This study examined DNA relatedness among various serovars of Yersinia ruckeri, the bacterium causing enteric redmouth disease in farmed salmonid fish. Researchers performed DNA hybridization assays on nitrocellulose filters using 66 isolates representing six serovars plus untyped strains. Strains from serovars I, II, III, V, and VI demonstrated high DNA relatedness (>70% relative binding ratios), indicating they belong to a single genetic species despite serological and biochemical differences. Notably, sorbitol-fermenting serovar II isolates showed close genetic relationship to the original serovar I strains (>70% RBR). Serovar IV and certain untyped isolates that fermented unusual sugars like arabinose and rhamnose were excluded from Y. ruckeri due to low hybridization values. Colony hybridization confirmed the species identity while distinguishing Y. ruckeri from related bacteria like Hafnia alvei and Yersinia enterocolitica. The results support broadening the species description of Y. ruckeri to include sorbitol-fermenting and serologically diverse isolates, with implications for aquaculture disease surveillance and diagnostic protocols.

Key findings

• All authentic Y. ruckeri serovars (I, II, III, V, VI) showed >70% DNA relatedness and form a single genetic species despite serological and biochemical differences
• Sorbitol-fermenting serovar II isolates are genetically closely related to original serovar I strains, validating their classification as Y. ruckeri
• Serovar IV and certain arabinose-fermenting isolates should be excluded from Y. ruckeri based on low DNA hybridization values (<8%)
• Colony hybridization using Y. ruckeri DNA probes provides a rapid diagnostic method to distinguish the pathogen from phenotypically similar bacteria like Hafnia alvei and Yersinia enterocolitica
• The high genetic similarity among serovars supports expanding Y. ruckeri species definition to encompass previously uncertain isolates, improving aquaculture health monitoring