Critical Role for Molecular Iron in Coxiella burnetii Replication and Viability.

, the causative agent of Query (Q) fever in humans, is a highly infectious obligate intracellular bacterium. Following uptake into a host cell, replicates within a phagolysosome-derived compartment referred to as the -containing vacuole (CCV). During infection, exhibits tropism for tissues related to iron storage and recycling (e.g., the liver and splenic red pulp), suggesting that pathogen physiology is linked to host iron metabolism. Iron has been described to have a limited role in virulence regulation, despite evidence that infected host cells increase expression of transferrin receptors, thereby suggesting that active iron acquisition by the bacterium occurs upon infection. Through the use of host cell-free culture, was separated from the host cell in order to directly assess the role of different forms of iron in replication and viability, and therefore virulence. Results indicate that tolerates molecular iron over a broad concentration range (i.e., ∼0.001 to 1 mM) and undergoes gross loss of viability upon iron starvation. protein synthesis and energy metabolism, however, occur nearly uninhibited under iron concentrations not permissive to replication. Despite the apparent absence of genes related to acquisition of host-associated iron-containing proteins, replication is supported by hemoglobin, transferrin, and ferritin, likely due to release of iron from such proteins under acidic conditions. Moreover, chelation of host iron pools inhibited pathogen replication during infection of cultured cells. Host organisms restrict the availability of iron to invading pathogens in order to reduce pathogen replication. To counteract the host's response to infection, bacteria can rely on redundant mechanisms to obtain biologically diverse forms of iron during infection. appears specifically dependent on molecular iron for replication and viability and exhibits a response to iron akin to bacteria that colonize iron-rich environments. Physiological adaptation of to the unique acidic and degradative environment of the CCV is consistent with access of this pathogen to molecular iron.