CsrA Supports both Environmental Persistence and Host-Associated Growth of Acinetobacter baumannii

<i>Acinetobacter baumannii</i> is an opportunistic and frequently multidrug-resistant Gram-negative bacterial pathogen that primarily infects critically ill individuals. Indirect transmission from patient to patient in hospitals can drive infections, supported by this organism's abilities to persist on dry surfaces and rapidly colonize susceptible individuals. To investigate how <i>A. baumannii</i> survives on surfaces, we cultured <i>A. baumannii</i> in liquid media for several days and then analyzed isolates that lost the ability to survive drying. One of these isolates carried a mutation that affected the gene encoding the carbon storage regulator CsrA. As we began to examine the role of CsrA in <i>A. baumannii</i>, we observed that the growth of Δ<i>csrA</i> mutant strains was inhibited in the presence of amino acids. The Δ<i>csrA</i> mutant strains had a reduced ability to survive drying and to form biofilms but an improved ability to tolerate increased osmolarity compared with the wild type. We also examined the importance of CsrA for <i>A. baumannii</i> virulence. The Δ<i>csrA</i> mutant strains had a greatly reduced ability to kill <i>Galleria mellonella</i> larvae, could not replicate in <i>G. mellonella</i> hemolymph, and also had a growth defect in human serum. Together, these results show that CsrA is essential for the growth of <i>A. baumannii</i> on host-derived substrates and is involved in desiccation tolerance, implying that CsrA controls key functions involved in the transmission of <i>A. baumannii</i> in hospitals.