also known as: Flexibacter columnaris, Bacillus columnaris, Cytophaga columnaris, cottonmouth disease, mouth "fungus", cottony mouth disease, rainbow trout fry syndrome
Flavobacterium columnare is a Gram-negative bacterium, best known for columnaris. The bacterium can infect a very wide range of cooler temperature freshwater fish species. The bacterium can infect most host tissues, but is often diagnosed by gill and mouth infections with yellowish secretions and bacterial masses that form microscopic "columns" (or sometimes cotton-like masses visible to the naked eye). Flavobacterium columnare infections can cause disease in both cultured fish and free-ranging populations, with hatchery-based young trout and salmon species especially vulnerable. Flavobacterium can be diagnosed using culturing in reduced nutrient media.