dStreptococcus iniae causes severe septicemia and meningitis in farmed fish and is also occasionally zoonotic. Vaccination against S. iniae is problematic, with frequent breakdown of protection in vaccinated fish. The major protective antigens in S. iniae are the polysaccharides of the capsule, which are essential for virulence. Capsular biosynthesis is driven and regulated by a 21-kb operon comprising up to 20 genes. In a long-term study, we have sequenced the capsular operon of strains that have been used in autogenous vaccines across Australia and compared it with the capsular operon sequences of strains subsequently isolated from infected vaccinated fish. Intriguingly, strains isolated from vaccinated fish that subsequently become infected have coding mutations that are confined to a limited number of genes in the cps operon, with the remainder of the genes in the operon remaining stable. Mutations in strains in diseased vaccinated fish occur in key genes in the capsular operon that are associated with polysaccharide configuration (cpsG) and with regulation of biosynthesis (cpsD and cpsE). This, along with high ratios of nonsynonymous to synonymous mutations within the cps genes, suggests that immune response directed predominantly against capsular polysaccharide may be driving evolution in a very specific set of genes in the operon. From these data, it may be possible to design a simple polyvalent vaccine with a greater operational life span than the current monovalent killed bacterins.

Recently identified virulence factors can lead to new approaches, such as vaccination programs at fish farms.

Background: Binding of serum components by surface M-related proteins, encoded by the emm genes, in streptococci constitutes a major virulence factor in this important group of organisms. The present study demonstrates fibrinogen binding by S. iniae, a Lancefield non-typeable pathogen causing devastating fish losses in the aquaculture industry and an opportunistic pathogen of humans, and identifies the proteins involved and their encoding genes.

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A bacterium was isolated from the mid-gut of healthy black tiger shrimp, Penaeus monodon, based on a large zone of inhibition in mixed culture on solid medium. The isolate was a Gram-positive, motile spore former, with an optimum pH range for growth in tryptone soya broth containing 2% NaCl of between pH 6 and 9. The bacterium was highly salt tolerant with concentrations between 0% and 8% having no detrimental effect on growth. The isolate was identified as Bacillus pumilus based on physiological capabilities using the API50CHB and Biolog systems. Amplification and sequencing of the 16S rRNA gene followed by phylogenetic analysis confirmed its identity. The Bacillus pumilus isolate was strongly inhibitory against the marine bacterial pathogens Vibrio alginolyticus, V. mimicus and V. harveyi, and weakly inhibitory against V. parahaemolyticus in cross-streaking assays on solid medium. The organism was marginally self-inhibitory, and inhibited B. licheniformis and B. subtilis. The suitability of the B. pumilus isolate for use as a probiotic in farmed shrimp was further supported by the absence of any of the known B. cereus enterotoxin genes. Based on these in vitro results, in vivo safety and efficacy trials are underway to determine suitability of the novel strain as a commercial probiotic.