HIV inhibition by lactobacilli: easier in a test tube than in real life.

A lactobacillus-dominant vaginal microbiota has been shown to decrease heterosexual HIV transmission. Nunn et al. now report that a vaginal microbiota dominated by Lactobacillus crispatus is associated with a relative inability of HIV pseudoviral particles to transverse cervicovaginal mucus (CVM) in vitro [mBio 6(5):e01084-15, 2015, doi:10.1128/mBio.01084-15]. The purported inhibitory mechanism is the interaction between carboxyl groups present on HIV and in CVM that occurred only under acidic conditions when carboxyl groups were protonated. L. crispatus produces high levels of lactic acid and results in the lowest vaginal pH when it is the dominant vaginal bacterium. In addition, high levels of lactic acid inhibit the proliferation of other bacteria that might negatively affect CVM structure. The utility of enhancing L. crispatus dominance to inhibit HIV transmission awaits assessment of the influence of ejaculated semen on this property and investigations on the role of Lactobacillus products such as d-lactic acid in this property.