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

Abstract

The number of nosocomial infections with multidrug-resistant (MDR) strains of Acinetobacter baumannii (Ab) rises worldwide. Hence, MDR Ab became the leading pathogen for which the development of novel antibiotics is of utmost importance. The extreme antibiotic resistance of Ab is especially provided by the composition of its outer membrane (OM). Tightly regulated and specific outer membrane proteins (OMPs) contribute to the efficient barrier function as well as the virulence of Ab. Upon biogenesis, nascent OMPs are guided through the periplasm to the β-barrel assembly machinery (BAM) by periplasmic chaperones. The BAM finally enables insertion of OMPs into the OM. In most Gram-negative bacteria SurA, Skp and DegP represent the major periplasmic chaperones. In Escherichia coli (Ec), Yersinia enterocolitica (Ye) as well as Pseudomonas aeruginosa (Pa) a reduced OM integrity, as well as an increased susceptibility against antibiotics could be observed upon the deletion of SurA. Therefore, the aim of this work was to determine the relevance of SurA and the two other periplasmic chaperones, Skp and DegP in Ab. Markerless single gene knockout mutants for surA, skp and degP were created in AB5075, a highly virulent MDR Ab strain, and analyzed with regards to consequences on OM integrity, antibiotic susceptibility and virulence. In contrast to what has been observed for other Gram-negative pathogens, in Ab only weak phenotypes resulted. Also, the concurrent lack of surA + skp, surA + degP, and skp + degP, which is lethal in Ec, was tolerated well by AB5075. Most surprisingly, even a triple knockout strain, lacking all three periplasmic chaperones was viable. A mass spectrometric analysis revealed no significant reduction in the abundance of the major OMPs of Ab including OmpA and CarO. Thus, our findings suggest the existence of mechanisms that allow Ab to tolerate or compensate for the concurrent lack of the three periplasmic chaperones SurA, Skp and DegP.