Effect of lytic peptides on selected fish bacterial pathogens*

Abstract. Two synthetic lytic peptides, derivatives of the naturally occurring lytic peptide cecropin B, were tested for their effect on eight pathogenic fish bacteria. All bacteria were grown on tryptic soy agar at 30°C. Four levels (dilutions) of lytic peptides were incubated with viable cells in the log phase for 1 h. Standard plate counts were made after 24±2h with control plates (no toxin) representing 100% survival. Significant differences (P<0.05) in the toxicity of the two peptides were found for six out of eight bacterial pathogens. Peptide concentration or dilution level also caused significant difference (P<0.01) in bacterial counts. Peptide type by dilution level interactions were significant (P<0.01) for only two bacterial pathogens. The LD₅₀. in molar concentrations of the lytic peptides on the bacterial pathogens, ranged from approximately 10^?6 to 10^?9. The possible role of utilizing these peptides to enhance disease resistance in aquaculture is discussed.