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The FCT regions of Streptococcus pyogenes strains encode a variety of cell wall-anchored surface proteins that bind the extracellular matrix of the human host and/or give rise to pilus-like appendages. Strong linkage is evident between transcription-regulatory loci positioned within the FCT and emm regions and the emm pattern genotype marker for preferred infection of the throat or skin. These findings provide a basis for the hypothesis that FCT region gene products contribute to tissue-specific infection. In an initial series of steps to address this possibility, the FCT regions of 13 strains underwent comparative sequence analysis, the gene content of the FCT region was characterized for 113 strains via PCR, and genetic linkage was assessed. A history of extensive recombination within FCT regions was evident. The emm pattern D-defined skin specialist strains were highly homogenous in their FCT region gene contents, whereas the emm pattern A-C-defined throat specialist strains displayed a greater variety of forms. Most pattern A-C strains harbored prtF1 (75%) but lacked cpa (75%). In contrast, the majority of emm pattern D strains had cpa (92%) but lacked prtF1 (79%). Models based on FCT and emm region genotypes revealed the most parsimonious pathways of evolution. Using niche-determining candidate genes to infer phylogeny, emm pattern E strains-the so-called generalists, which lack a strong tissue site preference-occupied a transition zone separating most throat specialists from skin specialists. Overall, population genetic analysis supports the possibility that the FCT region gives rise to surface proteins that are largely necessary, but not always sufficient, to confer tissue site preference for infection.

FCT region genes of Streptococcus pyogenes encode surface proteins that include fibronectin-and collagenbinding proteins and the serological markers known as T antigens, some of which give rise to pilus-like appendages. It remains to be established whether FCT region surface proteins contribute to virulence by in vivo models of infection. In this study, a highly sensitive and ecologically relevant humanized mouse model was used to measure superficial skin infection. Three genes encoding FCT region surface proteins essential for Tserotype specificity were inactivated. Both the ⌬cpa and ⌬prtF2 mutants were highly attenuated for virulence when topically applied to the skin following exponential growth but were fully virulent when delivered in stationary phase. In contrast, the ⌬fctA mutant was virulent at the skin, regardless of its initial growth state. Immunoblots of cell extracts revealed anti-FctA-reactive, ladder-like polymers characteristic of streptococcal pili. In addition, FctA formed a heteropolymer with the putative collagen-binding protein Cpa. The ⌬fctA mutant showed a loss in anti-Cpa-reactive polymers, whereas anti-FctA-reactive polymers were reduced in the ⌬cpa mutant. The findings suggest that both FctA and Cpa are required for pilus formation, but importantly, an intact pilus is not essential for Cpa-mediated virulence. Although it is an integral part of the T-antigen complex, the fibronectin-binding protein PrtF2 is not covalently linked to the FctA-and Cpa-containing heteropolymer derived from cell extracts. The data provide direct evidence that streptococcal T antigens function as virulence factors in vivo, but they also reveal that a pilus-like structure is not essential for the most common form of streptococcal skin disease.Pili can function to mediate bacterial adherence to host surfaces and facilitate horizontal gene transfer (14,31,43). Although well studied among many gram-negative organisms, pilus-like surface structures are becoming increasingly recognized as a component of gram-positive bacteria (26,42,45). Included among pilus-bearing bacteria are at least three species of Streptococcus that are important human pathogens (3,23,24,28).Genes encoding pilus-associated proteins map to the FCT region of Streptococcus pyogenes (28), a highly diversified portion of the genome that contains several well-studied genes (cpa, prtF1, and prtF2) encoding microbial surface cell recognition adhesion matrix molecules (MSCRAMMs) (5,12,21,32). The gene encoding the T6 antigen, which is an antigenic target of a major serological typing scheme, maps to the FCT regions of some isolates, where it forms part of the pilus structure (5, 28, 34). The T6 protein and other FCT region gene products are anchored to the peptidoglycan cell wall via specialized sortases, whose genes also lie within the FCT region (1, 2). Some isolates of S. pneumoniae have a genomic region, called a pathogenicity islet, that is similar in its general layout to the FCT region of S. pyogenes (16,40); at least two of these pneumococcal ...

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