Amyl-1-18, an octadecapeptide derived from a-amylase (Amyl-1) of rice (Oryza sativa L. japonica), is a novel cationic a-helical antimicrobial peptide (AMP) that contains two lysine and two arginine residues. The antimicrobial activity of Amyl-1-18 against human pathogens was quantitatively evaluated using a chemiluminescence method that measures ATP derived from viable cells. Of the ten kinds of human pathogens, Amyl-1-18 exhibited antimicrobial activity against nine. Its 50% growth-inhibitory concentrations (ICs50) against Porphyromonas gingivalis, Propionibacterium acnes, Pseudomonas aeruginosa, Candida albicans, and Streptococcus mutans were 13, 19, 50, 64, and 77 mu M, respectively. Amyl-1-18 had little or no hemolytic activity even at 500 mu M, and showed negligible cytotoxicity up to 1200 mu M. The degree of 3,3'-dipropylthiadicarbocyanine iodide release from P. gingivalis cells induced by the addition of Amyl-1-18 was significantly lower than that induced by the addition of melittin. Flow cytometric analysis showed that the percentages of P. aeruginosa, S. mutans, and C. albicans cells stained with propidium iodide (PI), a DNA-intercalating dye, were 89%, 43%, and 3%, respectively, when Amyl-1-18 was added at a concentration equal to its 43IC(50). Therefore, the antimicrobial activity of Amyl-1-18 against P. aeruginosa and S. mutans appears to be mainly attributable to its membrane-disrupting activity. In contrast, its antimicrobial activity against P. gingivalis and C. albicans most likely depends upon interactions with intracellular targets other than their cell membranes. Collectively, these results indicate that Amyl-1-18 is useful as a safe and potent AMP against the pathogens described above in many fields of healthcare. (C) 2015 Wiley Periodicals, Inc.