In the vagina and endocervix, Neisseria gonorrhoeae must interact with complex microflora. Among these are lactobacilli, which may inhibit the growth of gonococci. Lactobacillus acidophilus, which produce H2O2 (LB+), and L. acidophilus and Lactobacillus casei, which do not produce H2O2 (LB-), were coincubated with catalase-positive and -deficient strains of N. gonorrhoeae. When the incubation medium was maintained at pH 7.3, neither LB+ nor LB- affected gonococcal growth. However, LB+ caused a significant increase in expression of gonococcal catalase, which could be offset by exposure of the bacteria to exogenous catalase. When coincubation medium was at lower pH (4.8-5.0), there was a significant decrease in gonococcal survival and catalase activity, which was only partly reversed by exogenous catalase. Lysates of LB+ also effectively inhibited gonococcal catalase. This inhibition was retained upon heating of the lysate to 100 degrees C for 15 min but was lost with proteinase K treatment. Thus, LB+ may inhibit growth of gonococci by acidification of the environment, secretion of H2O2, and production of protein inhibitors.