Structural elucidation of the outer core tetrasaccharide isolated from the LPS of Rhizobium leguminosarum bv. trifolii strain 24

Characterization of Isomers of Lipid A from Pseudomonas aeruginosa PAO1 by Liquid Chromatography with Tandem Mass Spectrometry with Higher-Energy Collisional Dissociation and Ultraviolet Photodissociation

Lipopolysaccharides (LPS) constitute the outermost layer of Gram-negative bacteria and consequently play an important role in bacterial infections. In order to address public health issues posed by Gram-negative bacteria, it is necessary to elucidate the structure of the molecular actors at the forefront of infections. LPS virulence and toxicity are partially modulated by lipid A, a hydrophobic saccharolipid that anchors LPS to the bacterial outer membrane. Understanding the lipid A structure is inherently intertwined with understanding its role as an endotoxin. Accordingly, several successful strategies incorporating tandem mass spectrometry have been applied toward the structural analysis of lipid A. Herein, a shotgun HCD strategy was applied toward the characterization of the lipid A profile of Pseudomonas aeruginosa PAO1. This analysis was enhanced by the development of an LC-MS/MS approach to eliminate isomeric signals in the MS/MS spectra that confounded characterization. Importantly, combining reverse phase chromatography with HCD and ultraviolet photodissociation analyses of the lipid A profile revealed the presence of previously unreported lipid A acyl chain positional isomers. Altogether, these strategies provide the most in-depth structural and molecular characterization of PAO1 lipid A to date.

A Mutation in the Mesorhizobium loti oatB Gene Alters the Physicochemical Properties of the Bacterial Cell Wall and Reduces Survival inside Acanthamoeba castellanii

In our previous report, we had shown that the free-living amoeba Acanthamoeba castellanii influenced the abundance, competiveness, and virulence of Mesorhizobium loti NZP2213, the microsymbiont of agriculturally important plants of the genus Lotus. The molecular basis of this phenomenon; however, had not been explored. In the present study, we demonstrated that oatB, the O-acetyltransferase encoding gene located in the lipopolysaccharide (LPS) synthesis cluster of M. loti, was responsible for maintaining the protective capacity of the bacterial cell envelope, necessary for the bacteria to fight environmental stress and survive inside amoeba cells. Using co-culture assays combined with fluorescence and electron microscopy, we showed that an oatB mutant, unlike the parental strain, was efficiently destroyed after rapid internalization by amoebae. Sensitivity and permeability studies of the oatB mutant, together with topography and nanomechanical investigations with the use of atomic force microscopy (AFM), indicated that the incomplete substitution of lipid A-core moieties with O-polysaccharide (O-PS) residues rendered the mutant more sensitive to hydrophobic compounds. Likewise, the truncated LPS moieties, rather than the lack of O-acetyl groups, made the oatB mutant susceptible to the bactericidal mechanisms (nitrosative stress and the action of lytic enzymes) of A. castellanii.

Structure of the Lipopolysaccharide from the Bradyrhizobium sp. ORS285 rfaL Mutant Strain

The importance of the outer membrane and of its main constituent, lipopolysaccharide, in the symbiosis between rhizobia and leguminous host plants has been well studied. Here, the first complete structural characterization of the entire lipopolysaccharide from an O‐chain‐deficient Bradyrhizobium ORS285 rfaL mutant is achieved by a combination of chemical analysis, NMR spectroscopy, MALDI MS and MS/MS. The lipid A structure is shown to be consistent with previously reported Bradyrhizobium lipid A, that is, a heterogeneous blend of penta‐ to hepta‐acylated species carrying a nonstoichiometric hopanoid unit and possessing very‐long‐chain fatty acids ranging from 26:0(25‐OH) to 32:0(31‐OH). The structure of the core oligosaccharide region, fully characterized for the first time here, is revealed to be a nonphosphorylated linear chain with methylated sugar residues, with a heptose residue exclusively present in the outer core region, and with the presence of two singly substituted 3‐deoxy‐d‐manno‐oct‐2‐ulosonic acid (Kdo) residues, one of which is located in the outer core region. The lipid A moiety is linked to the core moiety through an uncommon 4‐substituted Kdo unit.