Intrauterine infection with plasmid-free Chlamydia muridarum reveals a critical role of the plasmid in chlamydial ascension and establishes a model for evaluating plasmid-independent pathogenicity.

Intravaginal infection with plasmid-competent but not plasmid-free Chlamydia muridarum induces hydrosalpinx in mouse upper genital tract, indicating a critical role of the plasmid in chlamydial pathogenicity. To evaluate the contribution of the plasmid to chlamydial ascension and activation of tubal inflammation, we delivered plasmid-free C. muridarum directly into the endometrium by intrauterine inoculation. We found that three of the six mouse strains tested, including CBA/J, C3H/HeJ, and C57BL/6J, developed significant hydrosalpinges when 1 × 10(7) inclusion-forming units (IFU) of plasmid-free C. muridarum were intrauterinally inoculated. Even when the inoculum was reduced to 1 × 10(4) IFU, the CBA/J mice still developed robust hydrosalpinx. The hydrosalpinx development in CBA/J mice correlated with increased organism ascension to the oviduct following the intrauterine inoculation. The CBA/J mice intravaginally infected with the same plasmid-free C. muridarum strain displayed reduced ascending infection and failed to develop hydrosalpinx. These observations have demonstrated a critical role of the plasmid in chlamydial ascending infection. The intrauterine inoculation of the CBA/J mice with plasmid-free C. muridarum not only resulted in more infection in the oviduct but also stimulated more inflammatory infiltration and cytokine production in the oviduct than the intravaginal inoculation, suggesting that the oviduct inflammation can be induced by plasmid-independent factors, which makes the hydrosalpinx induction in CBA/J mice by intrauterine infection with plasmid-free C. muridarum a suitable model for investigating plasmid-independent pathogenic mechanisms.