DISTRIBUTION OF PLASMID- AND CHROMOSOME-MEDIATED IRON UPTAKE SYSTEMS IN VIBRIO-ANGUILLARUM STRAINS OF DIFFERENT ORIGINS

We investigated the incidence of plasmid-mediated and chromosome-mediated iron uptake systems in strains of Vibrio anguillarum that belong to serotypes O1 and O2 and were isolated from different fish species and in different geographic areas. All of the strains gave positive reactions in CAS agar medium and in the Arnow test, which indicated that catechol types of siderophores were produced. The majority of V. anguillarum serotype O1 strains harbored a 65-kb plasmid similar to plasmid pJM1 from strain 775, which encodes the siderophore anguibactin and its outer membrane receptor, protein OM2. All of the isolates harboring this plasmid promoted the growth of an anguibactin-deficient receptor-proficient mutant derived from strain 775, but none of these isolates promoted the growth of mutants lacking receptor OM2. Furthermore, under iron-limiting conditions all of these strains induced outer membrane proteins that were identical in size to protein OM2 of strain 775. In contrast, none of the serotype O2 strains contained a high-molecular-weight plasmid, but all of them induced the growth of mutants defective in the anguibactin-mediated system regardless of the presence or absence of receptor OM2. The serotype O2 strains, but not the plasmid-bearing serotype O1 strains, also induced the growth of Salmonella typhimurium enb-1 which utilizes only enterobactin as a siderophore. Moreover, the pattern of iron-regulated outer membrane proteins in serotype O2 isolates was totally different from the pattern in the serotype O1 strains harboring the 65-kb plasmid. Interestingly, several plasmidless strains belonging to serotype O1 behaved in the same way as serotype O2 strains in cross-feeding experiments and produced the same iron-regulated outer membrane protein patterns. These results indicate that regardless of their origins all V. anguillarum strains belonging to serotype O2 contain similar chromosome-mediated iron-sequestering systems. However, in the serotype O1 strains both chromosome- and plasmid-encoded iron uptake systems occur, but there is a significant predominance of the plasmid-mediated system.