PHENOTYPIC CHARACTERISTICS AND VIRULENCE OF VIBRIO-ANGUILLARUM-RELATED ORGANISMS

The phenotypic, molecular, and virulence properties of 46 Vibrio angui llarum-related (VAR) strains isolated from diseased fish and shellfish and from the environment were investigated. Twelve reference strains belonging to the 10 serotypes of V. anguillarum and the vibrio splendi dus type strain were included for comparison. Numerical taxonomy studi es allowed us to group the isolates into four phena. The main phenotyp ic traits to differentiate VAR strains from V. anguillarum were fermen tation of arabinose and mannitol, indole and Voges-Proskauer reactions , gelatin and casein hydrolysis, hemolytic activity, growth at 37 and 4-degrees-C, and resistance to ampicillin. Serological analysis confir med that phena I and II were composed mainly of strains of V. anguilla rum, while phena III and IV included VAR strains. Excluding the refere nce strains, the typeable isolates belonged to serotypes O3 (15 strain s), O4 (3 strains), and O5 (2 strains) of V. anguillarum. The infectiv ity trials showed that only 9 of a total of 24 strains tested displaye d virulence for rainbow trout. Virulent strains (50% lethal dose rangi ng from 10(2) to 10(6) cells) included V. anguillarum strains belongin g to serotypes O1 (one strain), O2 (one strain), O3 (three isolates), and O4 (one isolate) and only three strains of the VAR group. Sodium d odecyl sulfate-polyacrylamide gel electrophoresis analysis of lipopoly saccharide and outer membrane proteins showed heterogeneity not only a mong the 10 V. anguillarum serotypes but also within the VAR group. Im munoblot assays demonstrated a close relationship among V. anguillarum strains from the same serotype, while strains from different serotype s were not antigenically related. The VAR strains did not share antige nic components with the serotypes of V. anguillarum tested (serotypes O1 to O5). Plasmids were detected in only 19 of the total of 59 strain s. The majority of the strains carrying plasmids were grouped within p henon IV, in which plasmid bands of 27 and 36 MDa were found in all th e isolates. No correlation between the plasmid content of VAR microorg anisms and their phenotypic or virulence characteristics was observed. From these results it can be concluded that VAR strains associated wi th disease should be included together with V. anguillarum in the form ulation of vaccines against vibriosis.