Abstract
Lactococcus garvieae causes fatal haemorrhagic septicaemia in fish such as yellowtail. The comparative analysis of genomes of a virulent strain Lg2 and a non-virulent strain ATCC 49156 of L. garvieae revealed that the two strains shared a high degree of sequence identity, but Lg2 had a 16.5-kb capsule gene cluster that is absent in ATCC 49156. The capsule gene cluster was composed of 15 genes, of which eight genes are highly conserved with those in exopolysaccharide biosynthesis gene cluster often found in Lactococcus lactis strains. Sequence analysis of the capsule gene cluster in the less virulent strain L. garvieae Lg2-S, Lg2-derived strain, showed that two conserved genes were disrupted by a single base pair deletion, respectively. These results strongly suggest that the capsule is crucial for virulence of Lg2. The capsule gene cluster of Lg2 may be a genomic island from several features such as the presence of insertion sequences flanked on both ends, different GC content from the chromosomal average, integration into the locus syntenic to other lactococcal genome sequences, and distribution in human gut microbiomes. The analysis also predicted other potential virulence factors such as haemolysin. The present study provides new insights into understanding of the virulence mechanisms of L. garvieae in fish.
Highlights
IntroductionLactococcus garvieae (formerly Enterococcus seriolicida) is a major pathogen of fish and causes fatal haemorrhagic septicaemia in fish such as yellowtail and trout
Lactococcus garvieae is a major pathogen of fish and causes fatal haemorrhagic septicaemia in fish such as yellowtail and trout
From the present study including the whole genome analysis of the Lg2 and the ATCC 49156 genomes and their comparative analysis with other related genomes as well as the analysis of the capsule gene cluster in the less virulent strain Lg2-S, we strongly suggest that the capsule is crucial for pathogenesis of Lg2
Summary
Lactococcus garvieae (formerly Enterococcus seriolicida) is a major pathogen of fish and causes fatal haemorrhagic septicaemia in fish such as yellowtail and trout. L. garvieae has been isolated from buffaloes with mastitis [2], and clinical specimens of human blood, urine, and skin [3,4,5]. For this reason, L. garvieae is considered an emerging zoonotic pathogen. It has only been demonstrated that virulence of L. garvieae for fish is, in part, dependent on its ability to form a capsule [6,7,8]. L. garvieae isolated from diseased fish is classified serologically into two groups, KG2 and KG+, and capsulated L. garvieae (KG2) are more virulent in fish than non-capsulated L. garvieae (KG+) [7,8,9]
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