Inhibition of Staphylococcus aureus by H sub(2)O sub(2)-producing Lactobacillus gasseri isolated from the vaginal tract of cattle

Infectious diseases are the major reproductive complication during postpartum. The bacteria that colonize the postpartum uterus of cattle are specific as well as opportunistic pathogens that include Staphylococcus aureus. The antibiotics and antiseptic agents used in the treatment of postpartum acute infections induce residues in foods, spread of bacterial resistance, increase in financial costs and failures in defense mechanisms of the host. Preventive treatment with probiotic products could decrease the use of antibiotics in dairy farming systems. Lactobacilli are present in the vaginal microflora of healthy cows. They can prevent pathogen colonization by mechanisms such as the production of antagonistic substances as lactic acid, H sub(2)O sub(2,) or bacteriocins. The aim of the present study was the selection of H sub(2)O sub(2) generating lactobacilli from a group of 72 strains isolated from the vagina of cattle. Both Lactobacillus gasseriCRL1421 and Lactobacillus gasseri CRL1412, which share some probiotic properties, produce H sub(2)O sub(2), detected by the plate colorimetric method. They were chosen to study the kinetics of H sub(2)O sub(2) production under different culture conditions. Both microorganisms produced greater amounts of H sub(2)O sub(2) in aerated than in static cultures. As L. gasseri CRL1421 had a greater capacity to generate H sub(2)O sub(2), associative cultures with this strain and S. aureus were conducted. A significant decrease in the growth of the pathogen was detected after culture for 6 h, this effect being greater under aerated conditions. The addition of catalase to mixed cultures partially abolished the inhibition, an effect that could be attributed to the combined action of H sub(2)O sub(2) and other antagonistic metabolites. The simultaneous addition of catalase and NaOH to these cultures restored S. aureus growth. This observation suggests that the inhibition was produced by the combination of H sub(2)O sub(2) and lactic acid, both released by the lactobacilli. Electron microphotographs showed the damage caused by the lactobacilli supernatant on the pathogenic cells. The treatment of S. aureus with lactic acid and hydrogen peroxide evidenced that each metabolite produced a different type of morphological damage. The number of viable cells obtained agrees with the electron microscopy observations. The results support the idea that L. gasseri CRL1421 could be successfully included in a probiotic product to prevent S. au