The UDP N-Acetylgalactosamine 4-Epimerase Gene Is Essential for Mesophilic Aeromonas hydrophilaSerotype O34 Virulence

ABSTRACTMesophilic Aeromonas hydrophilastrains of serotype O34 typically express smooth lipopolysaccharide (LPS) on their surface. A single mutation in the gene that codes for UDP N-acetylgalactosamine 4-epimerase (gne) confers the O−phenotype (LPS without O-antigen molecules) on a strain in serotypes O18 and O34, but not in serotypes O1 and O2. The gnegene is present in all the mesophilic Aeromonasstrains tested. No changes were observed for the LPS core in a gnemutant from A. hydrophilastrain AH-3 (serotype O34). O34 antigen LPS contains N-acetylgalactosamine, while no such sugar residue forms part of the LPS core from A. hydrophilaAH-3. Some of the pathogenic features of A. hydrophilaAH-3 gnemutants are drastically reduced (serum resistance or adhesion to Hep-2 cells), and the gnemutants are less virulent for fish and mice compared to the wild-type strain. Strain AH-3, like other mesophilic Aeromonasstrains, possess two kinds of flagella, and the absence of O34 antigen molecules by gnemutation in this strain reduced motility without any effect on the biogenesis of both polar and lateral flagella. The reintroduction of the single wild-type gnegene in the corresponding mutants completely restored the wild-type phenotype (presence of smooth LPS) independently of the O wild-type serotype, restored the virulence of the wild-type strain, and restored motility (either swimming or swarming).