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Background  Vaginal lactobacilli protect the female genital tract by producing lactic acid, bacteriocins, hydrogen peroxide or a local immune response. In bacterial vaginosis, normal lactobacilli are replaced by an anaerobic flora and this may increase susceptibility to  Neisseria gonorrhoeae , a facultative anaerobe. Bacterial interference between vaginal lactobacilli and  N. gonorrhoeae  has not been studied in liquid medium under anaerobic conditions. By co-cultivating  N. gonorrhoeae  in the presence of lactobacilli we sought to identify the relative contributions of acidification and hydrogen peroxide production to any growth inhibition of  N. gonorrhoeae .    Methods  Three strains of  N. gonorrhoeae  distinguishable by auxotyping were grown in the presence of high concentrations (10 7 -10 8  cfu/mL) of three vaginal lactobacilli ( L. crispatus ,  L. gasseri  and  L. jensenii ) in an anerobic liquid medium with and without 2-( N -morpholino)-ethanesulfonic (MES) buffer.  Fusobacterium nucleatum  was used as an indicator of anaerobiosis. Bacterial counts were performed at 15, 20 and 25 h; at 25 h pH and hydrogen peroxide concentrations were measured.    Results  Growth of  F. nucleatum  to >10 8  cfu/mL at 25 h confirmed anaerobiosis. All bacteria grew in the anaerobic liquid medium and the addition of MES buffer had negligible effect on growth.  L. crispatus  and  L. gasseri  produced significant acidification and a corresponding reduction in growth of  N. gonorrhoeae . This inhibition was abrogated by the addition of MES.  L. jensenii  produced less acidification and did not inhibit  N. gonorrhoeae . Hydrogen peroxide was not detected in any experiment.    Conclusions  During anaerobic growth, inhibition of  N. gonorrhoeae  by the vaginal lactobacilli tested was primarily due to acidification and abrogated by the presence of a buffer. There was no evidence of a specific mechanism of inhibition other than acid production under these conditions and, in particular, hydrogen peroxide was not produced. The acidification potential of vaginal lactobacilli under anaerobic conditions may be their most important characteristic conferring protection against  N. gonorrhoeae  infection.

The role of acidification in the inhibition of Neisseria gonorrhoeae by vaginal lactobacilli during anaerobic growth