| Summary: | Antimicrobial peptides are potential molecules for the development of novel antibiotic agents. The ZorO toxin of a type I toxin−antitoxin system in <i>Escherichia coli</i> O157:H7 is composed of 29 amino acids and its endogenous expression inhibits <i>E. coli</i> growth. However, little is known about its inhibitory mechanism. In this study, we demonstrate that the ZorO localized in the inner membrane affects the plasma membrane integrity and potential when expressed in <i>E. coli</i> cells, which triggers the production of cytotoxic hydroxyl radicals. We further show that five internal amino acids (Ala−Leu−Leu−Arg−Leu; ALLRL) of ZorO are necessary for its toxicity. This result prompted us to address the potential of the synthetic ALLRL peptide as an antimicrobial. Exogenously-added ALLRL peptide to Gram-positive bacteria, <i>Staphylococcus aureus</i> and <i>Bacillus subtilis</i>, and a fungus, <i>Candida albicans</i>, trigger cell membrane damage and exhibit growth defect, while having no effect on Gram-negative bacterium, <i>E. coli</i>. The ALLRL peptide retains its activity under the physiological salt concentrations, which is in contrast to natural antimicrobial peptides. Importantly, this peptide has no toxicity against mammalian cells. Taken together, an effective and short peptide, ALLRL, would be an attractive antimicrobial to Gram-positive bacteria and <i>C. albicans</i>.
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