Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) are the most common bacterial sexually transmitted infections (STIs) worldwide. The primary infection site for both bacteria is the epithelium of the endocervix in women and the urethra in men; both can also infect the rectum, pharynx and conjunctiva. CT/NG co-infections are often asymptomatic and occur more frequently than expected by chance, suggesting biological interactions between the two pathogens. Despite the high rates of co-infection detected clinically, most animal models and in vitro studies have focused only on individual host-pathogen interactions, highlighting the need for new models to better understand potential CT/NG pathogenic interplay. We developed an in vitro CT/NG co-infection model to test whether NG alters chlamydial development and infectivity and whether reciprocal effects on NG occur.
In vitro, cervical epithelial cells (HeLa) were infected with Chlamydia trachomatis serovar E (CtE) and immediately co-infected with the N. gonorrhoeae strain FA1090 for the 40-hours chlamydial developmental cycle. Chlamydial infection and development were assessed by immunofluorescence and inclusion-forming unit (IFU) assays, while gonococcal adherence, invasion and intracellular survival were measured using gentamicin protection assay.
Our data show that NG co-infection exerted a marked anti-chlamydial effect, significantly reducing inclusion size and infectivity. In contrast, intracellular NG levels were higher during co-infection compared with NG single infection. These anti-chlamydial effects required viable NG and direct host-pathogen contact, but were independent of pH modulation, extracellular nutrient depletion, or host sphingomyelin perturbation. Notably, these effects were partially reversed by gentamicin treatment at the time of co-infection, supporting a contact-dependent mechanism involving viable gonococci.
Together, these findings reveal an antagonistic interaction during CT/NG co-infection, in which NG suppresses chlamydial development while enhancing its own internalization and survival. Understanding how CT/NG co-infection impacts pathogenesis, bacterial load, transmission, and disease severity could help identify new targets for therapeutics or vaccine strategies for STI control.