Contribution of the Locus of Heat Resistance to Growth and Survival of Escherichia coli at Alkaline pH and at Alkaline pH in the Presence of Chlorine.

Tongbo Zhu,Michael G. Gänzle,David J. Simpson,Lynn M. McMullen,Tracy Raivio,Zhiying Wang

doi:10.3390/microorganisms9040701

Abstract

The locus of heat resistance (LHR) confers resistance to extreme heat, chlorine and oxidative stress in Escherichia coli. This study aimed to determine the function of the LHR in maintaining bacterial cell envelope homeostasis, the regulation of the genes comprising the LHR and the contribution of the LHR to alkaline pH response. The presence of the LHR did not affect the activity of the Cpx two-component regulatory system in E. coli, which was measured to quantify cell envelope stress. The LHR did not alter E. coli MG1655 growth rate in the range of pH 6.9 to 9.2. However, RT-qPCR results indicated that the expression of the LHR was elevated at pH 8.0 when CpxR was absent. The LHR did not improve survival of E. coli MG1655 at extreme alkaline pH (pH = 11.0 to 11.2) but improved survival at pH 11.0 in the presence of chlorine. Therefore, we conclude that the LHR confers resistance to extreme alkaline pH in the presence of oxidizing agents. Resistance to alkaline pH is regulated by an endogenous mechanism, including the Cpx envelope stress response, whereas the LHR confers resistance to extreme alkaline pH only in the presence of additional stress such as chlorine.

Highlights

Escherichia coli are commensals in the intestine of humans and animals, but the species includes pathogenic strains that cause infections of the gastrointestinal and urinary tracts [1,2]
In the Enterobacterales, the locus of heat resistance (LHR) has to date only been identified in the family Enterobacteriaceae and the genus Yersinia [12]
Expression of the LHR was measured by quantification of mRNA of the genes orf1, yfdX1GI and kefBGI with RT-qPCR by using MG1655 lacZ::LHR as reference

Summary

Introduction

Escherichia coli are commensals in the intestine of humans and animals, but the species includes pathogenic strains that cause infections of the gastrointestinal and urinary tracts [1,2]. Food contamination by pathogenic E. coli may occur at any step of the farmto-fork continuum and is a significant contributor to foodborne disease [3,4]. Alkaline-chlorinated sanitizers are used to sanitize food contact surfaces [5]. The chlorine and alkaline treatments function through synergistic mechanisms. Hypochlorous acid (HOCl), the active component of chlorine, oxidizes cellular components and permeabilizes the cytoplasmic membrane [6,7,8]. Extreme alkaline pH denatures proteins and results in membrane permeabilization [9].

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