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Campylobacter jejuni is a human pathogenand aleading cause of food poisoning in North America and Europe. The exteriorsurface of the bacterial cell wall is attached to a polymeric coatof sugar molecules known as the capsular polysaccharide （CPS） thathelps protect the organism from the host immune response. The CPSis composed of a repeating sequence of common and unusual sugar residues.In the HS:11 serotype of C. jejuni, we identifiedtwo enzymes in the gene cluster for CPS formation that are utilizedfor the biosynthesis of UDP-α-N-acetyl-d-mannosaminuronic acid （UDP-ManNAcA）. In the first step, UDP-α-N-acetyl-d-glucosamine （UDP-GlcNAc） is epimerizedat C2 to form UDP-α-N-acetyl-d-mannosamine（UDP-ManNAc）. This product is then oxidized by a NAD+-dependentC6-dehydrogenase to form UDP-ManNAcA. In the HS:6 serotype （C. jejuni strain 81116）, we identified three enzymes thatare required for the biosynthesis of CMP-β-N-acetyl-d-neuraminic acid （CMP-Neu5Ac）. In the first step,UDP-GlcNAc is epimerized at C2 and subsequently hydrolyzed to form N-acetyl-d-mannosamine （ManNAc） with the releaseof UDP. This product is then condensed with PEP by N-acetyl-d-neuraminate synthase to form N-acetyl-d-neuraminic acid （Neu5Ac）. In the final step, CMP-N-acetyl-d-neuraminic acid synthase utilizes CTPto convert this product into CMP-Neu5Ac. A bioinformatic analysisof these five enzymes from C. jejuni serotypes HS:11and HS:6 identified other bacterial species that can produce UDP-ManNAcAor CMP-Neu5Ac for CPS formation.
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