Uncircumcised men harbor more bacteria around the head of the penis than do circumcised men, and the mix of microbial species is decidedly different in the two groups, researchers find.
Whether these changes in microbial numbers and diversity explain why circumcised men are less likely to get infected with HIV remains unclear. But the findings identify previously unknown differences that exist between a warm, moist environment under the foreskin and the comparatively dry surfaces found on circumcised men.
“It’s a very promising line of research,” says Robert Bailey, a biologist and epidemiologist at the University of Illinois at Chicago, who wasn’t part of the study team. “But it’s far from really providing the story on the mechanism for the increased risk of HIV infection through the foreskin.”
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The report, which appears April 16 in mBio, finds that when the foreskin is removed from the head of the penis, resident microbes become exposed to oxygen and many bacteria flee the scene.
To measure these changes, the researchers enlisted 156 uncircumcised, married men in Uganda and obtained swabs from under each man’s foreskin. Roughly half of the men were then randomly assigned to get circumcised. A year later, researchers again took swabs.
While there was little difference in the penis biota in the men at the beginning of the study, the later samples revealed substantially less bacteria in the circumcised group and changes in the diversity of the bacteria that remained. Levels of nine kinds of anaerobic bacteria, which need an environment devoid of oxygen, had decreased.
Circumcision provides heterosexual men with considerable, but not total, protection against infection by HIV and other sexually transmitted viruses (SN: 4/25/2009, p. 10). Does changing the microbiota of the penis account for this protection? “We can’t say that our study answers that question, but we’re definitely chipping away at it,” says microbiologist Lance Price of George Washington University in Washington and the Translational Genomics Research Institute in Flagstaff, Ariz.
Price and his coauthors suggest that high amounts of bacteria and the presence of poorly understood anaerobic microbes in uncircumcised men might contribute to inflammation, which would facilitate infection by HIV lodged in the foreskin.
Another alternative is that having additional bacteria present might contribute to ulcerations, which also promote viral entry, suggests Bailey.
One thing is clear, Price says. Circumcision changes the microbial ecology of the region. He offers a macroscale analogy: “If you cut down a lot of trees, you would expect the animals who live there to change,” he says.