Emergence of Novel Streptococcus iniae Exopolysaccharide-Producing Strains following Vaccination with Nonproducing Strains

Abstract

Streptococcus iniae is a major pathogen of fish, producing fatal disease among fish species living in very diverse environments. Recently, reoccurrences of disease outbreaks were recorded in rainbow trout (Oncorhynchus mykiss, Walbaum) farms where the entire fish population was routinely vaccinated. New strains are distinguished from previous strains by their ability to produce large amounts of extracellular polysaccharide that is released into the medium. Present findings indicate that the extracellular polysaccharide is a major antigenic factor, suggesting an evolutionary selection of strains capable of extracellular polysaccharide production.

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

“…Coding mutations in the capsular operon correlate with autogenous vaccine failure. Reoccurrence of infection by S. iniae in vaccinated fish has been reported previously in Israel (4), and this appeared to be a result of changes in capsular polysaccharide and secreted exopolysaccharide (11). We sequenced the complete capsular operon in two postvaccine strains from Israel that had differing random amplified polymorphic DNA (RAPD) profiles (4) but that were apparently serologically cross-reactive (4, 11) and found a single coding mutation in cpsD (Table 1) resulting in a change from proline to leucine at amino acid (aa) position 123.…”

“…We found mutations in the capsular operon that coincided with infection in previously vaccinated stocks. These genetic changes likely lead to changes in polysaccharide biosynthesis (6,11). Intriguingly, changes were restricted to a limited suite of genes within the ϳ20-gene operon.…”

“…The Israeli team reported that these two isolates (named KPF177 [QMA0188] and KPF404 [QMA0186]) were serologically similar when tested with rabbit antisera using Ouchterlony immunodiffusion, but they differ in RAPD profiles (4). Each of these strains has been used in autogenous vaccines, and a novel serotype which differed in EPS production arose (11). It would be interesting to investigate the CPS operon of the newly emerged strain to determine whether coding mutations in the capsular biosynthesis machinery that led to increased polysaccharide overexpression or failure to anchor the membrane have arisen.…”

“…Even in completely enclosed recirculating systems, economics dictate that the system cannot be shut down and disinfected completely; thus, populations of the pathogen are likely to remain in biofilms on the pipework, tanks, and filters. Second, the infectious agent S. iniae is highly variable (6,12,26), with its major antigenic determinant, polysaccharide capsule (6,18,21), being highly polymorphic: novel capsular serotypes have already led to vaccine failure in fish farms in several parts of the world, including Australia (1,4,11,26). Finally, in Australia, there is no licensed generic vaccine against S. iniae, so autogenous vaccines ("custom" vaccines prepared from an isolate taken directly from the original farm and used only on that farm) are routinely employed to prevent streptococcosis.…”

Comparative genome analysis of Streptococcus iniae DX09 reveals new insights into niche adaptation and competitive host colonisation ability

“…C5a peptidase [8], capsule [1,[9][10][11]. phosphoglucomutase [12], exopolysaccharide [13] and α-enolase [14,15], which are potentially involved in the infection process. A number of candidate S. iniae vaccines have been developed based on these factors [2].…”