Osmolytes Contribute to pH Homeostasis of Escherichia coli

Ryan D Kitko,Gian M Garduque,Jessica C Wilks,Joan L Slonczewski,Stefan Bereswill

doi:10.1371/journal.pone.0010078

Abstract

BackgroundCytoplasmic pH homeostasis in Escherichia coli includes numerous mechanisms involving pH-dependent catabolism and ion fluxes. An important contributor is transmembrane K+ flux, but the actual basis of K+ compensation for pH stress remains unclear. Osmoprotection could mediate the pH protection afforded by K+ and other osmolytes.Methods and Principal FindingsThe cytoplasmic pH of E. coli K-12 strains was measured by GFPmut3 fluorimetry. The wild-type strain Frag1 was exposed to rapid external acidification by HCl addition. Recovery of cytoplasmic pH was enhanced equally by supplementation with NaCl, KCl, proline, or sucrose. A triple mutant strain TK2420 defective for the Kdp, Trk and Kup K+ uptake systems requires exogenous K+ for steady-state pH homeostasis and for recovery from sudden acid shift. The K+ requirement however was partly compensated by supplementation with NaCl, choline chloride, proline, or sucrose. Thus, the K+ requirement was mediated in part by osmolarity, possibly by relieving osmotic stress which interacts with pH stress. The rapid addition of KCl to strain TK2420 suspended at external pH 5.6 caused a transient decrease in cytoplasmic pH, followed by slow recovery to an elevated steady-state pH. In the presence of 150 mM KCl, however, rapid addition of another 150 mM KCl caused a transient increase in cytoplasmic pH. These transient effects may arise from secondary K+ fluxes occurring through other transport processes in the TK2420 strain.ConclusionsDiverse osmolytes including NaCl, KCl, proline, or sucrose contribute to cytoplasmic pH homeostasis in E. coli, and increase the recovery from rapid acid shift. Osmolytes other than K+ restore partial pH homeostasis in a strain deleted for K+ transport.

Highlights

The enteric neutralophile E. coli maintains a cytoplasmic pH within a narrow range, approximately pH 7.4 to 7.8, when grown over a large range of environmental pH from pH 5 to 9 [1,2,3]
Osmolytes other than K+ restore partial pH homeostasis in a strain deleted for K+ transport
The effect of osmolytes on the cytoplasmic pH is important to understanding the survival of food pathogens such as E. coli O157, whose survival in extreme acid is enhanced by high NaCl [9]

Summary

Introduction

The enteric neutralophile E. coli maintains a cytoplasmic pH within a narrow range, approximately pH 7.4 to 7.8, when grown over a large range of environmental pH from pH 5 to 9 [1,2,3]. The regulation of cytoplasmic pH during acid stress depends on catabolic acid consumption and ion transport [5]. The effect of osmolytes on the cytoplasmic pH is important to understanding the survival of food pathogens such as E. coli O157, whose survival in extreme acid is enhanced by high NaCl [9]. Both E. coli and Salmonella enterica show prolonged survival in model acidic food broths with NaCl concentrations up to 4% [10,11]. Cytoplasmic pH homeostasis in Escherichia coli includes numerous mechanisms involving pH-dependent catabolism and ion fluxes. Osmoprotection could mediate the pH protection afforded by K+ and other osmolytes

Methods

Results

Conclusion