Her thesis summary, however, includes the observation that further investigation needs to be carried out, particularly in relation to the transmissibility of LA-MRSA, the strain which was recently identified in a weaner in Northern Ireland.

“S. aureus has multiple ways of thwarting the host immune system,” she concluded. “The bacterium is able to colonize various hosts silently and under certain conditions cause infections of different severity. Various methods have been used to characterize S. aureus both as colonizer and as infectious agent. S. aureus and especially MRSA have been of world-wide importance for many years. The latest branch of MRSA is the LA-MRSA, which have been emerging in the past decade. This group shows a broader host-spectrum compared to most other MRSA and a different virulence profile with fewer toxin-encoding genes.

“Little is known about the ecology of ST398 (LA-MRSA) on farms. However, it is assumed that the use of antibiotics in the production animal industry has been the key force, driving the emergence and spread of MRSA ST398. Even though some studies have shown that ST398 transmits less frequently among humans than human S. aureus strains, the transmissibility of ST398 still needs further investigation.

“ST398 has been the most commonly reported MRSA strain associated with livestock in recent years, but knowledge on colonization and transmission of LA-MRSA in pigs is limited and mainly based on observational field surveys. We are only at the beginning of understanding the role of these strains in the epidemiology of human S. aureus ST398 colonization and disease.”

The National Food Institute, in its general assessment of the PhD work, added: “These studies have resulted in new methods, high-throughput approaches, which can identify genes important for the survival of MRSA in pigs. High-throughput approaches can identify those genes in the total gene pool of the bacteria which are essential, or the presence of which is advantageous, for the bacteria under some given circumstances.

“The fact that MRSA can spread from animals to humans, where they may result in infections, has caused great concern in recent years. LA-MRSA ST398 is a new type and has turned out to be particularly successful in colonisation of pigs, from where it may transmit to humans.

“Several central questions still remain unanswered, which makes it difficult to control the spread of this MRSA type. LA-MRSA ST398 is zoonotic, i.e. it can be transferred directly from animals to humans and cause disease. Thus, it is not sufficient to eradicate the bacteria from humans.

“LA-MRSA ST398 has proven to be particularly successful in colonization of pigs. By studying which genes are essential for the bacteria in pigs it may be possible for researchers to identify the factors important for the bacterium to colonize on pigs. We still don’t know, however, which specific genetic factors in this MRSA type facilitate the spread from animals to humans.”

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