Lactobacilli reduce cell cytotoxicity caused by Streptococcus pyogenes by producing lactic acid that degrades the toxic component lipoteichoic acid.

Lactobacilli are known to prevent colonization by many pathogens; nevertheless, the mechanisms of their protective effect are largely unknown. In this work, we investigated the role of lactobacilli during infection of epithelial cells with group A streptococci (GAS). GAS cause a variety of illnesses ranging from noninvasive disease to more severe invasive infections, such as necrotizing fasciitis and toxic shock-like syndrome. Invasion of deeper tissues is facilitated by GAS-induced apoptosis and cell death. We found that lactobacilli inhibit GAS-induced host cell cytotoxicity and shedding of the complement regulator CD46. Further, survival assays demonstrated that lactic acid secreted by lactobacilli is highly bactericidal toward GAS. In addition, lactic acid treatment of GAS, but not heat killing, prior to infection abolishes the cytotoxic effects against human cells. Since lipoteichoic acid (LTA) of GAS is heat resistant and cytotoxic, we explored the effects of lactic acid on LTA. By applying such an approach, we demonstrate that lactic acid reduces epithelial cell damage caused by GAS by degrading both secreted and cell-bound LTA. Taken together, our experiments reveal a mechanism by which lactobacilli prevent pathogen-induced host cell damage.