In a recent study published in Cell Reports Medicine journal, researchers in the United States (US) used a shotgun metagenomic approach to characterize urethral microbiomes of specimens from healthy men without infections or inflammations of the urethra. They did so to understand the significance and composition of the distal urethral microbiome.
Although the epithelia of the penile urethra contain immune cells that can perceive and mount immune responses against microorganisms, the penile urethra can get infected by viruses, bacteria, and other pathogens that are sexually transmitted.
While the female reproductive tract and gastrointestinal tract are colonized by commensal microbial communities that protect these regions from infections, it is unclear whether the penile urethra harbors a microbiome that plays a protective role.
The penile urethral microbiome is rarely tested in healthy humans due to the painful nature of sampling. Additionally, non-invasive methods, such as urine samples, do not provide an accurate idea of the diverse microbial communities that could exist in the penile urethra.
Therefore, the available information on the penile urethral microbiome comes from studies involving men with sexually transmitted diseases, making it difficult to ascertain the role of a healthy penile urethral microbiome.
About the study
In the present study, the researchers obtained penile urethral samples from healthy men identified using strict inclusion/exclusion criteria involving clinical examinations, urethral inflammations, tests for sexually transmitted infections, and behavioral surveys. A validation cohort comprising vaginal samples was also included in the study.
Shotgun metagenomic sequencing characterized the microbiome from the penile urethral and vaginal samples. Furthermore, the researchers also evaluated the roles of sexual behaviors and sexually transmitted infection risk factors in the composition of the penile urethral microbiome.
The clustering of the sequence data was used to organize the specimens into urethrotypes. Membership in one of the two urethrotypes was examined in the context of the findings from the behavioral survey. This was done to determine if the urethrotypes were associated with specific sexually transmitted infection risk factors or demographic characteristics.
Based on the findings that urethrotype two was associated with vaginal sex, the association between sexual behaviors and specific bacteria was explored further.
The results reported that Corynebacterium species and some lactic acid bacteria were found in most penile urethral specimens. When the urethrotypes were examined in association with sexual behaviors, specific sexually transmitted infection risk factors, and demographic characteristics, the findings indicated that while infection risk factors and demographics showed no significant associations, vaginal sex in the previous two months to one year was associated with urethrotype two.
In contrast, penetrative penile oral or anal sex was not associated with any of the urethrotypes, and bacteria associated with dysbiosis in the vagina were present in the penile urethral specimens of only those men who reported having vaginal sex. This suggested that sexual behavior was linked to the variation in species composition of the urethral microbiome.
Understanding the role of the penile urethral microbiome in urethral health is essential since it determines the approaches taken to prevent or treat sexually transmitted diseases.
If the penile urethral microbiome plays a significant role in maintaining urethral health, using broad-spectrum antibiotics as prophylactic treatment of sexually transmitted diseases might result in antimicrobial resistance. However, if the penile urethral microbiome does not have significant protective roles, prophylactic treatment might be essential in male and female urogenital health.
The association between vaginal sex and the penile urethral microbiome has also been reported by various other studies that have found bacterial vaginosis-associated bacteria in urethral swabs and urine samples of asymptomatic men.
However, the findings from the present study also suggested that the core bacteria that are established without any association with sexual exposures inhabit the region of the urethra known as the urethral meatus, which has higher oxygen availability. In contrast, the bacterial vaginosis-associated bacteria inhabit the penile urethral region, a mucin-rich area, and do not have overlapping niches with the core bacteria.
Overall, the findings indicated that the male urethra contains a simple microbiome comprising core bacteria that are not established through sexual behaviors, but exposure to the vaginal microbiome during sex can reshape the penile urethral microbiome.
Furthermore, the male urethral microbiome can harbor pathobionts that can cause infections in the female urogenital tract.