Study of the ecological interactions between commensal bacteria and microbial pathogens that co-inhabit the human urogenital tract has recently gathered momentum due to next generation sequencing. Using this technology, researchers have expanded our understanding of what constitutes a ‘normal’ vagina or ‘healthy’ male urethra and have identified characteristics of bacterial composition that are associated with infection. For example, using data from 16S rRNA sequencing of vaginal samples collected from asymptomatic North American women from four ethnic groups (white, black, Hispanic, and Asian), Ravel et al.
] identified five bacterial ‘community types’, four of which were dominated by one of four species of Lactobacillus
, Lactobacillus crispatus
, Lactobacillus gasseri
, or Lactobacillus jensenii
). The fifth community type had low proportions of lactic acid bacteria and high proportions of strictly anaerobic organisms. Interestingly, the proportions of each community group varied significantly by ethnic group, as did the vaginal pH. A follow-up study that used the same data set but restricted analysis to the eleven T. vaginalis
-positive women in the cohort indicated that the presence of T. vaginalis
in asymptomatic women could be associated with low Lactobacillus
community types [39
]. Unfortunately, no association of a particular T. vaginalis
genotype with community type was found, due to the low number of parasite samples tested.
Less well studied is the microbiome of the male urogenital tract. A recent study cloned and sequenced 16S rRNA amplicons of the microbiota of first-catch urine [40
], which has been shown to be indicative of the male urogenital tract microbiome [41
]; a second study used 16S rRNA pyrosequencing to identify microbes isolated from the coronal sulci of circumcised and uncircumcised Ugandan men [42
]. Despite differences in sequencing depth, both studies found that bacterial communities are complex and that a single, characteristic microbial community is not apparent. Furthermore, an overlap was found between the male urethral microbiome and the microbial communities of the superficial skin, colon, and vagina. The microbiomes of men infected with STIs tended to cluster together, distinct from those of non-infected men, and often included unidentified bacteria associated with female genital tract pathology [40
]. Unfortunately, neither study specifically considered an association between T. vaginalis
and the male urogenital tract microbiome.
The ability to characterize the diversity of microbial community types that inhabit the male and female urogenital tracts is enormously powerful, and will permit investigators to determine if disturbances in microbiomes allow sexually transmitted pathogens to infect sexual partners opportunistically, or if they cause specific types of disruption to urogenital microbial communities. Establishing causality will require following patient urogenital microbiomes longitudinally in the context of sexual activity, and hormonal changes, as reported recently [43
]. In addition, these techniques can be used to understand the impact that metronidazole treatment (or other antibiotic usage) may have on the vaginal microbial community, and the short- and long-term implications for vaginal health [44
]. Ultimately, a robust in vivo
model system is needed to facilitate investigation of ecological interactions between the microbial community and T. vaginalis
. Although mouse [46
] and Macaca
] have been described, they are less than ideal, and a rodent model with a humanized urogenital tract, or an in vitro
system composed of differentiated vaginal cells, immune cells and commensal bacteria, will be needed to consider the role of microbial ecology in regulating T. vaginalis
infection and virulence. By understanding the influence of particular bacteria on successful vaginal colonization, it may be possible to tailor preventative measures by identifying women most susceptible to T. vaginalis
infection, and modify their indigenous microbial communities to protect them against infection.