Serotyping and genetic evaluation of rainbow trout-derived Flavobacterium columnare isolates

Flavobacterium columnare is the etiological agent of columnaris disease in rainbow trout Oncorhynchus mykiss. F. columnare has recently been speciated into 4 independent columnaris disease-causing Flavobacterium species. Several F. columnare isolates from different geographic origins have been tested for variations in virulence and genetic diversity via multilocus sequence analysis of housekeeping genes. These studies have been limited in their ability to differentiate between highly variable and evolving regions of genetic diversity or to reliably predict differences in virulence between strains. To initiate a serotyping scheme, polyclonal antibodies were generated to F. columnare strains CSF-298-10, MS-FC-4 and PSFC-081215-1, and 7 serotypes were identified based on Western blot analysis of 54 F. columnare strains. Genome sequencing identified a 35-40 kb region encoding 32-36 putative lipopolysaccharide (LPS)-related genes that support 7 serotypes. Genes within this region encode molecules associated with the Lipid A, core region and O-antigen. All isolates were derived from columnaris outbreaks in the Columbia River of Washington State and the Snake River region of Idaho (USA). Serotyping and regional genome analyses allow for rapid and focused determination of isolate comparisons for population heterogeneity, LPS evolution and strain tracking. This is of the utmost importance when considering that all of these isolates were derived from columnaris outbreaks within the main US rainbow trout-producing regions. Knowledge of genetic and serotype diversity will help inform development of therapeutic strategies and vaccines.