Exploration of the Anti-biofilm Effects of Fish Antimicrobial Peptides Against Common Aquaculture Pathogens

Abstract

Aquaculture’s rapid growth and commercialization has been accompanied by increased spread of fish-related diseases that threaten aquaculture’s production capabilities. Diseases caused by Gram-negative bacteria such as, Aeromonas hydrophila and Flavobacterium columnare, are particularly problematic and are responsible for tremendous losses in fish. These bacterial fish pathogens exist not only in a free-living state but also occur readily as biofilms. Biofilms are microbial communities incased in a self-produced organic matrix and are much more resistant to conventional antimicrobial agents than their free-living counterparts. The prevalence of these pathogenic bacteria as biofilms in the aquaculture setting has encouraged the exploration of alternative anti-biofilm agents. The innate immune system of fish and other organisms produce peptides that are effective in combatting a wide range of pathogens. These peptides are collectively known as antimicrobial peptides (AMPs) and they may prove to be useful anti-biofilm agents due to their broad-spectrum capabilities. These peptides have been characterized among many different fish species and their effect on different Gram-negative bacteria has been evaluated. However, there exists a gap of knowledge in their anti-biofilm capabilities. The current study has assessed the activity of different teleost AMPs against Aeromonas hydrophila, Aeromonas veronii, Flavobacterium columnare and Escherichia coli. The NK-lysins (NK-1, NK-2, etc.) inhibited the growth of E. coli and A. veronii and were particularly effective against biofilm formation in these two pathogens. The NK-lysins tended to show an expected dose response in that higher concentrations showed greater inhibition of E. coli and A. veronii. The Piscidins inhibited E. coli and F. columnare total growth and showed effectiveness against their respective biofilm formation. Unlike the NK-lysins, the Piscidins inhibited biofilm formation of E. coli and F. columnare at the lowest tested concentration. The positive control melittin demonstrated significant total and biofilm inhibition towards all four bacteria; some of the AMPs in this test showed results comparable to melittin. These results suggest that several of the AMPs might serve as effective therapeutic agents against bacteria in aquaculture settings due to their ability to inhibit both free-living and biofilm-encased bacteria and are worthy of further study. Future studies should examine mechanisms of action regarding biofilm inhibition, while also observing activity under more natural conditions.