The Mla pathway in Acinetobacter baumannii has no demonstrable role in anterograde lipid transport

Abstract

The asymmetric outer membrane (OM) of Gram-negative bacteria functions as a selective permeability barrier to the environment. Perturbations to OM lipid asymmetry sensitize the cell to antibiotics. As such, mechanisms involved in lipid asymmetry are fundamental to our understanding of OM lipid homeostasis. One such mechanism, the Maintenance of lipid asymmetry (Mla) pathway has been proposed to extract mislocalized glycerophospholipids from the outer leaflet of the OM and return them to the inner membrane (IM). Work on this pathway inAcinetobacter baumanniisupport conflicting models for the directionality of the Mla system being retrograde (OM to IM) or anterograde (IM to OM). Here, we show conclusively thatA. baumannii mlamutants exhibit no defects in anterograde transport. Furthermore, we identify an allele of the GTPaseobgEthat is synthetically sick in the absence of Mla; providing another link between cell envelope homeostasis and stringent response.