NEW YORK – The urethral microbiome of healthy men likely gets colonized with microbes from the female reproductive tract following vaginal sex, according to a new study.
According to the authors, it had been unknown how specific sexual behaviors affected the microbiome composition of the penile urethra. "Sexual behavior has been ignored in most human microbiome studies," co-lead authors Evelyn Toh of Indiana University and Yue Xing of Loyola University Chicago wrote, along with their colleagues.
The study, published in Cell Reports Medicine on Tuesday, showed that microbes associated with bacterial vaginosis (BV), a condition that is currently not considered an STD, also dwell in the urethra of men who reported having vaginal sex. These BV-causing microbes lived in the male urethra for over 60 days without causing any symptoms.
"Our data really lays the foundation for the evidence that bacterial vaginosis is really a sexually transmitted disease," co-author Stephen Jordan, a researcher at Indiana University, told GenomeWeb. This could mean that future partners of these men could be at risk of BV, he added.
For their study, the researchers collected urethral swab samples from 164 male volunteers. After testing them for various STIs and applying other criteria, they whittled them down to 110 samples from healthy men who didn’t have symptoms of urethral inflammation and infection, which they sequenced using shotgun metagenomics.
A total of 117 different bacterial and 26 viruses were detected in the samples. Most contained bacteria such as lactic acid bacteria and Corynebacterium spp., which created a core microbiome that had been found in several previous studies.
However, the common perception among doctors and researchers had been that urine and urethra are sterile places. "What we are showing is that [they are] not sterile," Jordan said.
In addition to the core microbiome, the urethra of men who reported having vaginal sex also harbored high levels of DNA that belonged to bacteria such as Gardnerella vaginalis and Atopobium vaginae, which are associated with BV, or Streptococcus agalactiae and Streptococcus anginosis, which have been linked to aerobic vaginitis.
Another important finding was that female reproductive tract bacteria such as Lactobacillus spp., which have protective effects in females, were not detected in men who had reported vaginal sex.
"We don’t see any of the good vaginal bacteria; we only see the bad ones. We don't know why this happens," said co-corresponding author David Nelson, a professor of microbiology and immunology at Indiana University.
Meanwhile, microbes associated with oral and anal sex were not found in samples of men who indulged in these sexual behaviors.
Researchers said further studies are required to confirm if the BV-associated microbes get transmitted to female partners of men who harbor them. If these studies show that men with BV are infecting their partners, it might make sense to treat these men with antibiotics, Jordan said.