Abstract
The outer membrane (OM) of Gram-negative bacteria serves as a selective permeability barrier that allows entry of essential nutrients while excluding toxic compounds, including antibiotics. The OM is asymmetric and contains an outer leaflet of lipopolysaccharides (LPS) or lipooligosaccharides (LOS) and an inner leaflet of glycerophospholipids (GPL). We screened Acinetobacter baumannii transposon mutants and identified a number of mutants with OM defects, including an ABC transporter system homologous to the Mla system in E. coli. We further show that this opportunistic, antibiotic-resistant pathogen uses this multicomponent protein complex and ATP hydrolysis at the inner membrane to promote GPL export to the OM. The broad conservation of the Mla system in Gram-negative bacteria suggests the system may play a conserved role in OM biogenesis. The importance of the Mla system to Acinetobacter baumannii OM integrity and antibiotic sensitivity suggests that its components may serve as new antimicrobial therapeutic targets.
Highlights
Gram-negative bacteria are enveloped by two lipid bilayers, separated by an aqueous periplasmic space containing a peptidoglycan cell wall
This conclusion is based on the observation that newly synthesized GPLs accumulate at the IM of mla mutants, akin to how LPS molecules accumulate at the inner membrane in bacteria with mutations in the lpt genes encoding the LPS ABC transport system
We identified strains with mutations in genes required for maintenance of the Acinetobacter baumannii outer membrane (OM) barrier by screening transposon mutants for the development of a blue colony phenotype on agar plates containing the chromogenic substrate BCIP-Toluidine
Summary
Cassandra Kamischke[1], Junping Fan[1], Julien Bergeron[4,5], Hemantha D. 5 Kulasekara[1], Zachary D.
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