Figure 1. Channel catfish displaying skin lesions typically associated with columnaris disease.
The Flavobacterium columnare genome sequencing project is being conducted by Mark Lawrence, D.V.M., Ph.D. and Attila Karsi, Ph.D., of the Mississippi State University College of Veterinary Medicine, and David Dyer, Ph.D. and Allison Gillaspy, Ph.D., of the Laboratory for Genomics and Bioinformatics. These studies are supported by USDA/CSREES grant #2006-35600-16571.
Flavobacterium columnare is a long, slender gram-negative rod in the family Flavobacteriaceae, one of the main phyletic lines within the Bacteroidetes group from the domain Bacteria (3). Colonies are yellow-pigmented and typically rhizoid in appearance, although 3 slightly different colony morphologies have been described (9). Like other members of the Flavobacteriaceae, F. columnare is capable of gliding motility, the mechanism of which has been partially characterized in the related species Flavobacterium johnsoniae (5-7).
F. columnare is considered ubiquitous in the warmwater aquatic environment, and it is the causative agent of columnaris disease in fish. Columnaris disease is common throughout the world and infects most species of freshwater fish, including aquaculture species, wild fish populations, and ornamental fish. Commercially important aquaculture species affected include channel catfish, common carp, tilapia, rainbow trout, and eel. In particular, it is the second most prevalent bacterial disease affecting the channel catfish industry, which accounts for 85-90% of all fin fish aquaculture production in the United States. Case submissions from the past seven years at the College of Veterinary Medicine Aquatic Diagnostics Laboratory located at Stoneville, MS, indicated that columnaris disease is more commonly diagnosed than enteric septicemia of catfish (ESC), the most prevalent disease of catfish.
Columnaris disease is most commonly associated with stress, such as high temperatures, elevated organic loads, crowded conditions, and excessive handling (10, 11), which has made the development of a reliable challenge model difficult. These stressful environmental conditions are common in commercial aquaculture where producers are working to maximize production. The disease commonly occurs in channel catfish when water temperatures are in the range of 25 to 32C in the spring, summer, and fall.
Disease generally begins as an external infection on the skin, fins, gills, or oral cavity. On the skin and fins, lesions are characterized by dull, grayish-white or yellow erosive lesions that can progress to deep ulcers in the underlying muscle (Figure 1). Host inflammatory response in the underlying tissue is characteristically nonexistent. On the gills and in the oral cavity, brown to yellowish-brown necrotic lesions are often present (Figure 2), which often is concurrent with systemic infection and subacute mortalities (8). In some cases, systemic infection with little or no visible external or internal pathological signs may occur; in these cases, the pathogen can be isolated from the internal organs of fish (4). F. columnare infections can be chronic and cause lingering, gradually increasing mortalities in channel catfish, but more often, the disease appears suddenly and causes mortalities within a few days (1).
The F. columnare genome consists of a single circular chromosome with an estimated G+C content of 32% (2). The average reported genome size of bacterial species in the genus Flavobacterium has been reported as 4.1 ± 1 Mb (12). The F. columnare strain that was selected for sequencing is ATCC 49512.