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

Authors: Aksoy, Mediha; Beck, Benjamin

Submitted to: Journal of Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2017
Publication Date: 5/3/2017
Citation: Aksoy, M., Beck, B.H. 2017. Antimicrobial activity of chitosan and a chitosan oligomer against bacterial pathogens of warmwater fish. Journal of Applied Microbiology. 122:1570-1578.

Interpretive Summary: Fish producers are eager for new non-antibiotic strategies to prevent and treat costly diseases on farms. Three of the most problematic pathogens of the United States farmed catfish industry are Aeromonas hydrophila, Edwardsiella ictaluri and Flavobacterium columnare. In the present study we evaluated the antibacterial activity of two naturally occurring non-antibiotic compounds called chitosan and chitosan oligosaccharide lactate, which are derived from the shells of invertebrates such as shrimp. We found that both of these compounds were effective at stopping bacterial growth and were capable of killing bacteria in laboratory tests. We further characterized the mechanism of action and determined that these compounds act by coating bacteria and disrupting the protective barrier function of their outer membranes. These findings are a crucial first step in developing strategies to exploit the antibacterial activity of chitosan-based approaches for combating disease outbreaks in aquaculture.

Technical Abstract: 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 kinetics of cell proliferation and viability of bacteria were measured over time. The mode of their antimicrobial action on gram negative bacterial pathogens was investigated with an emphasis on their ability to bind to the cell surface and weaken barrier functions. Both CS and COL exhibited antibacterial activity against all three bacteria tested and their activity was dose-dependent. Chitosan 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 over a 72 h incubation period. 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 18h to 48h 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 bacteria. CS and COL molecules interact with bacterial surfaces due to the attraction of opposing charges. Significance and Impact of the Study: These findings indicate that chitosan-based strategies are promising candidates for exploration as alternatives to antibiotics for mitigating disease outbreaks in cultured fish.