Circumcision not only reduces the risk of HIV transmission in men, but also drastically changes the bacteria on the penis, reducing the risk of HIV and bacterial infection transmission to female partners, suggests a study published in PloS ONE and reported on by United Press International. The study was part of a larger initiative by the National Institutes of Health to examine the “human microbiome,” or microbes that exist on and within the human body.

“Our randomized trials have shown that male circumcision prevents HIV infection in men and protects their female partners from vaginal infections, especially bacterial vaginosis,” said senior author Ronald H. Gray, MD, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

After circumcision, the glans of the penis is exposed to more air and, specifically, oxygen, according to the article. This is important, Gray explained, because “[anaerobic] bacteria, which cannot grow in the presence of oxygen, have been implicated in inflammation and a number of infections affecting both men and women.”