The microbiome inside the penis can be altered by vaginal sex

The male urethra – the tube through which urine exits the body – is home to an array of bacteria, some of which are probably picked up during vaginal sex.

Evelyn Toh at the Indiana University School of Medicine and her colleagues swabbed the urethras of 110 men who had no sexually transmitted infections or urethra-related issues. The men, average age 28, came from a range of ethnic and racial backgrounds. Transgender people weren’t included in the study.

Of these men, 92 provided swabs with sufficient levels of bacterial DNA for further analysis.

The detected bacteria could be separated into two groups – those that can live in the presence of oxygen and therefore probably dwell near the tip of the penis, and those that cannot live when oxygen is present and therefore probably dwell higher up in the urethra.

The former group was found in most of the men’s swabs and are probably native to the penile urethra, according to the researchers.

The latter group, however, was dominated by bacteria that are often disrupted in the condition bacterial vaginosis (BV), a common cause of unusual vaginal discharge that is thought to come about when the vagina’s microbiome gets out of sync.

Of the 110 men, 75 identified as heterosexual, 22 as homosexual and 13 as “bisexual or other”.

Only the men who reported having vaginal sex had the BV-associated bacteria, such as Gardnerella vaginalis, suggesting that they acquired them during vaginal sex. These bacteria weren’t associated with oral or anal sex.

“The most important takeaway of our study is that it establishes a baseline for the flora of a healthy [penile] urethra,” says co-author David Nelson, also at the Indiana University School of Medicine.

Catriona Bradshaw at Monash University in Melbourne, Australia, says that this study details the bacterial composition of an infection-free penile urethra, which could serve as a baseline to measure whether a penile microbiome is healthy or infected.

The results also support the hypothesis that men who have vaginal sex without a condom may transfer certain bacteria from one woman to another, says Erica Plummer at Monash University.