Circumcision Alters the Penis Microbiome

Diminished bacterial diversity and abundance may help explain why circumcision is associated with reduced HIV infection.

Apr 18, 2013
Kate Yandell

CDC, C. GOLDSMITHA circumcised penis is less hospitable to anaerobic bacteria than an uncircumcised one, research published in mBio earlier this week found. The study suggests that changes in the penis’s microbial composition could explain why circumcision reduces men’s risk of contracting HIV.

Researchers compared 79 men from Rakai, Uganda, who volunteered to be circumcised, with 77 uncircumcised controls from the same community, measuring the microbial diversity of both groups’ penises before the circumcisions and a year after the procedures. (The control group also volunteered to be circumcised but was randomly assigned to undergo the procedure after the study was over.)

While microbial populations on all the men’s penises shifted over time, the men who had been circumcised showed a more pronounced change. Circumcision “selected for bacteria capable of surviving in the aerated circumcised microenvironment,” the researchers wrote in the paper. Prevalence and diversity of 12 taxa of anaerobic bacteria decreased most dramatically.

According to previous research, circumcision reduces men’s HIV risk by 50 to 60 percent, and the authors of the new study argued that this reduction could in part be explained by the decline in penis microbes. They hypothesized that the bacteria living under the foreskin may cause minor inflammatory immune responses. Inflammation caused by bacteria could stimulate Langerhans cells to present HIV particles to CD4+ T cells. Since HIV uses these cells to replicate, the authors reasoned, this immune response could increase the likelihood that HIV will take hold in the body. The researchers plan to test this hypothesis in further work, according to LiveScience.

“As a society, we’ve gotten used to thinking about alterations to the microbiome as having negative outcomes,” coauthor Lance Price, who researches microbial pathogens at the George Washington University School of Public Health, told the Los Angeles Times. “We think about the person who takes antibiotics in the hospital and ends up with an infection in their gut because we’ve knocked out the natural microbiota. But here’s a situation where we’re flipping that notion on its head. The disturbance of the microbiome could have a positive effect.”