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/sectored after 24 hours 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 switching frequency from opaque to translucent was increased 401-fold and 281-fold, respectively. The ompR mutant also exhibited sensitivity to sodium chloride in growth medium while 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 G. mellonella infection model while mutation of ompR had no effect on virulence of the translucent variant. IMPORTANCE A. baumannii is a well-known antibiotic resistant pathogen and many clinical isolates can only be treated by a very small number of antibiotics (including colistin) while some exhibit pan-resistance. 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, it 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 greater 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 attenuation of virulence in an invertebrate infection model.