An investigation into the bacterial aetiology of ‘black patch necrosis’ in Dover sole, Solea solea L.

. A recurring disease known as ‘black patch necrosis’ (BPN) has been the cause of severe mortalities in 0‐group and 1‐group Dover sole during the period 1974‐1978 at the Sea Fish Industry Authority's Marine Cultivation Unit at Hunter‐ston, Ayrshire, Scotland. Samples of diseased and healthy Dover sole tissue were examined microbiologically and a long thin filamentous Gram‐negative organism, strongly resembling Flexibacter columnar's , was repeatedly isolated from diseased tissue but was not found in healthy tissue. This organism was pathogenic to Dover sole, producing 100% mortality in 96 h at 17.5°C (±2°C) and the organism was reisolated from all the infected fish. The pathogenic isolate and reisolate were examined and compared with reference strain NCMB 1038 of Flexibacter columnarts , formerly known as Chondrococcus columnaris or Cytophaga columnaris. The isolate, reisolate and type culture were identical with respect to cell morphology, oxidase test, catalase production, production of H 2 S, casein hydrolysis, gelatin liquefaction, hydrolysis of Tween 20, degradation of tyrosine and starch hydrolysis. All bacterial types showed similar patterns of sensitivity to a range of antibiotics. It is proposed that a Flexibacter columnari's‐like organism is associated with the condition in farmed Dover sole known as ‘black patch necrosis’.