Effects of the spermicidal agent nonoxynol-9 on vaginal microbial flora

The use of nonoxynol-9--containing vaginal contraceptive preparations increases vaginal (and urethral) colonization by Escherichia coli. Nonoxynol-9 is toxic to various microorganisms, including Lactobacillus acidophilus, but has little or no direct effect on E. coli. L. acidophilus, which is present in the vaginas of most normal women, generates H2O2 which, when combined with peroxidase and a halide, was toxic to E. coli. This toxicity was inhibited by nonoxynol-9 due to the selective destruction of the lactobacilli. In contrast, at higher concentrations, nonoxynol-9 was toxic to E. coli when combined with peroxidase and a halide. This toxicity was shared with certain other nonionic detergents and was due to the formation of peroxides in the preparations on prolonged exposure to oxygen. E. coli colonization may, in part, reflect the balance between these opposing effects of nonoxynol-9 on the vaginal antimicrobial system. Further, damage to normal tissues by peroxides in nonoxynol-9 preparations needs to be considered.