Figure 6.

Hypothesis to explain the emergence of a new, unusually virulent subclone of serotype M3 GAS. Under this hypothesis, all isolates shown are multilocus enzyme electrophoretic type 2 (8). Relatively early in the 20th century, an ancestral strain acquired phage Φ315.2, a T12-like phage that encodes SSA (13). Subsequently, a single nucleotide mutation resulting in a single amino acid replacement (Val106Ile) produced the SpeA3 allele (16) encoded by phage Φ315.5. This ssa-positive, speA3-positive strain gained phage Φ315.4 that encodes Sla and SpeK, and disseminated widely in the mid/late 1980s. The hypothesis is based on data presented in this paper and published previously (8, 13, 14, 16, 28).