Evolution of the Capsular Operon of Streptococcus iniae in Response to Vaccination

Abstract

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.

Discovery of Streptococcus pneumoniae Serotype 6 Variants with Glycosyltransferases Synthesizing Two Differing Repeating Units

“…This serotype shift is probably useful in invading deeper tissues from the nasopharynx or in rapid responses to vaccination. Interestingly, evidence for such a capsule type shift by mutation has been recently described for Streptococcus iniae infecting vaccinated fish in fish farms (47).…”

On the origin of Group B Streptococcus from disease outbreaks in wild marine fish in Australia

“…The second method used was molecular serotyping, which explores the genes encoding the surface epitope determinants of the capsular polysaccharide (CPS) (Bentley et al, 2006;Honsa et al, 2008;Kong et al, 2008). Cps operon sequence typing is also physiologically relevant and mutation can be driven quite rapidly by host response, providing local scale resolution amongst isolates (Millard et al, 2012).…”

“…Mutations in the cps genes of the capsular operon change rapidly in response to intense selective pressure of the environment, vaccination and the host immune system (Millard et al, 2012). Mutations change the structure of the capsule resulting in new serotypes.…”

“…The capsular polysaccharides (CPSs) are the major virulence factors in GBS Martins et al, 2010;Millard et al, 2012) and are present in most encapsulated Gram-positive bacteria.…”

“…3.2), a set of isolates representative of the different clusters was selected and a buoyant density assay was determined in a continuous Percoll gradients in order to estimate the quantity of CPS expressed by the different strains as described previously (Locke et al, 2007;Millard et al, 2012). Briefly, mid-exponential-phase THB cultures of each strain were washed once in PBS and resuspended to an optical density at 600 nm (OD 600 ) of 2, and a 0.5 mL aliquot was layered onto standard isotonic Percoll gradients.…”