Characterization of M-Type-Specific Pilus Expression in Group A Streptococcus

In addition to providing a typing mechanism for group A Streptococcus (GAS) isolates (T typing), cell surface pilus production impacts GAS virulence characteristics, including adherence and immune evasion. The pilus biosynthesis genes are located in the fibronectin- and collagen-binding T-antigen (FCT) region of the genome, and nine different FCT types, encoding more than 20 different T types, have been described. GAS isolates are not uniform in their degree or pattern of pilus expression, as highlighted by pilus production being thermoregulated in isolates that harbor the FCT-type FCT-3 (e.g., M-types M3 and M49) but not in isolates that harbor FCT-2 (e.g., M-type M1). Here, we investigated the molecular basis underlying our previous finding that M3 GAS isolates produce pili in lower abundance than M1 or M49 isolates do. We discovered that, at least in part, the low pilus expression observed for M3 isolates is a consequence of the repression of pilus gene expression by the CovR/CovS two-component regulatory system and of an M3-specific mutation in the nra gene, encoding a positive regulator of pilus gene expression. We also discovered that the orthologous transcriptional regulators RofA and Nra, whose encoding genes are located within FCT-2 and FCT-3, respectively, are not functionally identical. Finally, we sequenced the genome of an M3 isolate that had naturally undergone recombinational replacement of the FCT region, changing the FCT and T types of this strain from FCT-3/T3 to FCT-2/T1. Our study furthers the understanding of strain- and type-specific variation in virulence factor production by an important human pathogen. IMPORTANCE Our ability to characterize how a pathogen infects and causes disease, and consequently our ability to devise approaches to prevent or attenuate such infections, is inhibited by the finding that isolates of a given pathogen often show phenotypic variability, for example, in their ability to adhere to host cells through modulation of cell surface adhesins. Such variability is observed between isolates of group A Streptococcus (GAS), and this study investigates the molecular basis for why some GAS isolates produce pili, cell wall-anchored adhesins, in lower abundance than other isolates do. Given that pili are being considered as potential antigens in formulations of future GAS vaccines, this study may inform vaccine design.

Automated summary

This study investigates why some isolates of group A Streptococcus (GAS) produce pili in lower abundance than others. It was found that the low pilus expression in M3 GAS isolates is a result of repression by the CovR/CovS regulatory system and a specific mutation in the nra gene. The study also reveals that the transcriptional regulators RofA and Nra, located in FCT-2 and FCT-3 respectively, are not functionally identical.