Characterization of mutants of the fish pathogen Flavobacterium psychrophilum

The present study focuses on the phenotypic characterization of several mutants of Flavobacterium psychrophilum, obtained from a transposon mutant library. This Gram-negative bacterium is the etiological agent of the "cold water disease", pathology that usually affects salmonids, mainly Oncorhynchus mykiss. This microorganism is considered a "fastidious bacterium" due to the difficulty to isolate it. Multiple secreted molecules have been described as contributors to virulence, such as proteases, toxins, and/or adhesins. In addition, F. psychrophilum has a particular way to move across wet surfaces, a physiological process known as gliding. In this research, four mutants were selected from a F. psychrophilum transposition library: (i) an ATPase chaperone activity (ClpB); (ii) a secretion system regulatory protein (PorY); (iii) a protein involved in exopolysaccharide synthesis (Wzc); (iv) a fungalysin with proteolytic activity (FpfB). Mutant characterization in some parameters (such as growth and colony spreading) showed that there were not significant differences between the mutants and the parental strain (except for the clpB mutant). Further analysis of the clpB mutant revealed differences in LD₅₀ and response to H₂O₂ and thermal shock compared with the parental strain.