Practice question

Lipopolysaccharide O antigen part structural component extends from core into extracellular environment. O side chain consists tandem repeats oligosaccharide units 2 to 6 sugars with diverse linkages deoxy sugars abequose tyvelose modifying antigenicity. Synthesis via Wzy dependent, ABC transporter, or synthase pathways assembled on undecaprenyl phosphate lipid carrier then ligated to lipid A core. Because repeats exposed and highly variable between strains due to glycosyl transferase gene hypervariation and horizontal transfer, immune system recognizes them as type specific antigens. Serological typing schemes Klebsiella K antigen capsule, Escherichia O antigen 187 types, Salmonella over 2500 serovars based on O and H flagellar antigens. O antigen variation influences complement deposition, serum resistance, bacteriophage adsorption, colonization niche. O side chain functions as antigenic determinant not as nutrient transporter porin channel transpeptidase inhibitor. Understanding O antigen diversity crucial diagnostics, vaccine design conjugate vaccines against Shigella dysenteriae, and tracking outbreak epidemiology via O serotyping molecular methods.