Antimicrobial peptides (AMPs), particularly those effective against methicillin-resistant Staphylococcus aureus (S. aureus) and antibiotic-resistant Pseudomonas aeruginosa (P. aeruginosa), are important alternatives to antibiotics. Typical peptide synthesis methods involving solid-phase sequential synthesis are slow and costly, which are obstacles to their more widespread application. In this paper, we synthesize peptides via ring-opening polymerization of alpha-amino acid N-carboxyanhydrides (NCA) using a transition metal initiator. This method offers high potential for inexpensive synthesis of substantial quantities of AMPs. Lysine (K) was chosen as the hydrophilic amino acid and alanine (A), phenylalanine (F), and leucine (L) as the hydrophobic amino acids. We synthesized five series of AMPs (i.e., P(KA), P(KL), P(KF), P(KAL), and P(KFL)), varied the hydrophobic amino acid content from 0 to 100%, and determined minimal inhibitory concentrations (MICs) against clinically important Gram-negative and Gram-positive bacteria and fungi (i.e., Escherichia coli (E. coli), P. aeruginosa, Serratia marcescens (S. marcescens), and Candida albicans (C. albicans). We found that P(K10F7.5L7.5) and P(K10F1.5) show the broadest activity against all five pathogens and have the lowest MICs against these pathogens. For P(K10F7.5L7.5), the MICs against E. coli, P. aeruginosa, S. marcescens, S. aureus, and C. albicans are 31 mu g/mL, 31 mu g/mL, 250 mu g/mL, 31 mu g/mL, and 62.5 mu g/mL, while for P(K10F15) the respective MICs are 31 mu g/mL, 31 mu g/mL, 250 mu g/mL, 31 mu g/mL, and 125 mu g/mL. These are lower than the MICs of many naturally occurring AMPs. The membrane depolarization and SEM assays confirm that the mechanism of microbe killing by P(K10F7.5L7.5) copeptide includes membrane disruption, which is likely to inhibit rapid induction of AMP-resistance in pathogens.