By Monte Morin, Los Angeles Times

April 15, 2013, 11:16 p.m.

Researchers say the foreskin can shelter troublesome bacteria, so its removal may bolster the immune system to keep the AIDS virus at bay.

Circumcision is known to reduce a man’s risk of HIV infection by at least half, but scientists don’t know why. A new study offers support for the theory that removing the foreskin deprives troublesome bacteria of a place to live, leaving the immune system in much better shape to keep the human immunodeficiency virus at bay.

A study of the bacteria living on the skin of the penis could point scientists toward a way of offering the HIV-preventing benefits of circumcision without having to do the procedure, researchers say. (Sean Gallup/Getty Images / April 16, 2013)

Anyone who has ever lifted a rock and watched as the earth beneath it was quickly vacated by legions of bugs and tiny worms would be familiar with the principle, said study leader Dr. Cindy Liu: After the foreskin is cut away, the masses of genital bacteria that once existed beneath it end up disappearing.

“It’s the same as if you clear-cut a forest,” said Liu, a pathologist at the Translational Genomics Research Institute in Flagstaff, Ariz. “The community of animals that once lived in that forest is going to change.”

Of particular note is that circumcision undercuts anaerobic bacteria, the microbes that thrive in oxygen-deprived environments, she said. By reducing the number of anaerobic bacteria, the body’s immune cells may be better able to destroy the virus — and less likely to fall prey to its Trojan horse-style of attack, the authors suggest.

Liu and her colleagues present their case in a paper published Tuesday in the journal mBio.

Numerous studies conducted over the last two decades have shown that male circumcision reduces the risk of HIV infection in men who have heterosexual intercourse by 50% to 60%. Some researchers have speculated that the foreskin is prone to tearing, giving the virus more routes of entry. Others have argued that removal of the foreskin simply reduces the surface area available to be infected.

Liu and coauthor Lance Price, a professor of environmental health sciences at George Washington University in Washington, suspected it had to do with the bacterial species that inhabit the coronal sulcus, the shallow groove behind the head, or glans, of the penis.

To establish a possible connection, study authors enrolled 156 Ugandan men in a randomized trial in which half of them were circumcised and the other half were not.

Study participants ranged in age from 15 to 49. While the prospect of undergoing circumcision as an adult might not appeal to many American men, 5,000 Ugandan males volunteered for the study. In a region where 1 in 6 people are infected with HIV, circumcision’s “powerful potential” to reduce the risk of infection was strong motivation, said coauthor Dr. Aaron Tobian, a health epidemiologist and pathologist who teaches at Johns Hopkins University School of Medicine.

On average, there was an 81% reduction in bacteria in the circumcised men one year after surgery, the researchers reported. Some of the biggest drops were recorded for anaerobic bacteria, they said.

Bacteria on the coronal sulcus fell by more than 33% in the circumcised men, Liu said.

Interestingly, the men in the uncircumcised control group also experienced a reduction in bacteria, but not to the degree that the circumcised men did, Liu said. This was probably because of health and hygiene information that was given to all study participants.

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