Deletion of Lytic Transglycosylases Increases Beta-Lactam Resistance in Shewanella oneidensis.

Jianhua Yin,Yiyang Sun,Juanping Qiu,Yijuan Sun,Haichun Gao,Zhiliang Yu

doi:10.3389/fmicb.2018.00013

Abstract

Production of chromosome-encoded β-lactamases confers resistance to β-lactams in many Gram-negative bacteria. Some inducible β-lactamases, especially the class C β-lactamase AmpC in Enterobacteriaceae, share a common regulatory mechanism, the ampR-ampC paradigm. Induction of ampC is intimately linked to peptidoglycan recycling, and the LysR-type transcriptional regulator AmpR plays a central role in the process. However, our previous studies have demonstrated that the expression of class D β-lactamase gene blaA in Shewanella oneidensis is distinct from the established paradigm since an AmpR homolog is absent and major peptidoglycan recycling enzymes play opposite roles in β-lactamase expression. Given that lytic transglycosylases (LTs), a class of peptidoglycan hydrolases cleaving the β-1,4 glycosidic linkage in glycan strands of peptidoglycan, can disturb peptidoglycan recycling, and thus may affect induction of blaA. In this study, we investigated impacts of such enzymes on susceptibility to β-lactams. Deletion of three LTs (SltY, MltB and MltB2) increased β-lactam resistance, while four other LTs (MltD, MltD2, MltF, and Slt2) seemed dispensable to β-lactam resistance. The double LT mutants ΔmltBΔmltB2 and ΔsltYΔmltB2 had β-lactam resistance stronger than any of the single mutants. Deletion of ampG (encoding permease AmpG) and mrcA (encoding penicillin binding protein 1a, PBP1a) from both double LT mutants further increased the resistance to β-lactams. Notably, all increased β-lactam resistance phenotypes were in accordance with enhanced blaA expression. Although significant, the increase in β-lactamase activity after inactivating LTs is much lower than that produced by PBP1a inactivation. Our data implicate that LTs play important roles in blaA expression in S. oneidensis.

Highlights

Beta-lactam antibiotics are the most widely used group of antibiotics, they target the penicillin-binding proteins (PBPs), eventually disrupting the peptidoglycan synthesis
SltY and MltF share 29 and 41% sequence identity with E. coli Slt70 and MltF, respectively. Both MltB and SO1994 are E. coli MltB homologs (36 and 35% identity respectively); both SO2564 and MltD are homologous with E. coli MltD (34 and 33% identity respectively). These results indicate that S. oneidensis may produce two MltB and MltD isozymes
The regulatory mechanism for blaA induction in S. oneidensis is distinct from the ampR-ampC paradigm

Summary

Introduction

Beta-lactam antibiotics are the most widely used group of antibiotics, they target the penicillin-binding proteins (PBPs), eventually disrupting the peptidoglycan synthesis. To combat these antibiotics, microorganisms have evolved multiple resistance mechanisms, including the direct inactivation or modification of antibiotics, protection of antibiotic targets, overexpression of drug efflux pumps, and reduction of permeability of the outer membrane (Blair et al, 2015). In Gram-negative bacteria, the production of β-lactamases is the predominant strategy of resistance to β-lactams. These enzymes, which resemble PBPs structurally and mechanistically, have ability to rapidly hydrolyze the β-lactams (Nicholas and Davies, 2012). Two cytoplasmic peptidoglycan intermediates can act as regulatory ligands for ampC induction by binding to AmpR, resulting in either activation or repression of ampC expression

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