Structural and Serological Studies of the O6-Related Antigen of Aeromonas veronii bv. sobria Strain K557 Isolated from Cyprinus carpio on a Polish Fish Farm, which Contains L-perosamine (4-amino-4,6-dideoxy-L-mannose), a Unique Sugar Characteristic for Aeromonas Serogroup O6

Amongst Aeromonas spp. strains that are pathogenic to fish in Polish aquacultures, serogroup O6 was one of the five most commonly identified immunotypes especially among carp isolates. Here, we report immunochemical studies of the lipopolysaccharide (LPS) including the O-specific polysaccharide (O-antigen) of A. veronii bv. sobria strain K557, serogroup O6, isolated from a common carp during an outbreak of motile aeromonad septicemia (MAS) on a Polish fish farm. The O-polysaccharide was obtained by mild acid degradation of the LPS and studied by chemical analyses, mass spectrometry, and 1H and 13C NMR spectroscopy. It was revealed that the O-antigen was composed of two O-polysaccharides, both containing a unique sugar 4-amino-4,6-dideoxy-l-mannose (N-acetyl-l-perosamine, l-Rhap4NAc). The following structures of the O-polysaccharides (O-PS 1 and O-PS 2) were established: O-PS 1: →2)-α-l-Rhap4NAc-(1→; O-PS 2: →2)-α-l-Rhap4NAc-(1→3)-α-l-Rhap4NAc-(1→3)-α-l-Rhap4NAc-(1→. Western blotting and an enzyme-linked immunosorbent assay (ELISA) showed that the cross-reactivity between the LPS of A. veronii bv. sobria K557 and the A. hydrophila JCM 3968 O6 antiserum, and vice versa, is caused by the occurrence of common α-l-Rhap4NAc-(1→2)-α-l-Rhap4NAc and α-l-Rhap4NAc-(1→3)-α-l-Rhap4NAc disaccharides, whereas an additional →4)-α-d-GalpNAc-associated epitope defines the specificity of the O6 reference antiserum. Investigations of the serological and structural similarities and differences in the O-antigens provide knowledge of the immunospecificity of Aeromonas bacteria and are relevant in epidemiological studies and for the elucidation of the routes of transmission and relationships with pathogenicity.