PhD defence V.O. (Valérie) Baede

Summary:

Methicillin-resistant Staphylococcus aureus (MRSA) is characterised by a limited number of genetic lineages, so-called clones. Some MRSA clones have spread on a global level and are responsible for a large number of MRSA infections in hospitals or the general community. Other MRSA clones have failed to spread successfully. This thesis focuses on the factors that may contribute to the successful or unsuccessful spread of epidemic MRSA clones and their rare counterparts. We focused on: bacterial factors, such as genes that lead to antibiotic resistance or genes that make MRSA more virulent; human factors, such as the immune system and the nasal microbiome; and external factors, such as the policy on antibiotic usage and infection prevention.

Several factors were found to contribute to the difference in the successful or unsuccessful spread of MRSA clones. Both antibiotic resistance and selective pressure due to antibiotic usage are important factors. In interaction with skin, both bacterial and human factors are important. Furthermore, all MRSA proved to be highly resistant to dehydration, which can benefit survival in the environment. 

We also looked at the effect of topical antibiotic treatment on S. aureus and other bacteria in the nose. Because this treatment disrupts the nasal microbiome for a long time, it should only be used when medically indicated in S. aureus carriers and not at all in noncarriers.

In this thesis we conclude that identification of epidemic and sporadic MRSA is hindered by large differences between MRSA surveillance programmes worldwide. We provide several recommendations to harmonise MRSA surveillance.

In summary, we conclude that multiple bacterial, human and external factors influence MRSA transmission and may drive potential transmission of epidemic clones. Successful transfer of MRSA clones is thus multifactorial and depends on a wide range of intrinsic and external factors.