The Chlamydiaceae are widespread pathogens of both humans and animals. Chlamydia trachomatis infection causes blinding trachoma and reproductive complications in humans. Chlamydia pneumoniae causes human respiratory tract infections and atypical pneumonia. Chlamydia suis infection is associated with conjunctivitis, diarrhea, and failure to gain weight in domestic swine. Chlamydial infections in humans and domesticated animals are generally controlled by antibiotic treatment—particularly macrolides (usually azithromycin) and tetracyclines (tetracycline and doxycycline). Tetracycline-containing feed has also been used to limit infections and promote growth in livestock populations, although its use has decreased because of growing concerns about antimicrobial resistance development. Because Sandoz and Rockey published an elegant review of chlamydial anti-microbial resistance in 2010, we will review the following: (i) antibiotic resistance in C. suis, (ii) recent evidence for acquired resistance in human chlamydial infections, and (iii) recent non-genetic mechanisms of antibiotic resistance that may contribute to treatment failure.

The obligate intracellular bacterium, Chlamydia trachomatis, and Herpes Simplex Virus Type-2 (HSV-2) are the leading sexually transmitted pathogens in the world. These infections are usually asymptomatic and clinically mild, but complications can be severe. Reports of dual detection of Chlamydia and HSV within the genital tracts of humans led our laboratory to develop an in vitro Chlamydia/HSV co-infection model. Little is known regarding the specific pathogenesis of Chlamydia and HSV co-infections, but HSV-super-infection of Chlamydia-infected cells caused the chlamydiae to deviate from their normal developmental cycle into a non-replicative state termed persistence, or the chlamydial stress response. Interactions between HSV envelope protein, gD with host cell junction protein, nectin-1, were enough to stimulate the departure from normal chlamydial development. Additional data also suggested that there might be differences between single infection and co-infection outcomes in vivo. Thus, two diverging hypotheses were investigated here: i) that host nectin-1 is required for normal chlamydial development; and ii) that pathogen shedding and/or disease progression in Chlamydia and HSV-2 co-infected animals will differ from that observed in singly-infected animals. Chlamydial infection of nectin-1 knockdown cell lines revealed no inhibition of chlamydial entry, but significant reductions in inclusion size and production of infectious chlamydiae. Additionally, nectin-1 knockout mice shed fewer Chlamydia compared to wild type mice. In other studies, we developed a novel in vivo Chlamydia and HSV-2 intravaginal super-infection model in BALB/c mice. Infection with Chlamydia muridarum, followed up to 9 days later by HSV-2 super-infection, both reduced HSV shedding and protected mice from HSV-induced fatal neurologic disease compared to HSV singly-infected animals. Protection is lost when: i) infected animals are no longer shedding C. muridarum; ii) when mice are inoculated with UV-inactivated C. muridarum; or iii) when viable chlamydiae are eliminated from the genital tract using antibiotics prior to HSV-2 super-infection. Altogether, we have determined that host nectin-1 is required for chlamydial development both in vitro and in vivo, and that chlamydial pre-infection protects mice from subsequent HSV infection. We predict that these observations may lead to novel approaches to prevent human infection by these two common sexually transmitted pathogens.