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Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has become a matter of concern worldwide, in particular in the USA. For the analysis of emergence and spread, clear definitions based on epidemiological origin are needed for discrimination between CA-MRSA, healthcare-associated community MRSA, and healthcare-associated MRSA (HA-MRSA). Although its role in pathogenesis is currently under debate, the capability for Panton-Valentine leukocidin formation is associated with the majority of CA-MRSA isolates from North America and from Europe. Most CA-MRSA isolates are attributed to clonal lineages different from HA-MRSA; there are, however, clonal lineages from which both HA-MRSA and CA-MRSA have been reported (e.g. ST1, ST5, ST8, and ST22); CA-MRSA ST8 (USA300), which is most frequent in the USA, has meanwhile been reported from Europe. CA-MRSA ST80 is widely disseminated in Europe; because of its pronounced oxacillin heteroresistance phenotype, cefoxitin-based assays are advisable for reliable detection. So far, CA-MRSA infections seem to be much less frequent in Europe than in the USA, where patients with particular predispositions and low social status are at especial risk.

The molecular population genetics and pathogenic potential of North American and European invasive strains of Streptococcus pyogenes were assessed. Isolates from recent invasive infections and from infections in the 1920s and 1930s were characterized for multilocus enzyme genotype and allelic variation in the gene (speA) that encodes streptococcal pyrogenic exotoxin (SPE) A (scarlet fever toxin). A subset of strains was studied for allelic variation in genes that encode SPE B and streptokinase. All contemporary strains assigned to electrophoretic types (ETs) 1 and 2 that synthesize SPE A have the speA2 and speA3 allelic variants, respectively, and their relative virulence in two mouse models is similar to that of strains of the same ET and M protein types recovered earlier. In contrast, ET 1 and 2 isolates from disease episodes in the 1920s and 1930s contain the speA1 allele. The data suggest there may be temporal and geographic variation in the occurrence of clone--virulence factor allele combinations, an observation that may in part explain fluctuations in disease frequency, severity, and character.