Gut Microbiome and Chlamydia pathogenicity in the upper genital tract Abstract Chlamydia trachomatis (CT) can cause long-lasting tubal fibrosis/adhesion/hydrosalpinx/infertility. However, the mechanism remains unclear. Studies based on C. muridarum (CM) mouse genital tract infection model have defined chlamydial ascension and tubal inflammation as two key pathogenic determinants. Both chlamydial and host factors that can affect chlamydial ascension and tubal inflammation have been identified. However, these factors cannot fully explain chlamydial pathogenicity in some cases. Some mice with both adequate ascending infection and tubal inflammation still fail to develop significant long-lasting hydrosalpinx. Clinically, only a fraction of women with high titers of anti-Chlamydia antibodies develop tubal infertility. Thus, efforts have been made to correlate genital microbiota profiles with susceptibility to chlamydial infection. However, it remains to determine whether lower genital tract microbiota can impact chlamydial pathogenicity in the upper genital tract. On the other hand, gut microbiota are associated with pathogenic fibrosis and can induce profibrotic/regulatory T cells as well as impact extra-GI tissue pathologies. However, the role of gut microbiome in chlamydial induction of tubal fibrosis has not been seriously investigated. We recently screened 11 different mouse strains for susceptibility to CM induction of tubal fibrosis/hydrosalpinx and found that intravaginal inoculation with CM failed to induce significant hydrosalpinx in DBA/2J mice despite the fact that CM both ascended to the oviduct and induced tubal inflammation. We took advantage of this surprising finding for evaluating the impact of gut microbiota on chlamydial pathogenicity. When we used an oral antibiotic cocktail (Abx) consisting of vancomycin and gentamycin (that cannot penetrate mammalian cell membranes and are unable to affect chlamydial infection) to induce gut dysbiosis, the DBA/2J mice became highly susceptible to induction of hydrosalpinx by intravaginal inoculation with CM. Furthermore, the oral Abx-promoted CM pathogenicity correlated with both gut dysbiosis and increased profibrotic T cell responses but the genital microbiota or chlamydial infections were not altered by the same oral Abx regime. We hypothesize that the oral Abx regime may promote chlamydial pathogenicity in the upper genital tract by inducing gut dysbiosis that favors the development of T cells able to enhance tubal fibrosis. Testing this hypothesis will open up a new dimension for investigating chlamydial pathogenicity, add a new example to the growing list of mechanisms by which gut microbiome regulates extra-gut tissue pathology, lay a foundation for in-depth understanding on how gut microbial species regulate fibrotic responses and finally provide correlates for designing human studies.