An ompR-envZ Two-Component System Ortholog Regulates Phase Variation, Osmotic Tolerance, Motility, and Virulence in Acinetobacter baumannii Strain AB5075

Recently, a novel phase-variable colony opacity phenotype was discovered in Acinetobacter baumannii strain AB5075, where colonies interconvert between opaque and translucent variants. Opaque colonies become mottled or sectored after 24 h of growth due to translucent variants arising within the colony. This easily distinguishable opaque-colony phenotype was used to screen for random transposon insertions that increased the frequency of sectoring at a time point when wild-type colonies were uniformly opaque. A colony was identified that contained multiple papillae of translucent variants, and the insertion in this mutant mapped to an ortholog of the two-component system response regulator ompR. Subsequent investigation of in-frame deletions of ompR and the sensor kinase envZ (located adjacent to ompR) showed that the switching frequency from opaque to translucent was increased 401-and 281-fold, respectively. The ompR mutant also exhibited sensitivity to sodium chloride in growth medium, whereas the envZ mutation did not elicit sensitivity to sodium chloride. Mutation of either gene reduced motility in A. baumannii strain AB5075, but a mutation in both ompR and envZ produced a more profound effect. The ompR and envZ genes were cotranscribed but were not subject to autoregulation by OmpR. Both ompR and envZ mutant opaque variants were attenuated in virulence in the Galleria mellonella infection model, whereas mutation of ompR had no effect on the virulence of the translucent variant. IMPORTANCE Acinetobacter baumannii is a well-known antibiotic-resistant pathogen; many clinical isolates can only be treated by a very small number of antibiotics (including colistin), while some exhibit panresistance. The current antimicrobial arsenal is nearing futility in the treatment of Acinetobacter infections, and new avenues of treatment are profoundly needed. Since phase variation controls the transition between opaque (virulent) and translucent (avirulent) states in A. baumannii, this may represent an Achilles' heel that can be targeted via the development of small molecules that lock cells in the translucent state and allow the host immune system to clear the infection. A better understanding of how phase variation is regulated may allow for the development of methods to target this process. The ompR-envZ two-component system ortholog negatively regulates phase variation in A. baumannii, and perturbation of this system leads to the attenuation of virulence in an invertebrate infection model.