Lipopolysaccharide modification in Gram-negative bacteria during chronic infection

Abstract

The Gram-negative bacterial lipopolysaccharide (LPS) is a major component of the outer membrane that plays a key role in host-pathogen interactions with the innate immune system. During infection, bacteria are exposed to a host environment that is typically dominated by inflammatory cells and soluble factors, including antibiotics, which provide cues about regulation of gene expression. Bacterial adaptive changes including modulation of LPS synthesis and structure are a conserved theme in infections, irrespective of the type or bacteria or the site of infection. In general, these changes result in immune system evasion, persisting inflammation and increased antimicrobial resistance. Here, we review the modifications of LPS structure and biosynthetic pathways that occur upon adaptation of model opportunistic pathogens (Pseudomonas aeruginosa, Burkholderia cepacia complex bacteria, Helicobacter pylori and Salmonella enterica) to chronic infection in respiratory and gastrointestinal sites. We also discuss the molecular mechanisms of these variations and their role in the host-pathogen interaction.

Keywords: Burkholderia cenocepacia; Helicobacter pylori; O antigen; Pseudomonas aeruginosa; adaptive mutation; cystic fibrosis; gastric ulcer; lipid A.

Graphical Abstract Figure.

The authors review modifications of lipopolysaccharide structure and biosynthetic pathways that occur upon bacterial adaptation to chronic respiratory and gastrointestinal infections.

Figure 1.

Cell envelope organization of Gram-negative bacteria. The cell envelope of Gram-negative bacteria is characterized by the presence of two lipid bilayers: the outer membrane (OM) and the cytoplasmic membrane (CM), which are separated by the periplasm, containing hydrolytic enzymes, binding proteins, chemoreceptors and the peptidoglycan cell wall. The OM is an asymmetric lipid bilayer. The outer leaflet of the OM contains mainly LPS molecules, which form contacts with integral outer membrane proteins (OMPs). The inner layer of the OM and the lipid layers of the cytoplasmic membrane contain phospholipids and membrane proteins.

Figure 2.

Simplified overview of the LPS biosynthesis. Lipid A-Kdo₂ is synthesized on the cytoplasmic surface of the cytoplasmic membrane. The rest of the core is assembled to the lipid A-Kdo₂ and MsbA flips the whole complex to the periplasmic side of the cytoplasmic membrane. The O antigen is synthesized by cytoplasmic membrane-associated enzyme complexes using C55 Und-P as an acceptor for chain assembly and is then flipped to the periplasmic face of the membrane by one of the three pathways: (1) Wzy dependent, (2) ABC transporter dependent or (3) synthase dependent. For simplicity, only the ABC-transporter pathway is represented. Once on the periplasmic side, the O antigen is linked to the lipid A-core by the WaaL ligase and the mature LPS molecule is then transported across the periplasm and inserted into the outer leaflet of the outer membrane by the Lpt (LPS transport) system, a complex that spans the Gram-negative cell envelope to deliver LPS to the outer membrane (E). OM, outer membrane; CM, cytoplasmic membrane.

Figure 3.

Lipid A modifications occurring in P. aeruginosa during adaptation to long-term chronic infection. The basic tetra-acylated lipid A structure can be modified by: deacylation by PagL; palmitoylation by PagP; acylation by HtrB; acylation by LpxO; addition of Arap4N by PmrAB on position 1 or 4′; and addition of phosphoethanolamine by ColRS on position 1 or 4′.

Figure 4.

Lewis antigen structures. Helicobacter pylori can produce type 1 (based on a β-(1,3)-linked galactose-GlcNAc sugar backbone) and type 2 (based on a β-(1,4)-linked galactose-GlcNAc sugar backbone) Lewis antigens. Le^a and Le^x are built by addition of a fucose residue to the GlcNAc sugar of the type 1 and type 2 backbone, through α-(1,4) or α-(1,3) linkages, respectively. Le^b and Le^Y are built by addition of a fucose residue through α-(1,2) linkage to Le^a and Le^x structures, respectively. Sialyl-Le^x (SLe^x) is built by addition of a sialyl group to the Le^x antigen by a α-(2,3) linkage.