Colistin Dependence in Extensively Drug-Resistant Acinetobacter baumannii Strain Is Associated with ISAjo2 and ISAba13 Insertions and Multiple Cellular Responses

Abstract

The nosocomial opportunistic Gram-negative bacterial pathogen Acinetobacter baumannii is resistant to multiple antimicrobial agents and an emerging global health problem. The polymyxin antibiotic colistin, targeting the negatively charged lipid A component of the lipopolysaccharide on the bacterial cell surface, is often considered as the last-resort treatment, but resistance to colistin is unfortunately increasing worldwide. Notably, colistin-susceptible A. baumannii can also develop a colistin dependence after exposure to this drug in vitro. Colistin dependence might represent a stepping stone to resistance also in vivo. However, the mechanisms are far from clear. To address this issue, we combined proteogenomics, high-resolution microscopy, and lipid profiling to characterize and compare A. baumannii colistin-susceptible clinical isolate (Ab-S) of to its colistin-dependent subpopulation (Ab-D) obtained after subsequent passages in moderate colistin concentrations. Incidentally, in the colistin-dependent subpopulation the lpxA gene was disrupted by insertion of ISAjo2, the lipid A biosynthesis terminated, and Ab-D cells displayed a lipooligosaccharide (LOS)-deficient phenotype. Moreover, both mlaD and pldA genes were perturbed by insertions of ISAjo2 and ISAba13, and LOS-deficient bacteria displayed a capsule with decreased thickness as well as other surface imperfections. The major changes in relative protein abundance levels were detected in type 6 secretion system (T6SS) components, the resistance-nodulation-division (RND)-type efflux pumps, and in proteins involved in maintenance of outer membrane asymmetry. These findings suggest that colistin dependence in A. baumannii involves an ensemble of mechanisms seen in resistance development and accompanied by complex cellular events related to insertional sequences (ISs)-triggered LOS-deficiency. To our knowledge, this is the first study demonstrating the involvement of ISAjo2 and ISAba13 IS elements in the modulation of the lipid A biosynthesis and associated development of dependence on colistin.

Novel Small Molecule Growth Inhibitor Affecting Bacterial Outer Membrane Reduces Extraintestinal Pathogenic Escherichia coli (ExPEC) Infection in Avian Model

“…Decreased levels of BamA and multiple other OM proteins were also observed with JB-95 (LptD inhibitor) treatment in E. coli ( 22 ). Similarly, downregulation of MlaA and LptD proteins was observed concurrently in LPS-deficient Acinetobacter baumannii ( 53 ).…”

A subclass of the IS1202family of bacterial insertion sequences targets XerCD recombination sites

“…For example, it was observed (15) that an IS 1202 -related IS was implicated in the deletion of the capsular polysaccharide locus ( cps ), the major known Streptococcus pneumoniae virulence factor, important for Streptococcal survival in the blood and strongly associated with antiphagocytic activity (16). While Chamoun et al (17) showed that insertion of IS Ajo2 into the lpxA gene, involved in lipid A biosynthesis, can result in colistin dependence which, they suggest, possibly leads to colistin resistance.…”

Maintenance of bacterial outer membrane lipid asymmetry: insight into MlaA

“…N. gonorrhoea strains from geographically distinct areas and different years (clinical isolates from Baltimore, 1991–1994, clinical isolates from Seattle, 2011–2013 and WHO 2016 reference strains) have different MlaA levels, whereas many clinical isolates of non-typeable H. influenzae have lower levels of MlaA [ 14 , 63 ]. Furthermore, in a study comparing an A. baumannii colistin-susceptible clinical isolate to its colistin-dependent subpopulation, downregulated expression of proteins involved in the maintenance of OM asymmetry, including MlaA, was observed in the latter isolates [ 102 ].…”