Abequose-containing O-antigen polysaccharide from Aeromonas sobria K221 (serogroup PGO1): structure, O-antigen gene cluster correlation, and Abe-transferase prediction.

Aeromonas species are important opportunistic pathogens in freshwater aquaculture, including Polish fish farms, where outbreak-associated isolates have been assigned to local provisional serogroups. Among these, PGO1 is frequently detected, but the structural and genetic diversity of the corresponding O-specific polysaccharides remains poorly understood. Here, we characterized Aeromonas sobria strain K221, a PGO1 isolate recovered from common carp during an outbreak of motile Aeromonas infection/septicaemia. The O-specific polysaccharide (OPS), isolated from LPS, was analysed by chemical methods and 1H/13C NMR spectroscopy, and its O-repeating unit was identified as a linear pentasaccharide containing β-GlcpNAc, 2-O-acetylated α-Rhap, and α-Abep residues. Bioinformatic analysis of the O-antigen gene cluster (OGC) revealed gene content consistent with the OPS structure and supported functional assignment of the biosynthesis locus, including a putative α-1,3-CDP-abequosyltransferase. Both the OPS and its OGC differed from those of other recently characterized PGO1 strains, indicating that K221 represents a distinct variant within this serogroup. The occurrence of 2-substituted abequose, not previously reported in Aeromonas O-polysaccharides or as a 2-substituted residue in bacterial O-polysaccharides, highlights an unusual structure-biosynthesis relationship. These findings reveal greater structural and genetic diversity within the Aeromonas PGO1 serogroup than previously recognized and expand current knowledge of bacterial O-specific carbohydrate polymers.