Phenotypic and Genetic Characterization of a Capsule-deficient Lactococcus formosensis M-T2 Isolated from Farmed Greater Amberjack

Abstract

In 2016, Lactococcus formosensis M-T2 was isolated from a farmed greater amberjack Seriola dumerili. Electron microscopy revealed the absence of capsule-like structures in M-T2. Infection and vaccination trials in greater amberjack showed that M-T2 was avirulent and that a formalin-killed vaccine prepared from M-T2 failed to provide protection against either L. garvieae or L. formosensis. Comparative genomic analysis between the virulent L. formosensis strain 122061 and M-T2 revealed differences at 23 loci, including a nonsense mutation in a glycerophosphotransferase gene potentially involved in capsular polysaccharide synthesis. Differences in plasmid composition were also observed among strains. These findings are expected to contribute to the identification of virulence-associated factors and vaccine antigens in L. formosensis.