Date Published: March 17, 2017
Publisher: Public Library of Science
Author(s): Hong Li, Tiandi Yang, Tingting Liao, Aleksandra W. Debowski, Hans-Olof Nilsson, Alma Fulurija, Stuart M. Haslam, Barbara Mulloy, Anne Dell, Keith A. Stubbs, Barry J. Marshall, Mohammed Benghezal, Karla J. F. Satchell.
Helicobacter pylori lipopolysaccharide promotes chronic gastric colonisation through O-antigen host mimicry and resistance to mucosal antimicrobial peptides mediated primarily by modifications of the lipid A. The structural organisation of the core and O-antigen domains of H. pylori lipopolysaccharide remains unclear, as the O-antigen attachment site has still to be identified experimentally. Here, structural investigations of lipopolysaccharides purified from two wild-type strains and the O-antigen ligase mutant revealed that the H. pylori core-oligosaccharide domain is a short conserved hexasaccharide (Glc-Gal-DD-Hep-LD-Hep-LD-Hep-KDO) decorated with the O-antigen domain encompassing a conserved trisaccharide (-DD-Hep-Fuc-GlcNAc-) and variable glucan, heptan and Lewis antigens. Furthermore, the putative heptosyltransferase HP1284 was found to be required for the transfer of the third heptose residue to the core-oligosaccharide. Interestingly, mutation of HP1284 did not affect the ligation of the O-antigen and resulted in the attachment of the O-antigen onto an incomplete core-oligosaccharide missing the third heptose and the adjoining Glc-Gal residues. Mutants deficient in either HP1284 or O-antigen ligase displayed a moderate increase in susceptibility to polymyxin B but were unable to colonise the mouse gastric mucosa. Finally, mapping mutagenesis and colonisation data of previous studies onto the redefined organisation of H. pylori lipopolysaccharide revealed that only the conserved motifs were essential for colonisation. In conclusion, H. pylori lipopolysaccharide is missing the canonical inner and outer core organisation. Instead it displays a short core and a longer O-antigen encompassing residues previously assigned as the outer core domain. The redefinition of H. pylori lipopolysaccharide domains warrants future studies to dissect the role of each domain in host-pathogen interactions. Also enzymes involved in the assembly of the conserved core structure, such as HP1284, could be attractive targets for the design of new therapeutic agents for managing persistent H. pylori infection causing peptic ulcers and gastric cancer.
Helicobacter pylori is well-adapted to survival in the human stomach mucosa and establishes persistent infection, which causes chronic gastritis and can lead to peptic ulcer disease and gastric adenocarcinoma [1,2]. Lipopolysaccharide (LPS), a highly acylated glycolipid compactly anchored in the outer leaflet of the outer membrane (OM), is a key factor for H. pylori to establish colonisation and persistence in the gastric niche [3–7].
In this study, a combination of genetic and structural analysis of H. pylori LPS from wild-type and mutant strains enabled the experimental identification of the O-antigen attachment site and the precise assignment of the core-oligosaccharide and O-antigen domains. In addition, HP1284 is proposed to encode the Hep III transferase of the LPS core domain, based on structural analysis of corresponding mutant LPS in two strains, sequence homology to the Hep III transferase WaaQ of H. influenza, and the reduction in the size of the core-oligosaccharide of the double HP1284/waaL mutant compared to the single waaL mutant. Deletion of HP1284 and the O-antigen ligase waaL led to a moderate decrease in resistance to polymyxin B and loss of colonisation in the mouse model.