Characterization of RelA in<i>Acinetobacter baumannii</i>

Abstract

In response to nutrient depletion, the RelA and SpoT proteins generate the signaling molecule (p)ppGpp, which then controls a number of downstream effectors to modulate cell physiology. In <i>Acinetobacter baumannii</i> strain AB5075, a <i>relA</i> ortholog (<i>ABUW_3302</i>) was identified by a transposon insertion that conferred an unusual colony phenotype. An in-frame deletion in <i>relA</i> (Δ<i>relA</i>) failed to produce detectable levels of ppGpp when amino acid starvation was induced with serine hydroxamate. The Δ<i>relA</i> mutant was blocked from switching from the virulent opaque colony variant (VIR-O) to the avirulent translucent colony variant (AV-T), but the rate of AV-T to VIR-O switching was unchanged. In addition, the Δ<i>relA</i> mutation resulted in a pronounced hypermotile phenotype on 0.35% agar plates. This hypermotility was dependent on the activation of a LysR regulator ABUW_1132, which was required for expression of AbaR, a LuxR family quorum-sensing regulator. In the Δ<i>relA</i> mutant, ABUW_1132 was also required for the increased expression of an operon composed of the <i>ABUW_3766-ABUW_3773</i> genes required for production of the surfactant-like lipopeptide acinetin 505. Additional phenotypes identified in the Δ<i>relA</i> mutant included (i) cell elongation at high density, (ii) reduced formation of persister cells tolerant to colistin and rifampin, and (iii) decreased virulence in a <i>Galleria mellonella</i> model.<b>IMPORTANCE</b><i>Acinetobacter baumannii</i> is a pathogen of worldwide importance. Due to the increasing prevalence of antibiotic resistance, these infections are becoming increasingly difficult to treat. New therapies are required to combat multidrug-resistant isolates. The role of RelA in <i>A. baumannii</i> is largely unknown. This study demonstrates that like in other bacteria, RelA controls a variety of functions, including virulence. Strategies to inhibit the activity of RelA and the resulting production of ppGpp could inhibit virulence and may represent a new therapeutic approach.