Indole Pulse Signalling Regulates the Cytoplasmic pH of E. coli in a Memory-Like Manner

Ashraf Zarkan,Jehangir Cama,David K Summers,Jinbo Zhu,Ulrich F Keyser,Santiago Caño-Muñiz,Kareem Al Nahas

doi:10.1038/s41598-019-40560-3

Ashraf Zarkan, Jehangir Cama + Show 5 more

Open Access

https://doi.org/10.1038/s41598-019-40560-3

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Abstract

Bacterial cells are critically dependent upon pH regulation. Here we demonstrate that indole plays a critical role in the regulation of the cytoplasmic pH of Escherichia coli. Indole is an aromatic molecule with diverse signalling roles. Two modes of indole signalling have been described: persistent and pulse signalling. The latter is illustrated by the brief but intense elevation of intracellular indole during stationary phase entry. We show that under conditions permitting indole production, cells maintain their cytoplasmic pH at 7.2. In contrast, under conditions where no indole is produced, the cytoplasmic pH is near 7.8. We demonstrate that pH regulation results from pulse, rather than persistent, indole signalling. Furthermore, we illustrate that the relevant property of indole in this context is its ability to conduct protons across the cytoplasmic membrane. Additionally, we show that the effect of the indole pulse that occurs normally during stationary phase entry in rich medium remains as a “memory” to maintain the cytoplasmic pH until entry into the next stationary phase. The indole-mediated reduction in cytoplasmic pH may explain why indole provides E. coli with a degree of protection against stresses, including some bactericidal antibiotics.

Highlights

Bacteria have evolved to grow over a wide range of external pH, while maintaining their cytoplasmic pH within a narrow range; a phenomenon known as pH homeostasis[1]
To test whether the difference was due to some component present in the complex (LB) medium but absent in M9/Glucose, trace elements, Vitamin B1 and casamino acids were added to M9 but the cytoplasmic pH remained unchanged at 7.8
Our experiments demonstrate the existence of an indole-mediated mechanism for the regulation of cytoplasmic pH in E. coli

Summary

Introduction

Bacteria have evolved to grow over a wide range of external pH, while maintaining their cytoplasmic pH within a narrow range; a phenomenon known as pH homeostasis[1]. The fluorescence distributions of pHluorin and mCherry show single peaks for both the WT (Fig. 2a) and the ∆tnaA mutant samples (Fig. 2b) suggesting relative uniformity of cytoplasmic pH among cells within each culture. To investigate whether indole persistent signalling was responsible for regulating the cytoplasmic pH of E. coli, cultures of BW25113 ∆tnaA in LB were supplemented with a range of indole concentrations from 30 μM to 0.5 mM, and the cytoplasmic pH was measured using the fluorimeter method.

Results

Conclusion