Population structure of methicillin-susceptible Staphylococcus aureus (MSSA) in Portugal over a 19-year period (1992-2011).

Despite their clinical relevance, few studies have addressed the epidemiology of methicillin-susceptible S. aureus (MSSA). In particular, it is not clear how MSSA population structure has evolved over time and how it might have been shaped by the emergence of MRSA in the community (CA-MRSA). In the present study we have evaluated the MSSA population structure over time, its geographical distribution and relatedness with MRSA in Portugal. A total of 465 MSSA from infection and colonization, collected over a 19-year period (1992-2011) in the northern, central and southern regions of Portugal were analyzed. Isolates were characterized by spa typing and multilocus-sequence typing (MLST). Isolates with predominant spa types were characterized by pulsed-field gel electrophoresis (PFGE). Isolates relatedness was analyzed by eBURST and BURP. The 172 spa types found among the 465 MSSA were grouped into 18 spa-CC (clonal complexes). Ten clonal types were more prevalent (40 %): one major clone (ST30-t012) was present in the entire study period and all over the country and the other nine were intermittently detected over time (ST5-t002, ST8-t008, ST15-t084, ST34-t166, ST72-t148, ST1-t127, ST7-t091, ST398-t571 and ST34-t136). Interestingly, three MSSA clonal types observed only after 1996 were closely related with CA-MRSA epidemic strains (ST8-t008, ST72-t148 and ST1-t127) found currently in Portugal. The MSSA population in Portugal is genetically diverse; however, some dominant clonal types have been established and widely disseminated for almost two decades. We identified MSSA isolates that were related with emergent CA-MRSA clones found in Portugal.