This project aimed to study epidemiology and genetic diversity of methicillin resistant Staphylococcus aureus (MRSA) and methicillin resistant Staphylococcus sciuri (MRSS). Indeed, while S. aureus is recognized as a pathogen potentially responsible of nosocomial infection in humans or mastitis in bovines, S. sciuri is one of the most ancient species in the genus Staphylococcusmainly and considered as a harmless environmental species though it has also been recovered from some cases of infections in humans and animals. In this project we followed two surveys in Belgium over MRSA in poultry in 2011 and in healthy bovines in 2012. During these surveys, two isolation methods have also be compared. One of these isolation methods uses two enrichment broths and is proposed by the EFSA though considered as not very sensitive. The alternative method is very similar to the first one except that only one enrichment broth is used. Additionally to this, MRSS have been isolated during the survey in poultry. Additionally to this, investigation on the prevalence of methicillin resistant Staphylococcus sciuri (MRSS) in poultry and in farm animals has been performed.
Our results show the importance of methicillin resistant Staphylococci in livestock though with different estimated prevalence depending on the farm animal and the age group. We have also shown the high prevalence of MRSS in food producing animals. Indeed MRSS prevalence in poultry outnumbered the prevalence of MRSA. While MRSA was shown to appear occasionally in chicken, MRSS were more common. MRSA prevalence differs also among origin. Indeed, the prevalence estimated during the bovine survey was much higher than that of poultry. This prevalence was also depending on the age group of the animals. Whereas no significant difference could be highlighted between the prevalence in broilers and layers, the prevalence of MRSA in dairy and meat cows was significantly lower compared to veal calves.
Furthermore, our results showed that, such as S. aureus, S. sciuri is often found to be multi-resistant to antibiotics and as such, it may serve as a reservoir for antimicrobial resistance or eventually virulence genes. Indeed, since MRSA and MRSS are commonly found in farm animals, they might share a common gene pool. However, some differences in the antimicrobial resistance profile were noted. While MRSA were mostly resistant to tetracycline and trimethoprim; MRSS were more resistant to fusidic acid, tiamulin and quinupristin/dalfopristin. However, trimethoprim and clindamycin resistance was high in both species. Furthermore, resistance to ciprofloxacin and chloramphenicol was low in MRSS and all isolates were sensitive to vancomycin in both cases.
Genetic diversity of both MRSA and MRSS was also assessed. In contrast to S. sciuri, there are well defined molecular tools available for S. aureus characterization allowing a better understanding of its behavior in different animal populations and of the population structure of this species. Spa-typing and MLST showed the presence of a broad range of different MRSA types in bovines although the livestock-associated (LA)-MRSA ST398 was the most common. However, unexpectedly, non ST398 MRSA has also been found both in bovines and in poultry. This type was determined as ST239 which is a common hospital acquired (HA)-MRSA. This support the hypothesis of a spill-over and spill-back between animals and humans that was set in this project. However, all these tools are not available for S. scuri and our studies had to rely on SCCmec typing and PFGE for subtyping of the population.
SCCmec typing in MRSA and MSSA was different, while MRSA mostly harbor SCCmec type IV and V, MRSS carried mostly SCCmec type III and a non-typable cassette. In poultry, these cassettes seemed to be associated to the animal origin since the non-typable SCCmec was mostly found in layers while the type III was mainly recovered in broilers. This repartition was also supported by the PFGE profiles since two major clusters appeared in the dendrogram and corresponded to the origin (broilers or layers) and to the SCCmec type. This study showed that, even if some diversity was observed in the profiles, some strains were common to different farm which indicates a possible strain sharing between farms in the same country
