Chlamydia trachomatis (Ct) is an intracellular energy-parasitic bacterium that requires
ATP derived from infected cells for its growth. Meanwhile, depending on the O2 concentration, the host cells change their mode of
ATP production between oxidative phosphorylation in mitochondria (Mt) and glycolysis; this change depends on signaling via
reactive oxygen species (ROS) produced by
NADPH oxidases (NOXs) as well as Mt. It has been proposed that Ct correspondingly switches its source of acquisition of
ATP between host-cell Mt and glycolysis, but this has not been verified experimentally. In the present study, we assessed the roles of host-cell NOXs and Mt in the intracellular growth of CtL2 (L2 434/Bu) under normoxia (21% O2) and
hypoxia (2% O2) by using several inhibitors of NOXs (or the downstream molecule) and Mt-dysfunctional (Mtd) HEp-2 cells. Under normoxia,
diphenyleneiodonium, an inhibitor of ROS diffusion, abolished the growth of CtL2 and other Chlamydiae (CtD and C. pneumoniae). Both
ML171 (a pan-NOX inhibitor) and GLX351322 (a NOX4-specific inhibitor) impaired the growth of CtL2 under normoxia, but not
hypoxia. NOX4-knockdown cells diminished the bacterial growth.
SB203580, an inhibitor of the NOX4-downstream molecule p38MAPK, also inhibited the growth of CtL2 under normoxia but not
hypoxia. Furthermore, CtL2 failed to grow in Mtd cells under normoxia, but no effect was observed under
hypoxia. We conclude that under normoxia, Ct requires functional Mt in its host cells as an
ATP source, and that this process requires NOX4/p38MAPK signaling in the host cells. In contrast to
hypoxia, crosstalk between NOX4 and Mt via p38MAPK may be crucial for the growth of Ct under normoxia.