( TIME.com ) -- Promising trials hinted that circumcision could lower rates of HIV infection , but until now , researchers did n't fully understand why . Now , in a study published in the journal mBio , scientists say that changes in the population of bacteria living on and around the penis may be partly responsible . Relying on the latest technology that make sequencing the genes of organisms faster and more accessible , Lance Price of the Translational Genomics Research institute ( TGen ) and his colleagues conducted a detailed genetic analysis of the microbial inhabitants of the penis among a group of Ugandan men who provided samples before circumcision and again a year later . TIME.com : If circumcision rates keep falling , health costs and infections will spike . While the men showed similar communities of microbes before the operation , 12 months later , the circumcised men harbored dramatically fewer bacteria that survive in low oxygen conditions . They also had 81 % less bacteria overall compared to the uncircumcised men , and that could have a dramatic effect on the men 's ability to fight off infections like HIV , says Price . Previous studies showed that circumcised men lowered their risk of transmitting HIV by as much as 50 % , making the operation an important tool in preventing infection with the virus . Why ? A high burden of bacteria could disrupt the ability of specialized immune cells known as Langerhans cells to activate immune defenses . Normally , Langerhans are responsible for grabbing invading microbes like bacteria or viruses and presenting them to immune cells for training , to prime the body to recognize and react against the pathogens . But when the bacterial load increases , as it does in the uncircumcised penile environment , inflammatory reactions increase and these cells actually start to infect healthy cells with the offending microbe rather than merely present them . TIME.com : Can new circumcision devices help fight AIDS in Africa ? That may be why uncircumcised men are more likely to transmit HIV than men without the foreskin , says Price , since the Langerhans cells could be feeding HIV directly to healthy cells . His group is also investigating how changes in the levels of cytokines , which are the signaling molecules that immune cells use to communicate with each other , might be influenced by bacterial populations . `` There is a real revolution going on in our understanding of the microbiome , '' says Price , who is also professor of occupational and environmental health at George Washington University . `` The microbiome is almost like another organ system , and we are just scratching the surface of understanding the interplay between the microbiome and the immune system . '' Previous work suggested that changes in the bacterial populations in the gut , for example , could affect obesity , and other studies found potential connections between microbial communities and the risk for cancer , asthma and other chronic conditions . TIME.com : Study identifies four new genetic markers for severe childhood obesity . This study was originally published on TIME.com . Why circumcision lowers risk of HIV . & copy 2012 TIME , Inc. TIME is a registered trademark of Time Inc. Used with permission .
The decreased risk may be due to changes in bacteria on and around the penis . A high level of bacteria may disrupt specialized immune systems . Uncircumcised men are more likely to transmit HIV than married men .