Proadrenomedullin N-terminal 20 peptide (PAMP) is a regulatory peptide that is found in various cell types. It is involved in many biological activities and is rich in basic and hydrophobic amino acids, a common feature of antimicrobial peptides (AMPs). In this study, the cell selectivity and antimicrobial mechanism of PAMP and its C-terminal peptide, PAMP(9-20), were investigated. PAMP and PAMP(9-20) displayed potent antimicrobial activity (minimum inhibitory concentration: 4-32 mu M) against standard bacterial strains, but showed no hemolytic activity even at the highest tested concentration of 256 mu M. PAMP(9-20) showed 2- to 4-fold increase in antimicrobial activity against gram-negative bacteria compared to PAMP. Cytoplasmic membrane depolarization, leakage of calcein dye from membrane mimic liposomes, SYTOX Green uptake, membrane permeabilization, and flow cytometry studies indicated that the major target of PAMP and PAMP(9-20) is not the microbial cell membrane. Interestingly, laser-scanning confocal microscopy demonstrated that FITC-labeled PAMP and PAMP(9-20) enter the cytoplasm of Escherichia coli similar to buforin-2, and gel retardation assay indicated that PAMP and PAMP(9-20) effectively bind to bacterial DNA. These results suggest that the intracellular target mechanism is responsible for the antimicrobial action of PAMP and PAMP(9-20). Collectively, PAMP and PAMP(9-20) could be considered promising candidates for the development of new antimicrobial agents. (C) 2020 Elsevier Inc. All rights reserved.