Plasmid-mediated histamine biosynthesis in the bacterial fish pathogen Vibrio anguillarum

Histamine production in bacteria-contaminated fish is the result of the presence of bacterial histidine decarboxylase activity, which converts histidine present in muscle proteins to histamine. The fish pathogen Vibrio anguillarum harbors a plasmid-encoded histidine decarboxylase gene (angH) that is essential for biosynthesis of the siderophore anguibactin. However, the role of angH in histamine biosynthesis by this pathogen has not been fully determined. Thus, the objectives of this study were to monitor the production and release of histamine by the wild-type as well as by a plasmidless strain and angH isogenic mutants generated by allelic exchange. Reverse transcription polymerase chain reaction showed that only the wild-type strain expressed angH, while no angH message was detected in the mutants and the plasmidless derivative. The iron uptake-deficient phenotype of one of the angH mutants confirmed the location of the mutation and the unique role of this gene in iron acquisition. Thin-layer chromatography, gas chromatography, and mass spectrometry showed that histamine was released by the strain harboring a wild-type angH gene when grown in excess histidine. This biogenic amine was not detected in the culture supernatants of the plasmidless derivative and the angH mutant when cultured under the same experimental conditions. These results indicate that angH is essential for histamine biosynthesis in V. anguillarum, a compound responsible for food poisoning and potentially involved in bacterial virulence. Thin-layer chromatography of wild-type culture supernatants and beta-galactosidase assays using the isogenic angH mutant demonstrated that the expression of this gene is independent of the histidine concentration of the medium under both iron-rich and iron-limiting conditions.