The role of the periplasmic chaperones SurA, Skp and DegP for fitness, outer membrane integrity and virulence of Acinetobacter baumannii AB5075: same-same, but different?

The outer membrane (OM) of Gram-negative bacteria efficiently protects from harmful environmental influences such as antibiotics, disinfectants or dryness. A main constituent of the OM are integral OM ß-barrel proteins (OMPs) and lipopolysaccharide (LPS), or lipooligosaccharide (LOS) in the case of Acinetobacter baumannii (Ab). In Gram-negative bacteria such as E. coli (Ec), Y. enterocolitica (Ye) and P. aeruginosa (Pa), both the insertion of OMPs and LPS/LOS depend directly or indirectly on a sophisticated protein biogenesis pathway. This pathway comprises the SecYEG translocon enabling inner membrane (IM) passage, the chaperones SurA, Skp and DegP that facilitate the passage of ß-barrel OMPs through the periplasm, and the ß-barrel assembly machinery (BAM) complex that facilitates insertion into the OM. In Ec, Ye and Pa especially the deletion of SurA leads to a loss of OM integrity resulting in sensitization to antibiotics, and reduced virulence. In search of targets that could be exploited to develop compounds that interfere with OM integrity in Ab, we employed the multidrug-resistant strain AB5075 to generate single gene knockout strains lacking individual periplasmic chaperones. In contrast to Ec, Ye and Pa, AB5075 tolerates the lack of SurA, Skp or DegP with only weak phenotypes. Unlike in Ec, where knockouts of SurA+Skp and SurA+DegP are conditionally lethal, all double knockouts were well tolerated by AB5075. Strikingly, even a triple knockout strain of AB5075, lacking SurA+Skp+DegP, was viable.