Misidentification of Vibrio cholerae O155 isolated from imported shrimp as O serogroup O139 due to cross-agglutination with commercial O139 antisera.

Fish and shellfish products imported into Denmark are routinely analyzed for pathogenic Vibrio spp., particularly Vibrio cholerae, if products originate from subtropical or tropical areas. A V. cholerae strain that agglutinated commercial O139 antiserum but not the O1, Inaba, or Ogawa antisera was isolated from imported raw frozen shrimp. The toxigenicity of the strain was analyzed, and the results of a polymerase chain reaction showed that the V. cholerae strain did not contain the virulence genes ctx, tcpA, and zot, which are normally found in V. cholerae O1 and O139. The strain was resistant to colistin and spectinomycin. The high susceptibility of the strain to antimicrobial agents was confirmed by the lack of an SXT element, a self-transmissible, chromosomal genetic element that is normally present in 0139 strains and encodes resistance to sulfonamides, trimethoprim, and streptomycin. The strain contained two plasmids, in contrast to other O139 strains, which normally do not contain plasmids. The characteristics of the strain led to further agglutination testing with other antisera that are not commercially available, and the strain was found to agglutinate O155 antiserum in repeated testing. Manufacturers of 0139 antiserum should be aware of the closely related O antigens of the O139, O22, and O155 serogroups and should be aware that their commercial diagnostic O139 antiserum must be absorbed to remove cross-reacting agglutinins of O22 and O155 strains.