| Summary: | <i>Edwardsiella tarda</i> can cause fatal gastro-/extraintestinal diseases in fish and humans. Overuse of antibiotics has led to antibiotic resistance and contamination in the environment, which highlights the need to find new antimicrobial agents. In this study, the marine peptide-N6 was amidated at its C-terminus to generate N6NH2. The antibacterial activity of N6 and N6NH2 against <i>E. tarda</i> was evaluated in vitro and in vivo; their stability, toxicity and mode of action were also determined. Minimal inhibitory concentrations (MICs) of N6 and N6NH2 against <i>E. tarda</i> were 1.29–3.2 μM. Both N6 and N6NH2 killed bacteria by destroying the cell membrane of <i>E. tarda</i> and binding to lipopolysaccharide (LPS) and genomic DNA. In contrast with N6, N6NH2 improved the stability toward trypsin, reduced hemolysis (by 0.19% at a concentration of 256 μg/mL) and enhanced the ability to penetrate the bacterial outer and inner membrane. In the model of fish peritonitis caused by <i>E. tarda</i>, superior to norfloxacin, N6NH2 improved the survival rate of fish, reduced the bacterial load on the organs, alleviated the organ injury and regulated the immunity of the liver and kidney. These data suggest that the marine peptide N6NH2 may be a candidate for novel antimicrobial agents against <i>E. tarda</i> infections.
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