By changing the bacterial communities of the penis, circumcision substantially lowers HIV risk among men, finds a new study.
According to scientists at the Translational Genomics Research Institute (TGen) and Johns Hopkins University the strongest evidence for a cause-and-effect relationship between circumcision and HIV risk reduction came from three randomized-control trials in sub-Saharan Africa, where the circumcision rate is relatively low and the HIV infection rate is relatively high.
All the studies showed more than 40 percent reduction in HIV acquisition among circumcised men.
The new study found that circumcision - the removal of the foreskin, or prepuce, from the penis - eliminates an area of mucous membrane and dramatically changes the penile bacterial ecosystem.
"Our randomized trials have shown that male circumcision prevents HIV infection in men and protects their female partners from vaginal infections, especially bacterial vaginosis. It is possible that the virtual elimination of anaerobic bacteria by circumcision contributes to these benefits of the procedure," said Ronald H. Gray, a renowned epidemiologist at Johns Hopkins and the scientific paper's senior author.
The research team believes that circumcision reduces the amount of mucosal tissue exposed to vaginal secretions during heterosexual intercourse and thus may reduce the potential interactions between the virus and its target immune cells.
Circumcision results in a process called keratinization, whereby the top layer of the inner foreskin becomes thicker, which may provide additional protection for the underlying target immune cells.
Moreover, circumcision-associated physiological changes of the penis - including lower moisture and oxygen availability around the head of the penis - may reduce the number of pro-inflammatory anaerobic bacteria that could make the target immune cells more vulnerable to HIV infection.
"The concept that there are good and harmful bacteria is essential to studying the human microbiome. Our work showed that the profile of the penile bacterial communities changed significantly after circumcision," said Dr. Cindy M. Liu, a medical doctor and researcher at both TGen and Northern Arizona University. She is the paper's other co-lead author.
The study is published in the scientific journal PLoS ONE.