During
infection, bacterial pathogens must adapt to a nutrient
metal-limited environment that is imposed by the host. The innate immune
protein calprotectin inhibits bacterial growth in vitro by chelating the divalent
metal ions zinc (Zn2+, Zn) and
manganese (Mn2+, Mn), but pathogenic bacteria are able to cause disease in the presence of this antimicrobial
protein in vivo. One such pathogen is Acinetobacter baumannii, a Gram-negative bacterium that causes
pneumonia and
bloodstream infections that can be complicated by resistance to multiple
antibiotics. A. baumannii inhibition by
calprotectin is dependent on
calprotectin Mn binding, but the mechanisms employed by A. baumannii to overcome Mn limitation have not been identified. This work demonstrates that A. baumannii coordinates transcription of an NRAMP family Mn transporter and a
urea carboxylase to resist the antimicrobial activities of
calprotectin. This NRAMP family transporter facilitates Mn accumulation and growth of A. baumannii in the presence of
calprotectin. A. baumannii is found to utilize
urea as a sole
nitrogen source, and
urea utilization requires the
urea carboxylase encoded in an operon with the NRAMP family transporter. Moreover,
urea carboxylase activity is essential for
calprotectin resistance in A. baumannii Finally, evidence is provided that this system combats
calprotectin in vivo, as deletion of the transporter impairs A. baumannii fitness in a mouse model of
pneumonia, and this fitness defect is modulated by the presence of
calprotectin. These findings reveal that A. baumannii has evolved mechanisms to subvert host-mediated
metal sequestration and they uncover a connection between
metal starvation and metabolic stress.
IMPORTANCE: Acinetobacter baumannii is a bacterium that causes bloodstream,
wound, urinary tract, and
pneumonia infections, with a high disease burden in intensive care units. Treatment of A. baumannii
infection is complicated by resistance to most
antibiotics in use today, and resistance to last-resort
therapies has become commonplace. New treatments for A. baumannii
infection are desperately needed, but our current understanding of the bacterial factors required to cause
infection is limited. We previously found that the abundant innate immune
protein calprotectin inhibits the growth of A. baumannii by withholding essential metals. Despite this, A. baumannii is still able to infect wild-type mice, which produce
calprotectin during
infection. Here, we identify factors employed by A. baumannii during
infection to overcome
calprotectin-mediated
metal sequestration. Moreover, we expose a connection between
metal starvation and metabolism that may be a "chink in the armor" of A. baumannii and lead to new treatment options.