Sickeningly Sweet: L-rhamnose stimulates Flavobacterium columnare biofilm formation and virulence.

Flavobacterium columnare, the causative agent of columnaris disease, causes substantial mortality worldwide in numerous freshwater finfish species. Due to its global significance and impact on the aquaculture industry continual efforts to better understand basic mechanisms that contribute to disease are urgently needed. The current work sought to evaluate the effect of L-rhamnose on the growth characteristics of F. columnare. While we initially did not observe any key changes during the total growth of F. columnare isolates tested when treated with L-rhamnose, it soon became apparent that the difference lies in the ability of this carbohydrate to facilitate the formation of biofilms. The addition of different concentrations of L-rhamnose consistently promoted the development of biofilms among different F. columnare isolates; however, it does not appear to be sufficient as a sole carbon source for biofilm growth. Our data also suggest that iron acquisition machinery is required for biofilm development. Finally, the addition of different concentrations of L-rhamnose to F. columnare prior to a laboratory challenge increased mortality rates in channel catfish (Ictalurus punctatus) as compared to controls. These results provide further evidence that biofilm formation is an integral virulence factor in the initiation of disease in fish.

Antimicrobial activity of chitosan and a chitosan oligomer against bacterial pathogens of warmwater fish.

AIM: The antibacterial activities of chitosan (CS) and its derivative chitosan oligosaccharide lactate (COL) were evaluated against Aeromonas hydrophila, Edwardsiella ictaluri and Flavobacterium columnare, three highly pathogenic bacteria of warmwater finfish. METHODS AND RESULTS: The magnitude and mode of antimicrobial action on Gram-negative bacterial pathogens was investigated with an emphasis on examining the inhibition of bacterial growth and the weakening of barrier functions. Both CS and COL exhibited antibacterial activity against all three bacteria tested and their activity was dose-dependent. CS and COL completely inhibited growth of A. hydrophila at 0·8% and E. ictaluri and F. columnare at 0·4% or higher concentrations. COL was more effective in killing or inhibiting the growth of all bacteria tested. CS and COL molecules have the ability to interact with bacterial surfaces via adsorption. This was confirmed by initial decreases in the conductivity of CS or COL treated bacterial cell solutions. A marked re-increase in conductivity from 18 to 48 h was documented, which was due to the leakage of cellular ions into the solution through damaged bacterial cell membranes. CONCLUSION: Both CS and COL exhibited antibacterial activity against all three bacterial species through a sequential process beginning with adsorption to bacterial surfaces culminating in the leakage of intracellular constituents and cell death. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings indicate that CS-based strategies are promising candidates for exploration as alternatives to antibiotics for mitigating disease outbreaks in cultured fish.

A comparison of high- and low-virulence Flavobacterium columnare strains reveals differences in iron acquisition components and responses to iron restriction.

Flavobacterium columnare, the causative agent of columnaris disease causes substantial mortality worldwide in numerous freshwater finfish species. Due to its global significance, an improved understanding of the factors that contribute to virulence is urgently needed. In a laboratory challenge, we found that significantly greater mortality was observed in channel catfish Ictalurus punctatus (Rafinesque) challenged with isolate LSU-066-04 (LSU) as compared to fish challenged with isolate LV-359-01 (LV). Strikingly, mortality was 100% in LSU-challenged fish, with all fish dying within the first 24 h after challenge, while mortality in the LV-challenged group was significantly lower with 26.7% of fish dying on days 1-4 post-challenge. There were no differences in initial bacterial adhesion between the isolates at 1-2 h post-challenge; however, by 4 h LSU-challenged fish had a greater bacterial load on the gill. Next, to better understand this variation in virulence, we examined transcriptional and functional attributes related to iron acquisition. The isolates were differentially sensitive to iron restriction both in vitro and in vivo and the basal expression of TonB family member genes and a ferroxidase gene differed significantly. Our findings provide new insight into iron uptake and pathogen virulence, and offer promising new targets for columnaris prevention and treatment.

Kaolinitic clay protects against Flavobacterium columnare infection in channel catfish Ictalurus punctatus (Rafinesque).

Columnaris disease, caused by the bacterial pathogen Flavobacterium columnare, continues to be a major problem worldwide in both wild and cultured freshwater finfish. Despite the far-reaching negative impacts of columnaris disease, safe and efficacious preventatives and curatives for this disease remain limited. In this study, we evaluated the potential of kaolin (Al2 Si2 05 (OH)4 ), a type of clay, for the prevention of columnaris disease. Channel catfish, Ictalurus punctatus (Rafinesque), fingerlings were experimentally challenged with Flavobacterium columnare in untreated water or with water containing kaolin (1 g L(-1) ). Over the 7-day course of study, kaolin treatment led to significantly (P < 0.001) improved survival (96%) as compared to untreated fish (78% survival). Histological examination of the gills revealed that kaolin-treated fish had substantially less gill damage than untreated controls. Quantitative PCR analysis of gill tissue revealed that kaolin significantly reduced F. columnare adhesion (measured at 1 h post-challenge) and colonization (24 h post-challenge). Incubation of kaolin with F. columnare in vitro demonstrated that kaolin reduced the number of F. columnare cells in culture supernatants, presumably through the formation of physical complexes through adsorption. In summary, kaolin can improve survival, reduce gill pathologies and reduce bacterial attachment to key tissues associated with columnaris disease in channel catfish by binding to F. columnare.