Lactobacilli Reduce Cell Cytotoxicity Caused by Streptococcus pyogenes by Producing Lactic Acid That Degrades the Toxic Component Lipoteichoic Acid

Abstract

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.Streptococcus pyogenes (group A streptococci [GAS]) is the cause of many important human diseases ranging from mild superficial skin infections to life-threatening systemic diseases. The pathogen is a common colonizer of the mucosal layers in the mouth, nose, and pharynx. Colonization by GAS is often transient and asymptomatic; however, dissemination by local infection occasionally develops into severe systemic disease, such as streptococcal toxic shock syndrome (8, 24). A pathogenicity feature of GAS is the ability to induce cytotoxicity in human cells (33, 37), which facilitates bacterial entry into deeper tissues of the body (2, 9). Invasion of host cells (17,26,53), as well as many virulence factors of GAS, such as the surface component lipoteichoic acid (LTA) (12, 19-22, 49, 51) and the toxins streptolysin O and streptolysin S (11,29,45), is reported to induce host cell damage. In addition, pathogeninduced apoptosis and necrosis lead to shedding of the cell surface complement regulator CD46 (14, 25). Shed CD46 binds to the surface of GAS and increases survival of the bacteria in the blood (23, 27).The normal microbiota is an important first line of defense against invading pathogens. The Gram-positive lactobacilli colonize various parts of the body and are common inhabitants of the mucosal membranes in the oral tract (1, 3, 39). Lactobacilli are known to protect against colonization by many pathogens (44) and have also been reported to prevent cytotoxicity induced by bacterial pathogens (4,5,15,31). In addition, in a recent study, Lactobacillus rhamnosus GG, a commonly used probiotic strain, was reported to inhibit GAS invasion of host cells (36). Lactobacilli have further been reported to inhibit streptococcal growth (15,18,46,47). However, the mechanisms of t...

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