Comparative Structural and Antigenic Characterization of Genetically Distinct Flavobacterium psychrophilum O-Polysaccharides

John O Cisar,C Allen Bush,Gregory D Wiens

doi:10.3389/fmicb.2019.01041

Abstract

Little is known about the underlying basis of serotype specificity among strains of Flavobacterium psychrophilum, the agent of rainbow trout fry syndrome and bacterial cold-water disease. The identification of different heat-stable O-serotypes among strains of this gram-negative pathogen does, however, suggest structural variations in the O-polysaccharide (O-PS) moiety of cell surface lipopolysaccharide (LPS). A trisaccharide composed of L-rhamnose (L-Rha), 2-acetamido-2-deoxy-L-fucose (L-FucNAc) and 2-acetamido-4-R-2,4-dideoxy-D-quinovose (D-Qui2NAc4NR), where R represents a dihydroxyhexanamido derivative, was previously identified as the repeating unit of Fp CSF259-93 O-PS. Interestingly, the O-PS gene cluster of this strain and that of Fp 950106-1/1, which belongs to a different O-serotype, are identical except for wzy, which encodes the putative polymerase that links trisaccharide repeats into O-PS chains. We have now found from results of glycosyl composition analysis and high-resolution nuclear magnetic resonance, that the linkage of D-Qui2NAc4NR to L-Rha, which is α1-2 for Fp CSF259-93 versus β1-3 for Fp 950106-1/1, is the only structural difference between O-PS from these strains. The corresponding difference in O-serotype specificity was established from the reactions of rabbit and trout anti-F. psychrophilum antibody with purified O-PS and LPS. Moreover, LPS-based differences in antigenicity were noted between strains with O-PS loci identical to those of Fp CSF259-93 or Fp 950106-1/1, except for the genes predicted to direct synthesis of different R-groups in Qui2NAc4NR. The findings provide a framework for defining the genetic basis of O-PS structure and antigenicity and suggest that the repertoire of F. psychrophilum O-serotypes extends beyond what is presently recognized from serological studies of this important fish pathogen.

Highlights

Flavobacterium psychrophilum, the agent of rainbow trout fry syndrome and bacterial cold-water disease, poses a serious threat to the salmonid aquaculture industry (Cipriano and Holt, 2005; Starliper, 2011)
In agreement with previous results of MacLean et al (2001), the corresponding cross peaks in a long-range 1H-13C correlation spectrum of F. psychrophilum (Fp) CSF259-93 were between A-H1 and B-C3, between B-H1 and C-C2 and between C-H1 and A-C4 (Figure 1D). These findings indicate that the O-PS structures of Fp CSF259-93 and Fp 960106-1/1 are identical except for the linkage of D-Qui2NAc4NR to L-Rha, which is α1-2 for the former as opposed to β1-3 for the latter polysaccharide (Figure 1D)
The present findings provide a framework for defining the genetic basis of O-PS structure and antigenicity

Summary

Introduction

Flavobacterium psychrophilum, the agent of rainbow trout fry syndrome and bacterial cold-water disease, poses a serious threat to the salmonid aquaculture industry (Cipriano and Holt, 2005; Starliper, 2011). This threat is not from a single pathogenic entity but instead from several different host-specific serotypes (Mata et al, 2002; Izumi et al, 2003) and a wide range of multilocus sequence types (ST) (Nicolas et al, 2008). A genetic scheme for serotyping F. psychrophilum is needed to explore this possibility as well as other potential mechanisms of pathogenesis, host genetic resistance and immune protection

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Conclusion