The structure of Lipid A and composition of the oligosaccharides of the lipopolysaccharides of the bacterial species: Veillonella parvula

Abstract

Veillonella parvula is a Gram-negative bacterium known to be an abundant commensal coloniser of the human gut and mouth; it is linked to progression of childhood immune system but has also been found to be an opportunistic pathogen. Lipopolysaccharides (LPS) are glycoconjugates found in the outer membrane of almost all Gram-negative bacteria and are composed of three components: a glycolipid (lipid A), an oligosaccharide (the core region) and a repeating oligosaccharide unit (O-antigen).
Both Lipid A (the toxic component of LPS) and the polysaccharide side chains (the nontoxic but immunogenic portion of LPS) act as determinants of virulence in Gram-negative bacteria. Oantigens have adhered properties and these are resistance to phagocytes, protection toward to antigens and antigenic variation property. Lipid A act as an immune stimulator, which induces the biological responses of a specific organism
The LPS activates the host immune system, the lipid A through the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex in a structure dependent manner and the polysaccharide side chains, act as virulence determinants of the Gram-negative bacteria. Therefore, the structural elucidation of the LPS found in gut microbiota population is particularly interesting to understand the role of gut bacteria LPS in the activation and/ or suppression of the immune system response.
Successful separation and purification of the individual components from LPS (O-antigen, core- oligosaccharide and lipid A) of Veillonella parvula was achieved through a variety of chemical approaches, such as acetylated alditols derivatives, acetic acid hydrolysis and alkaline hydrolysis. Elucidation of the lipid A structure from a clinically isolated strain of V. parvula, grown within laboratory conditions, was achieved and gave insights into the high structural heterogeneity of lipid A from a gut bacterium. The most abundant species comprises a bisphosphorylated hexa-acylated species with a variety of iso-branched acyl chains.
Additionally, the saccharide composition of the LPS of two separately grown strains of Veillonella parvula have been characterised and compared to give an insight into the complexity of the carbohydrate components of LPS from a gut microbiota species.