A penicillin-binding protein inhibits selection of colistin-resistant, lipooligosaccharide-deficient Acinetobacter baumannii

Significance

Antimicrobial drug resistance is a major threat to public health. Gram-negative bacteria are exceptionally resistant to antibiotics because of their outer-membrane barrier. Glycolipids called lipopolysaccharide (LPS) or lipooligosaccharide (LOS) fortify the outer membrane from many antimicrobials and biocides and were thought to be essential for Gram-negative bacterial survival. The last-resort treatment for multidrug-resistant Gram-negative infections is colistin, which targets the lipid A domain of LPS/LOS to disrupt the membrane, but the emerging pathogen Acinetobacter baumannii can develop colistin resistance by inactivating lipid A biosynthesis. This analysis advances our understanding of lipid A/LOS essentiality in A. baumannii and identifies antimicrobial targets.

Abstract

The Gram-negative bacterial outer membrane fortifies the cell against environmental toxins including antibiotics. Unique glycolipids called lipopolysaccharide/lipooligosaccharide (LPS/LOS) are enriched in the cell-surface monolayer of the outer membrane and promote antimicrobial resistance. Colistin, which targets the lipid A domain of LPS/LOS to lyse the cell, is the last-line treatment for multidrug-resistant Gram-negative infections. Lipid A is essential for the survival of most Gram-negative bacteria, but colistin-resistant Acinetobacter baumannii lacking lipid A were isolated after colistin exposure. Previously, strain ATCC 19606 was the only A. baumannii strain demonstrated to subsist without lipid A. Here, we show that other A. baumannii strains can also survive without lipid A, but some cannot, affording a unique model to study endotoxin essentiality. We assessed the capacity of 15 clinical A. baumannii isolates including 9 recent clinical isolates to develop colistin resistance through inactivation of the lipid A biosynthetic pathway, the products of which assemble the LOS precursor. Our investigation determined that expression of the well-conserved penicillin-binding protein (PBP) 1A, prevented LOS-deficient colony isolation. The glycosyltransferase activity of PBP1A, which aids in the polymerization of the peptidoglycan cell wall, was lethal to LOS-deficient A. baumannii. Global transcriptomic analysis of a PBP1A-deficient mutant and four LOS-deficient A. baumannii strains showed a concomitant increase in transcription of lipoproteins and their transporters. Examination of the LOS-deficient A. baumannii cell surface demonstrated that specific lipoproteins were overexpressed and decorated the cell surface, potentially compensating for LOS removal. This work expands our knowledge of lipid A essentiality and elucidates a drug resistance mechanism.