The study, published in the online journal mBio, found men who were circumcised as part of the study had 33.3 percent less bacteria on the penis than those who remained uncircumcised one year after the study began.
Senior author Dr. Lance Price, director of the TGen Center for Microbiomics and Human Health, a Phoenix non-profit research organization, said the study, which involved 156 men in Rakai, Uganda, showed the decrease of bacteria was primarily found in 12 types of bacteria, most of which were intolerant to oxygen.
Past studies showed circumcision reduced female-to-male human immunodeficiency virus transmission, Price said.
This study suggests a possible mechanism for HIV protection -- the shift in the number and type of bacteria living on the penis.
Further studies will have to be done to demonstrate a change in the penis microbiome can help reduce the risk of HIV transmission, the study authors said.
"We know that male circumcision can prevent HIV and other diseases in heterosexual men, but it is important to know why," Price said in a statement. "We think that these dramatic changes in the penis microbiome may explain, at least in part, why male circumcision is protective."