Bacteriophages are viruses that infect bacteria. Their attachment is a two-step mechanism that involves reversible and irreversible binding. Phage A25 is known to reversibly bind the bacterial cell wall, yet the exact component (peptidoglycan, Group A Carbohydrate (GAC)) as well as the second receptor remain to be identified. Moreover, in vivo accessibility of this receptor has never been investigated. Group A Streptococcus (GAS) uses a plethora of surface proteins to bind human serum proteins which conceal the bacterial surface. In this work, we characterized the effect of serum on phage A25 infection and undertook the identification of its bacterial receptor.
We found that pooled human serum protects GAS against phage infection. Sera from immune donors were more protective, suggesting that specific antibodies could either target the receptor or amplify its masking by gathering serum proteins. Interestingly, we observed a limited protection with heat-treated serum, concomitant with weak binding of high molecular weight proteins, like fibronectin, to the bacterial surface.
Besides, we generated deletion mutant of genes coding for surface antigens and found that GlcNAc and glycerol phosphate of the GAC are not involved in phage infection. We also showed that, although the M protein is not the phage receptor, it contributes to the protection offered by the human serum. Finally, we also identified spontaneously phage-resistant mutants that were mainly revertant of a 1bp-frameshift which prevented capsule expression.
Understanding the interaction between GAS and its phages will enable to delve deeper into their key role in the evolution of GAS virulence.