Biochemical and cell-surface characteristics of Yersinia ruckeri in relation to the epizootiology and pathogenesis of infections in fish
Isolates of Yersinia ruckeri were obtained from Europe, North America, Australia and South Africa. The biochemical and serological characteristics of the isolates were investigated. Biochemically the isolates were extremely uniform although motile, Tween positive isolates could be differentiated from non-motile, Tween negative isolates; these were designated biotypes 1 and 2 respectively. With the exception of two isolates, biotype 2 isolates were confined to the U. K. Five 0-serotypes were recognised and an O-serotyping scheme is proposed; the relation of this scheme to previously described schemes is discussed. The geographic distribution of the different serotypes is also discussed. The lipopolysaccharide (LPS) and outer membrane protein (OMP) profiles of isolates were analysed by SDS-PAGE and Western-blotting using both rabbit and rainbow trout antisera. The relation of LPS-type to 0- serotype, as well as variation within LPS-types, is discussed. Based on interstrain variation in the molecular weight of a heat-modifiable protein and of peptidoglycan-associated (porin) proteins, an OMP-typing scheme was developed. Three major OMP-types comprised 95% of the isolates studied. Variation in biotype, serotype and OMP-type was used as an epizootiological tool, and six serotype 01 clonal groups were recognised which differed in their geographic distribution. The production of iron-regulated OMPs and siderophores was investigated. Four iron-regulated OMPs were produced in all of the isolates examined; siderophores appeared not to be produced by any of the isolates. Production of iron-regulated OMPs was not an important virulence determinant and appears to be a chromosomally-mediated factor. Resistance to the bactericidal effects of normal rainbow trout serum and virulence were also investigated. Serum-resistance was associated principally with two serotype 01 clonal groups and virulence was associated with the same two clonal groups. Other serotype 01 clonal groups and other serotypes were iii generally serum-sensitive and avirulent. Thus, serum-resistance is an important virulence determinant in this organism. The role of outer membrane components in serum-resistance and virulence is discussed.