When men or boys are circumcised, the operation reduces the abundance of bacteria living on the penis and might help explain why circumcision offers men some protection against HIV, according to a study in Uganda.
Researchers from the Translational Genomics Research Institute in Phoenix found that removing the foreskin causes a significant shift in the bacterial community or microbiome of the penis.
The international collaboration focused on 156 men in Rakai, Uganda — part of the world’s largest randomized-controlled trial on male circumcision. Researchers showed that men who were circumcised as part of the study had 33.3 percent less bacteria on their penis than those who remained uncircumcised one year after the study began.
Past studies have shown that circumcision reduces female-to-male HIV transmission, among other benefits. This study suggests a possible mechanism for HIV protection: the shift in the number and type of bacteria living on the penis.
“We think that these dramatic changes in the penis microbiome may explain, at least in part, why male circumcision is protective,” says researcher Lance Price.
The researchers hypothesize that penis bacteria in uncircumcised men facilitate HIV infection in two ways: by both recruiting more HIV target cells to the foreskin and by triggering another set of immune cells, the Langerhans cells, to deliver the virus to susceptible T-cells. Without this trigger, the Langerhans cells simply destroy the virus.