Staphylococcus aureus is a major pathogen responsible for both nosocomial and community acquired infections. The severity of these infections varied from local benign wounds to severe systemic diseases. The situation is also complicated with emergence of bacterial resistance to common antibiotics, such as methicillin. Endemic strains of MRSA carrying multiple resistance determinants have become a worldwide nosocomial problem only in the early 1980's, carrying a threefold attributable cost and a threefold excess length of hospital stay when compared with methicillin-susceptible S. aureus bacteraemia. Recent genetic advances have enabled identification and characterization of clinical isolates in real-time. These tools support infection control strategies to limit bacterial spreading and ensure the appropriate use of diminishing antibiotics. They are also attractive for understanding the epidemiology of MRSA and the relationship between genome content and virulence.

Further reading: Francois, P. and Schrenzel, J. (2008) Rapid Diagnosis and Typing of Staphylococcus aureus In: Staphylococcus: Molecular Genetics

Rapid detection and identification of MRSA is an absolute prerequisite to adopt prompt isolation measures. Until recently, microbiological methods dedicated to MRSA identification were based on the utilization of selective growth media, which are timeconsuming and preclude same-day diagnosis. For more than one decade, nucleic acid-based identification assays have demonstrated their usefulness and robustness for the detection of hardly cultivable, non-cultivable and even killed microorganisms, as well as for the identification of specific pathogens against the background of a complex microflora. The current view is still that molecular methods are used to supplement, but not to replace cultures. MRSA molecular detection nicely illustrates this paradigm: it provides early warning but cultures are still required for further antimicrobial susceptibility testing or epidemiological typing. Molecular assays based on target nucleic acid amplification, and especially real-time PCR, have proven rapid, affordable and successful in terms of sensitivity and specificity. Current challenges for MRSA screening are centred on the selection of the most appropriate assay, both in terms of feasibility (costs, technical expertise) and assay performance. One has to be especially careful when embarking on detection strategies that are based on mobile and highly variable genetic regions, such as the SCCmec insertion site. Indeed, iterative changes in the detection protocol to adapt for emerging variants might not only affect the performance of the assay but also open unpredictable and systematic breaches in the infection control programme. To date, with several decades of hindsight, we can firmly state that molecular variations will continuously emerge, but it is currently impossible to predict where these changes will emerge. As the molecular epidemiology may substantially vary from country to country, and possibly also between different regions, this underlines the importance of having an epidemiologic molecular surveillance, ideally as close as possible to our own lab practice.

Further reading: Francois, P. and Schrenzel, J. (2008) Rapid Diagnosis and Typing of Staphylococcus aureus In: Staphylococcus: Molecular Genetics