Background: Serotype M28 group A Streptococcus (GAS) isolates are non-randomly associated with life-threatening puerperal infections. All tested M28 GAS isolates harbor a 36.3-kb mobile genetic element termed the region of difference 2 (RD2). We are interested in assessing (i) the role of RD2 in the association of M28 isolates with puerperal infections, and (ii) whether barriers exist that prevent or reduce RD2 acquisition by non-M28 GAS isolates, in which RD2 is rarely present.
Methods: Gain- and loss-of-function studies were performed to assess whether RD2 impacts the disease potential of M28 isolates, with readouts from tissue culture-based adherence assays and a murine vaginal colonization model. To assess barriers to RD2 conjugation, filter-mating assays were used to quantify intra- and inter-serotype conjugation rates.
Results: We identified that RD2, in a strain/serotype-specific manner, enhances GAS adherence to human vaginal epithelial cells and colonization of the murine vagina. Intra-serotype (M28-M28) conjugation assays generated higher numbers of transconjugants than inter-serotype (M28-M1) assays. Use of a series of M1 (naturally make capsule) and M28 (naturally lack capsule) GAS derivatives showed that this phenomenon was not attributable to differences in capsule production. However, capsule did reduce the adherence ability of RD2-containing strains.
Conclusions: The data are consistent with RD2 promoting the association of serotype M28 GAS isolates with puerperal infections, and provide insight into the observed serotype-specific presence of RD2. While capsule production negatively correlated with the adherence ability of RD2-harboring strains, it was not a barrier for conjugation of RD2 between strains in vitro.