Objective: Antimicrobial peptides (AMPs) are front-line defenders of the innate immune system. As a consequence several bacterial pathogens have developed methods to subvert AMP activity. Streptococcus dysgalactiae subsp equisimilis (SDSE) possesses a gene, drsG, which encodes a protein with moderate homology to SIC and DRS in Streptococcus pyogenes. These proteins bind and disrupt the activity of several AMPs. The aim of the current study was to characterise the biological function of DrsG.
Methods: PCR, Southern hybridisation and bioinformatic analyses of draft SDSE genomes was used to examine the distribution of drsG in clinical SDSE isolates. Antibodies raised to recombinant DrsG were used to characterise native expression of the protein. Indirect ELISAs were subsequently used to examine the binding of DrsG to several AMPs, including LL-37, hBD-1 and hBD-2.
Results: Fourteen different SDSE emm-types were found to possess isolates harbouring drsG. However, not all isolates within these emm-types possessed DrsG, suggesting the gene may be part of a larger mobile genetic element. Alignment of translated drsG sequences also revealed DrsG to be highly conserved. In general, two variants of DrsG are present. The larger DrsGL possesses two separate repeat domains, the first of which contains of two repeat units. The smaller DrsGS lacks the first of the repeat domains. Analysis of culture supernatants demonstrated that both DrsGL and DrsGS are expressed and secreted into the culture supernatant. Indirect ELISA subsequently demonstrated that recombinant DrsG bound to LL-37, hBD-1, hBD-2, but not hBD-4. The addition of DrsG to bacterial culture also resulted in increased growth of SDSE in the presence of LL-37.
Conclusions: We have shown that DrsG binds to a number of AMPs, and may play an important role in protecting SDSE from the innate immune system.