Insights into penicillin-induced Chlamydia trachomatis persistence

Chlamydia persistence is a viable, but non-cultivable, growth stage, resulting in a long-term relationship with the infected host cell. In vitro, this condition can be induced by different stressor agents, including beta-lactam antibiotics, as penicillin. The aim of this study was to get new insights into the interactions between Chlamydia trachomatis (serovars D and L2) and the epithelial host cells (HeLa) during persistence condition. In particular, we evaluated the following aspects, by comparing the normal chlamydial development cycle with penicillin-induced persistence: (i) cell survival/death, (ii) externalization of phosphatidylserine, (iii) caspase 1 and caspase 3/7 activation, and (iv) reactive oxygen species (ROS) production by the infected cells. At 72 h post-infection, the cytotoxic effect displayed by CT was completely abolished for both serovars and for all levels of multiplicity of infection only in the cells with aberrant CT inclusions. At the same time, CT was able to switch off the exposure of the lipid phosphatidylserine on the surface of epithelial cells and to strongly inhibit the activation of caspase 1 and caspase 3/7 only in penicillin-treated cells. Forty-eight hours post-infection, CT elicited a significant ROS expression both in case of a normal cycle and in case of persistence. However, serovar L and penicillin-free infection activated a higher ROS production compared to serovar D and to penicillin-induced persistence, respectively. In conclusion, we added knowledge to the cellular dynamics taking place during chlamydial persistence, demonstrating that CT creates a suitable niche to survive, switching off signals able to activate phagocytes/leukocytes recruitment. Nevertheless, persistent CT elicits ROS production by the infected cells, potentially contributing to the onset of chronic inflammation and tissue damages.