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Background  The natural course of sexually transmitted infections caused by  Chlamydia trachomatis  varies between individuals. In addition to parasite and host effects, the vaginal microbiota might play a key role in the outcome of  C. trachomatis  infections. Interferon-gamma (IFN-γ), known for its anti-chlamydial properties, activates the expression of indoleamine 2,3-dioxygenase (IDO1) in epithelial cells, an enzyme that catabolizes the amino acid L- tryptophan into N-formylkynurenine, depleting the host cell’s pool of tryptophan. Although  C. trachomatis  is a tryptophan auxotroph, urogenital strains (but not ocular strains) have been shown in vitro to have the ability to produce tryptophan from indole using the tryptophan synthase ( trpBA ) gene. It has been suggested that indole producing bacteria from the vaginal microbiota could influence the outcome of  Chlamydia  infection.    Results  We used two in vitro models (treatment with IFN-γ or direct limitation of tryptophan), to study the effects of direct rescue by the addition of exogenous indole, or by the addition of culture supernatant from indole-positive versus indole-negative  Prevotella  strains, on the growth and infectivity of  C. trachomatis . We found that only supernatants from the indole-positive strains,  P. intermedia  and  P. nigrescens , were able to rescue tryptophan-starved  C. trachomatis . In addition, we analyzed vaginal secretion samples to determine physiological indole concentrations. In spite of the complexity of vaginal secretions, we demonstrated that for some vaginal specimens with higher indole levels, there was a link to higher recovery of the  Chlamydia  under tryptophan-starved conditions, lending preliminary support to the critical role of the IFN-γ-tryptophan-indole axis in vivo.    Conclusions  Our data provide evidence for the ability of both exogenous indole as well as supernatant from indole producing bacteria such as  Prevotella , to rescue genital  C. trachomatis  from tryptophan starvation. This adds weight to the hypothesis that the vaginal microbiota (particularly from women with lower levels of lactobacilli and higher levels of indole producing anaerobes) may be intrinsically linked to the outcome of chlamydial infections in some women.    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-016-0903-4) contains supplementary material, which is available to authorized users.

bmc microbiology

In vitro rescue of genital strains of Chlamydia trachomatis from interferon-γ and tryptophan depletion with indole-positive, but not indole-negative Prevotella spp.