| Summary: | A Vibrio anguillarum mutant (M93SmΩD) unable to grow in salmon gastrointestinal mucus was selected and characterized using transposon mutagenesis. The mutant failed to grow over a 24 h period in NSS supplemented with mucus (NSSM). Whereas V. anguillarum M93Sm was demonstrated to be highly virulent in Atlantic salmon, M93SmΩD was shown to be avirulent over a 14 d period. Fish injected with M93SmΩD were protected against vibriosis when challenged with virulent V. anguillarum M93Sm cells. The region of DNA containing the transposon insertion mutation was cloned and sequenced. The insertion was located within a 1.39 kbp open reading frame, which we termed mugA. Of several bacterial genera and species tested by Southern analysis, only strains of V. anguillarum were found to contain the mugA gene. Site directed insertion mutagenesis was performed using two different V. anguillarum strains, M93Sm4–13 and NB10, to further investigate the role of mugA in growth and virulence. Mutants of both strains exhibited delayed growth in mucus and delayed virulence in Atlantic salmon. These data suggested that mugA is involved in the ability of V. anguillarum to grow in mucus, and to cause disease in Atlantic salmon. A brown pigment producing V. anguillarum M93Sm transposon mutant (M93Sm4–13) was isolated while attempting to generate mini-Tn 10 mucus growth mutants. While it was demonstrated that M93Sm4–13 grows well in mucus and is highly virulent, this strain was shown to produce a brown pigment and to inhibit the growth of wild type V. anguillarum . The region of DNA containing the transposon mutation was cloned and sequenced. Additionally, the effect of a mugA mutation on the V. anguillarum M93Sm4–13 phenotype was investigated.
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