A series of studies conducted in sub-Saharan Africa have demonstrated a public health effect: circumcised men are less vulnerable to HIV infection than their uncircumcised counterparts. Why this is the case has been unclear, but a new study out of Johns Hopkins University suggests a possible reason: circumcision creates a hostile environment for bacteria, and that, in turn, inhibits HIV.
Several randomized-control studies in sub-Saharan Africa have demonstrated a correlation between circumcision and reduced HIV-risk; the largest of these, conducted in Uganda, was led by Johns Hopkins epidemiologist Ronald Gray, who is lead author on the current study. While conducting their research, Gray's team took penile swabs from all of their subjects, yielding data that are the basis of the new study. In examining the results of those swabs, the researchers found substantial differences between circumcised and uncircumcised penises.
Circumcision, or removal of the penile foreskin, makes substantial changes to the penis—and the most important of these are not cosmetic. In fact, Gray's team found that circumcised penises had much less anaerobic (non-oxygen processing) bacteria and more aerobic (oxygen-processing) bacteria, presumably because removal of the foreskin allows more oxygen to reach the head of the penis. Circumcised penises in fact had dramatically more aerobic bacteria, a fact that may well be connected to a degree of HIV prevention. The question is—how?
That's the tricky part. Gray et al cite several existing explanations for why circumcision may help in HIV prevention. Circumcision limits the exposure of delicate mucous membranes to HIV; circumcised penises thicken through the inner foreskin, giving the immune cells in the penis additional protection; or changing the penis by keeping it dryer and better oxygenated creates a more virus-hostile environment. But Gray and his team are honing in on the bacteria. "These bacteria," he says, "which cannot grow in the presence of oxygen, have been implicated in inflammation and a number of infections affecting both men and women. Our randomized trials have shown that male circumcision prevents HIV infection in men and protects their female partners from vaginal infections, especially bacterial vaginosis. It is possible that the virtual elimination of anaerobic bacteria by circumcision contributes to these benefits of the procedure."
Up next for this research team: determining whether particular bacteria can be identified with elevated HIV risk, and how they can be eliminated. This suggests the possibility of non-surgical ways of replicating the effect of circumcision. In the near term, Dr. Cindy Liu, a co-lead author on the current study, suggests that it's not enough to just cut and run. "Eliminating harmful bacteria may be only half of the needed action," she said. "Ensuring that the niche left by pre-circumcision anaerobic bacteria are filled with 'good' bacteria will also be critical."