Protein FOG is a moderate inducer of MIG/CXCL9, and group G streptococci are more tolerant than group A streptococci to this chemokine's antibacterial effect.

Streptococcus dysgalactiae subsp. equisimilis (group G streptococci; GGS) cause disease in humans but are often regarded as commensals in comparison with Streptococcus pyogenes (group A streptococci; GAS). The current study investigated the degree and kinetics of the innate immune response elicited by the two species. This was assessed as expression of the chemokine MIG/CXCL9 and bacterial susceptibility to its bactericidal effect. No significant difference in MIG/CXCL9 expression from THP-1 or Detroit 562 cells was observed when comparing whole GGS or GAS as stimuli. The study demonstrates that protein FOG was released from the bacterial surface directly and by neutrophil elastase. Expression of MIG/CXCL9 following stimulation with soluble M proteins of the two species (the recently described protein FOG of GGS and protein M1 of GAS) was reduced for protein FOG in both the monocytic and the epithelial cell line. When the antibacterial effects of MIG/CXCL9 were examined in conditions of increased ionic strength, MIG/CXCL9 killed GAS more efficiently than GGS. Also in the absence of MIG/CXCL9, GGS were more tolerant to increased salt concentrations than GAS. In summary, both GGS and GAS evoke MIG/CXCL9 expression but they differ in susceptibility to its antibacterial effects. This may in part explain the success of GGS as a commensal and its potential as a pathogen.