Some of bacteria that colonise men's penises may increase their risk of becoming infected with HIV, scientists have shown. This is the first time that penile bacterial are recognised as a risk factor for HIV in men – and they can be passed on to women during sex, also increasing their risk of infection.

Past research has revealed that the bacteria living in and on the human body – collectively known as microbiome – is linked to the development of different health conditions. However, few studies had ever been conducted to study the role of penile microbiome and how it affects men's health.

Recently, scientists had shown that in men who had been circumcised, penile microbiome changed after the procedure. Furthermore, there is robust evidence that circumcision reduces the risk heterosexually acquired HIV infection in men by approximately 60%.

So researchers believed there might be a link between the composition of men's penile microbiome and HIV infection risk, which they set out to investigate in a study now published in the journal mBio.

Penile bacteria hiding under the foreskin

The scientists worked with 182 uncircumcised heterosexual men in Rakai, Uganda, during a large, two-year clinical study. Over the course of the study, 46 men became infected with HIV, while 136 remained HIV-negative.

Analysing penile swabs collected from all the participants, the scientists studied how the penile microbiome differed between men who did and those who did not become HIV-infected during the trial. In particular, the researchers measured the abundance of penile bacteria living under the foreskin.

They found that a tenfold increase in four anaerobic bacteria - Prevotella, Dialister, Finegoldia and Peptoniphilus - correlated to a 54 to 63% increase in HIV risk. Anaerobic bacteria are bacteria which do not require oxygen for growth.

"What we find is that it's not about whether you have certain bacteria, but more about how much of these bacteria you have compared to other bacteria. We find that men who have high amounts of anaerobic bacteria are at higher risk of HIV. We think it's an exciting discovery because HIV remains a global health problem and we need to identify more risk factors, to find more ways to reduce the risk of infection for people," first author Cindy Liu, at the Milken Institute School of Public Health (US), told IBTimes UK.

The scientists also discovered that the penile microbiome may be increasing the risk of HIV by triggering the production of immune factors that lure the immune cells targeted by the virus to the penis.

"Immune cells are HIV's gateway to the human body. Our study suggests that some bacteria are triggering biochemical alarms that draw immune cells to the penis where they are more easily infected by HIV. With other pathogens this would be a good thing, because the immune cells could help fight infection, but in the case of HIV, it does just the opposite," co-senior author Lance Price at Milken Institute School of Public Health added.

These findings may aid the development innovative methods to reduce the risk of HIV transmission. Scientists could for instance try to develop techniques to selectively reduce the amount of anaerobic bacteria on the penis by increasing oxygen levels. Because these bacteria are known to be shared by heterosexual partners and are thus associated with HIV risk in women, doing this could help protect both men and women.

While the study does not encourage circumcision, it reveals a possible explanation as to why circumcised men are less at risk of HIV transmission - fewer anaerobic bacteria may be colonising their genitalia.

"Our study adds to the body of evidence to understand why men who are circumcised may be less at risk of HIV. We are not trying to say that all men should be circumcised, but we are trying to show a way in which circumcision may reduce HIV, and to identify new risk factors to show innovative ways through which we can reduce transmission," Liu concluded.