Recent studies in STIs have drawn attention to the impact of the state of the microbiome of the female genital tract (FGT) and susceptibility to STIs – and, in particular, the protective effect of the hydrogen peroxide-releasing microbe Lactobacillus crispatus (L.c.) (Antonio & Hillier/STIs) (A&H). Such STIs include, not only BV (53% less likely with L.c., according to A&H), but apparently also HIV, gonorrhea and HSV-2, which show, according to a Uganda based study (Francis & Grosskurth/STIs) an association of BV estimated at RR 2.35, 1.3, 1.69, respectively. A recent study of 236 young women participating in the S. African FRESH cohort found that, of the 24 of these having a FGT microbiome dominated by L.c., none figured among the 31 HIV seroconversions (STI blogs: Susceptibility of sub-Saharan women to HIV). Haggerty & Ness/STIs (http://sti.bmj.com/content/92/6/441?sid=674668c5-6489-4db7-8da7-dc541da48ec8), in the PID Evaluation and Clinical Health Study, find a strong (RR 4.7) association of certain FGT microbiotic bacteria with PID. Finally, a mouse based study by Gilbert & Lewis (STI/blogs: Bacterial vaginosis associated bacterium) seems to confirm the hypothesis, proposed in 2001 by Shahmanesh/STIs (http://sti.bmj.com/content/77/2/139), that Gardnerella may indirectly cause NGU (non-gonococcal urethritis) by triggering the emergence of covert Escheridia coli.

But if the importance now seems established of the composition of the woman’s FGT microbiome for the susceptibility to a whole range of STI (BV, HIV, Ng, HSV-2, PID, NGU), what are we to think of the finding of a recent paper in mBio (the journal of the American Society for Microbiology) that bacteria in the penile microbiome play a comparable role in the case of men – at least, in respect to HIV?

Liu & Price  http://mbio.asm.org/content/8/4/e00996-17.abstract?related-urls=yes&legid=mbio;8/4/e00996-17) uses data from 182 uncircumcised men from Rakai, Uganda, 46 of whom go on, over a two-year period, to develop HIV. In the case of five genera of bacteria suspected of playing a role in the association between male circumcision and reduced HIV risk, a strong association was discovered between their baseline prevalence in the penile microbiome and the risk of infection with HIV. With each 10-fold increase in the abundance of Prevotella, and Diliaster, for example, an increased risk was noted (AOR: 1.63 and 1.57, respectively). Other genera associated with increased risk were Peptoniphilus, Finegoldia, Porphyromonas, Mobiluncus, Peptostreptococcus, and Murdochiella. HIV risk was also found to be associated with inflammatory markers – especially interleukin-8. Presence of the latter increased significantly with densities of anaerobic bacteria. The greatest risk was observed where three are more cytokines were detected. These findings lead the researchers to conclude that the response of the immune system to shifts in the penile microbiome facilitate infection by HIV.

Of course, this is precisely the mechanism that may account for the increased susceptibility to STI (including HIV) in women with FGT deficient in L.c.. However, the problem can be largely resolved in the case of men by means of circumcision.

The authors also make the interesting point that the bacterial dysbiosis that they observe in the male microbiome may be passed on to women through sexual activity – so that the genital microbiome is, to some extent, ‘shared’ between them. Presumably, this could also mean that voluntary male medical circumcision reduces this risk of HIV transmission for women as well as men to the extent that women are at less risk of infection with bacteria that render them susceptible to HIV.