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...