Flavobacterium columnare is a thin Gram-negative rod bacterium of the genus Flavobacterium. Fish with columnaris usually have Brown to yellowish-brown lesions (sores) on their gills, skin and/or fins. The bacteria attach to the gill surface, grow in spreading patches, and eventually cover individual gill filaments. This results in cell death. A characteristic lesion produced by columnaris is a pale white band encircling the body, often referred to as saddleback condition. As the infection progresses, a yellowish-brown ulcer often is found in the center of the “saddle”.