Streptococcus iniae cpsG alters capsular carbohydrate composition and is a cause of serotype switching in vaccinated fish.
Streptococcus iniae causes septicaemia and meningitis in marine and freshwater fish wherever they are farmed in warm-temperate and tropical regions. Although serotype specific, vaccination with bacterins (killed bacterial cultures) is largely successful and vaccine failure occurs only occasionally through emergence of new capsular serotypes. Previously we showed that mutations in vaccine escapes are restricted to a limited repertoire of genes within the 20-gene capsular polysaccharide (cps) operon. cpsG, a putative UDP-galactose 4-epimerase, has three sequence types based on the insertion or deletion of the three amino acids leucine, serine and lysine in the substrate binding site of the protein. To elucidate the role of cpsG in capsular polysaccharide (CPS) biosynthesis and capsular composition, we first prepared isogenic knockout and complemented mutants of cpsG by allelic exchange mutagenesis. Deletion of cpsG resulted in changes to colony morphology and cell buoyant density, and also significantly decreased galactose content relative to glucose in the capsular polysaccharide as determined by GC-MS, consistent with epimerase activity of CpsG. There was also a metabolic penalty of cpsG knockout revealed by slower growth in complex media, and reduced proliferation in whole fish blood. Moreover, whilst antibodies raised in fish against the wild type cross-reacted in whole cell and cps ELISA, they did not cross-opsonise the mutant in a peripheral blood neutrophil opsonisation assay, consistent with reported vaccine escape. We have shown here that mutation in cpsG results in altered CPS composition and this in turn results in poor cross-opsonisation that explains some of the historic vaccination failure on fish farms in Australia.
The University of Queensland, School of Biological Sciences and Centre for Marine Science, Brisbane, Queensland, Australia. Electronic address: candice.heath@uqconnect.edu.au.
Candice
C
Heath
School of Chemistry and Molecular Biosciences, Brisbane, Queensland, Australia. Electronic address: christine.gillen@gmail.com.
Christine M
CM
Gillen
Australian Institute for Biotechnology and Nanotechnology, Brisbane, Queensland, Australia. Electronic address: p.chrysanthopoulos@uq.edu.au.
Panagiotis
P
Chrysanthopoulos
School of Chemistry and Molecular Biosciences, Brisbane, Queensland, Australia. Electronic address: mark.walker@uq.edu.au.
Mark J
MJ
Walker
The University of Queensland, School of Biological Sciences and Centre for Marine Science, Brisbane, Queensland, Australia. Electronic address: a.barnes@uq.edu.au.
Andrew C
AC
Barnes
D000818:Animals; D017756:Aquaculture; D016667:Bacterial Capsules / Q000737:chemistry / Q000276:immunology; D001428:Bacterial Vaccines / Q000276:immunology; D002241:Carbohydrates / Q000737:chemistry / Q000276:immunology; D005393:Fish Diseases / Q000235:genetics / Q000276:immunology / Q000382:microbiology*; D005399:Fishes; D009154:Mutation; D009876:Operon / Q000235:genetics*; D011135:Polysaccharides, Bacterial / Q000737:chemistry / Q000276:immunology*; D065288:Serogroup; D013290:Streptococcal Infections / Q000276:immunology / Q000382:microbiology / Q000662:veterinary*; D000070058:Streptococcus iniae / Q000276:immunology*; D014534:UDPglucose 4-Epimerase / Q000235:genetics / Q000378:metabolism; D014611:Vaccination / Q000662:veterinary
D001428:Bacterial Vaccines; D002241:Carbohydrates; D011135:Polysaccharides, Bacterial; D014534:UDPglucose 4-Epimerase
Aquaculture; Capsule; Epimerase; Polysaccharide; Streptococcus iniae; Vaccine