Increased Osmolarity in Biofilm Triggers RcsB-Dependent Lipid A Palmitoylation in Escherichia coli.

Christophe Beloin,Matthew R Parsek,Jean-Marc Ghigo,Magdalena Szczesny

doi:10.1128/mbio.01415-18

Abstract

ABSTRACTBiofilms are often described as protective shelters that preserve bacteria from hostile surroundings. However, biofilm bacteria are also exposed to various stresses and need to adjust to the heterogeneous physicochemical conditions prevailing within biofilms. In Gram-negative bacteria, such adaptations can result in modifications of the lipopolysaccharide, a major component of the outer membrane characterized by a highly dynamic structure responding to environmental changes. We previously showed that Gram-negative biofilm bacteria undergo an increase in lipid A palmitoylation mediated by the PagP enzyme, contributing to increased resistance to host defenses. Here we describe a regulatory pathway leading to transcriptional induction of pagP in response to specific conditions created in the biofilm environment. We show that pagP expression is induced via the Rcs envelope stress system independently of the Rcs phosphorelay cascade and that it requires the GadE auxiliary regulator. Moreover, we identify an increase in osmolarity (i.e., ionic stress) as a signal able to induce pagP expression in an RcsB-dependent manner. Consistently, we show that the biofilm is a hyperosmolar environment and that RcsB-dependent pagP induction can be dampened in the presence of an osmoprotectant. These results provide new insights into the adaptive mechanisms of bacterial differentiation in biofilm.

Highlights

Biofilms are often described as protective shelters that preserve bacteria from hostile surroundings
We previously reported that E. coli biofilm bacteria show a higher level of lipid A palmitoylation on LPS gels than planktonic bacteria
To further confirm the correlation between biofilm formation and transcriptional induction of pagP expression in biofilm, we compared the levels of pagP expression in BW25113 and MG1655 E. coli K-12 backgrounds carrying or not the biofilm-promoting F conjugative plasmid [18]

Summary

Introduction

Biofilms are often described as protective shelters that preserve bacteria from hostile surroundings. We show that the biofilm is a hyperosmolar environment and that RcsB-dependent pagP induction can be dampened in the presence of an osmoprotectant These results provide new insights into the adaptive mechanisms of bacterial differentiation in biofilm. We investigated the mechanisms of a widespread and biofilm-associated chemical modification of the lipopolysaccharide (LPS), a major component of all Gram-negative bacterial outer membranes This modification corresponds to the incorporation, mediated by the enzyme PagP, of a palmitate chain into lipid A (palmitoylation) that reduces bacterial recognition by host immune responses. Bacteria have evolved different mechanisms enabling them to sense and respond to nutrient and stress variations This capacity for adaptation is important during the development of highly heterogeneous biofilms, in which bacteria are exposed to a variety of physicochemical gradients rarely encountered by free-living organisms [1]. This report provides new insights into how physiological adaptations to the local biofilm microenvironment can contribute to increased stress resistance in biofilm bacteria

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Conclusion