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.