Lactobacillus-mediated inhibition of clinical toxic shock syndrome Staphylococcus aureus strains and its relation to acid and peroxide production

The inhibitory activities of 39 strains representing 20 different species of Lactobacillus toward a menstrual toxic shock syndrome (TSS) Staphylococcus aureus archetype strain MN8 were investigated. Nearly every strain (38 of 39) produced an inhibitory effect under both aerobic and anaerobic conditions when assayed on agar medium. In addition, the MN8 inhibition was conserved against at least 10 other clinical TSS S. aureus isolates and, interestingly, required actively growing cultures of Lactobacillus (verified with a two-well co-culture system in broth medium). This general uniform inhibition could be ameliorated by organic buffer (PIPES) supplied in the growth medium and, with only one exception, MRS medium adjusted with non-organic acid (HCl) failed to support growth of TSS strains at or below pH 5.5. By comparison, the vast majority of lactobacilli in this study decreased culture pH to a range of 4–5. Hydrogen peroxide production by the lactobacilli was also assessed and verified by two different methodologies revealing a broad spectrum of phenotypes that, contrary to reports touting its effectiveness, did not seem to correspond with our inhibition studies. Furthermore, resistances to peroxide by MN8, other TSS strains, and a subset of lactobacilli used in this study were nearly identical whereas the S. aureus collection was slightly more sensitive to racemic lactic acid than the lactobacilli. Collectively, these data suggest that the underlying inhibition toward Staphylococcus is generally conserved in Lactobacillus sp. and is related to a common factor in this genus involving promotion of acidic conditions.