Multiple roles for a novel RND‐type efflux system in Acinetobacter baumannii AB5075

Colony opacity phase variation in Acinetobacter baumannii strain AB5075 is regulated by a reversible high‐frequency switch. Transposon mutagenesis was used to generate mutations that decreased the opaque to translucent switch and a gene encoding a predicted periplasmic membrane fusion component of a resistance–nodulation–cell division (RND)‐type efflux system was isolated. This gene was designated arpA and immediately downstream was a gene designated arpB that encodes a predicted membrane transporter of RND‐type systems. A nonpolar, in‐frame deletion in arpA resulted in a 70‐fold decrease in the opaque to translucent switch. An arpB::Tc mutant exhibited a 769‐fold decrease in the opaque to translucent switch. However, the translucent to opaque switch was largely unchanged in both the arpA and arpB mutants. The arpA and arpB mutants also exhibited increased surface motility in the opaque form and the arpB mutant exhibited increased susceptibility to aminoglycosides. The arpA and arpB mutants were both attenuated in a Galleria mellonella model of virulence. A divergently transcribed TetR‐type regulator ArpR was capable of repressing the arpAB operon when this TetR regulator was overexpressed. The arpR gene was also involved in regulating the opaque to translucent switch as an in‐frame arpR mutation decreased this switch by 1,916‐fold.