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Columnaris disease is an acute to chronic bacterial infection that affects anadromous salmonids and virtually all species of warmwater fishes. Davis (1922), who first described the disease, named it columnaris because the causal bacterial cells seen in wet mounts of affected gills and fins were arranged in columnar aggregations. Ordal and Rucker (1944) were the first to isolate the causal organism and, based on cellular morphology, identified it as a myxobacterium. Organisms classified in the order Myxobacterales are long, thin gram-negative rods that are motile on agar media by a creeping or flexing motion. They have a life cycle composed of vegetative cells, microcysts (resting cells), and fruiting bodies, or only vegetative cells and microcysts. Ordal and Rucker (1944) reported that the myxobacterium from columnaris disease produced both fruiting bodies and microcysts and named the organism Chondrococcus columnaris. Garnjobst (1945) studied strains of the columnaris bacterium and reported that microcysts were present but not fruiting bodies. Because fruiting bodies could not be demonstrated, she placed the organism in the genus Cytophaga and suggested Cytophaga columnaris. However, in the eighth edition of Bergey's Manual of Determinative Bacteriology (Buchanan and Gibbons 1974), it was stated that the columnaris bacterium produced neither fruiting bodies nor microcysts. Therefore it was removed from the Myxobacterales, placed in the order Cytophagales, and renamed Flexibacter columnaris.