The genus Flexibacter, within the family Cytophagaceae, consists of Gram-negative, long, slender bacteria which are capable of gliding motility when in contact with a solid surface. The taxonomy of the gliding bacteria is unclear (Reichenbach & Dworkin 1981) but is is gradually being resolved, releaving three important fish pathogens, Flexibacter psychrophilus (Borg 1960, Pacha & ordal 1970, Bernardet & Grimond 1989) and F. columnaris (Becker & Fujihara 1978) in freshwater fish and F. maritimus (masumura & Wakabayashi 1977, Hikida et al. 1979, Wakabayashi et al. 1984, 1986), in marine species.
Flexibacter psychrophilus (syn. Cytophaga psychrophila) forms slender rods 0.3-0.5 x 1.0-7.0 um with rounded ends. Optimum growth is at 15 degrees C with no growth at 30 degreesC. Growth occurs on solid media of low nutrient content and proteins such as gelatin and casein are actively decomposed. The mol % G+C in the DNA is 33-34. It is the causative agent of bacterial cold-water disease, a septicaemic infection of salmonids occurring particularly in north-west USA. Outbreaks among alevins may result in 30-35% mortalities: in older fish losses are 20% or less.