| Summary: | <p>Abstract</p> <p>Background</p> <p>A few lineages of Group A streptococci (GAS) have been associated with a reemergence of severe invasive streptococcal disease in developed countries. However, the majority of the comparisons between invasive and non-invasive GAS isolates have been performed for collections of reduced genetic diversity or relied on limited typing information to distinguish clones. We characterized by several typing methods and compared a collection of 160 isolates recovered from normally sterile sites with 320 isolates associated with pharyngitis and recovered in the same time period in Portugal.</p> <p>Results</p> <p>Although most of the isolates belonged to clones that were equally prevalent in invasive infections and pharyngitis, we identified markers of invasiveness, namely the <it>emm</it> types 1 and 64, and the presence of the <it>speA</it> and <it>speJ</it> genes. In contrast, <it>emm</it>4, <it>emm</it>75, and the <it>ssa</it> and <it>speL/M</it> genes were significantly associated with pharyngitis. There was a strong agreement between the <it>emm</it> type, the superantigen (SAg) genes and the clusters defined by pulsed-field gel electrophoresis (PFGE) profiling. Therefore, combinations of particular <it>emm</it> types and SAg genes frequently co-occurred in the same PFGE cluster, but there was no synergistic or antagonistic interaction between them in determining invasiveness. Only macrolide-susceptible PFGE clones were significantly associated with invasive infections or pharyngitis, while the clones of resistant isolates sharing all other molecular properties analyzed were equally prevalent in the two groups of isolates.</p> <p>Conclusions</p> <p>This study confirmed the importance of the widely disseminated <it>emm</it>1-T1-ST28 clone in invasive infections but also identified other clones linked to either invasive infections (<it>emm</it>64-ST164) or pharyngitis (<it>emm</it>4-T4-ST39), which may be more limited in their temporal and geographical spread. Clonal properties like some <it>emm</it> types or SAg genes were associated with disease presentation, highlighting the importance of bacterial genetic factors to the outcome of GAS infections, although other, yet unidentified factors may also play an important role.</p>
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