Spontaneous mutations in the CsrRS two-component regulatory system of Streptococcus pyogenes result in enhanced virulence in a murine model of skin and soft tissue infection

CsrS/CsrR is a 2-component system in Streptococcus pyogenes that negatively regulates hyaluronic capsule and several exotoxins. To detect spontaneous mutations in csrRS, mucoid and large colony variants of M1 strain MGAS166 w ere isolated from experimental murine skin infections. By use of complement ation with a csrRS(+) plasmid, relevant mutations were also detected in 7 o f 12 human clinical isolates. The presence of spontaneous mutants in mouse infection was associated with larger, more necrotic lesions. Most spontaneo us changes in CsrR resulted from single amino acid substitutions, whereas m ost csrS mutations were frameshift or nonsense mutations. In 2 instances, I S1548 insertions were found in csrS. Experimental inoculation of mixtures o f wild-type (wt) and csrRS 2 bacteria yielded larger, more necrotic lesions than did either strain at twice the inoculum, which suggests that these va riants may exhibit pathogenic synergy. Spontaneous emergence of csrRS 2 mut ants in vivo enhances the virulence of wt bacteria and increases severity o f murine skin infection.