Role of AmiA in the Morphological Transition of Helicobacter pylori and in Immune Escape

Catherine Chaput,Ivo G Boneca,Nadège Cayet,Stéphanie Guadagnini,Catherine Werts,Agnès Labigne,Stephen E Girardin,Chantal Ecobichon,Dominique Mengin-Lecreulx,Marie-Christine Prévost

doi:10.1371/journal.ppat.0020097

Abstract

The human gastric pathogen Helicobacter pylori is responsible for peptic ulcers and neoplasia. Both in vitro and in the human stomach it can be found in two forms, the bacillary and coccoid forms. The molecular mechanisms of the morphological transition between these two forms and the role of coccoids remain largely unknown. The peptidoglycan (PG) layer is a major determinant of bacterial cell shape, and therefore we studied H. pylori PG structure during the morphological transition. The transition correlated with an accumulation of the N-acetyl-D-glucosaminyl-β(1,4)-N-acetylmuramyl-L-Ala–D-Glu (GM-dipeptide) motif. We investigated the molecular mechanisms responsible for the GM-dipeptide motif accumulation, and studied the role of various putative PG hydrolases in this process. Interestingly, a mutant strain with a mutation in the amiA gene, encoding a putative PG hydrolase, was impaired in accumulating the GM-dipeptide motif and transforming into coccoids. We investigated the role of the morphological transition and the PG modification in the biology of H. pylori. PG modification and transformation of H. pylori was accompanied by an escape from detection by human Nod1 and the absence of NF-κB activation in epithelial cells. Accordingly, coccoids were unable to induce IL-8 secretion by AGS gastric epithelial cells. amiA is, to our knowledge, the first genetic determinant discovered to be required for this morphological transition into the coccoid forms, and therefore contributes to modulation of the host response and participates in the chronicity of H. pylori infection.

Highlights

Helicobacter pylori is a human pathogen with an unique niche: the stomach
We describe the construction of a mutant strain with a mutation of the amiA gene—encoding a putative PG hydrolase—that is impaired in the accumulation of the GM-dipeptide motif; it is defective in the transition from spiral bacteria into coccoid forms
Epithelial Cell Response to H. pylori PG and Coccoid Forms Having demonstrated that the transition into coccoid forms is a process controlled by AmiA, we investigated the biological role of the coccoid forms

Summary

Introduction

Helicobacter pylori is a human pathogen with an unique niche: the stomach. The presence of this bacterium is always associated with chronic gastritis, and less often with severe duodenal ulcers, gastric adenocarcinoma, or mucosa-associated lymphoid tissue lymphoma. The coccoid forms appear in stationary phase and can be induced under stress conditions, for example, following modification of pH, O2 tension, or temperature [1,2], or exposure to antibiotics such as amoxicillin [3,4]. There is still controversy regarding the biological role of this form. Both forms are commonly observed in the human stomach [5,6]. Several groups have reported colonization of mice with coccoid bacteria and have subsequently isolated spiral bacteria from their stomachs, indicating that under certain conditions coccoids may revert back to spiral bacteria [16,17,18,19]

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