Use of an Animal Model of Group A Streptococcal Infection to Identify Factors Important for Virulence

The Group A Streptococcus (GAS) is a strict human pathogen responsible for a broad assortment of diseases ranging from pharyngitis (strep throat) and impetigo to necrotizing fasciitis (flesh eating disease) and streptococcal toxic shock syndrome (STSS). Virulence of the GAS is multifactorial, as it possesses an array of virulence factors regulated in a coordinated fashion by a complement of transcriptional regulators. A mouse model of streptococcal invasive skin infection was used to identify unknown factors important in GAS virulence. The first section of this study used this model to assess the effect of a mutation in the response regulator gene spt10R on virulence in vivo. The spt10R mutation was subsequently discovered to have a polar effect on the downstream ß-galactosidase BgaA. Disruption of bgaA was shown to lead to an attenuation of virulence in the mouse model as well as a reduction in the production of the cysteine protease SpeB. Complementation of the spt10R/bgaA double mutant with bgaA restored the expression of speB to wild type levels. A microarray analysis of the spt10R/bgaA mutant revealed significant transcriptional changes in genes involved in virulence and carbohydrate metabolism. In addition, spt10SR and bgaA were found to be part of a four-gene operon that is repressed by the CovR virulence regulator. A second avenue of study comparing various sequenced strains in the invasive skin infection model revealed a hypervirulent M3 strain MGAS315. This hypervirulence phenotype was lost upon in vitro passage similar to the passages used during directed mutagenesis of the strain. There was no remarkable phenotypic difference between MGAS315 and its passage-attenuated derivative in vitro. However, transcriptome and proteome analysis at mid- and late-logarithmic phases of growth revealed potential contributors to the hypervirulence phenotype. Upon passage through mice, the passage-attenuated strain was able to revert towards the high virulence phenotype. Overall, an animal model of streptococcal invasive skin infection was useful in the study and identification of factors important for virulence and may provide insight into the interaction of the GAS with its host.