Objective: Antimicrobial peptides (AMPS) have shown the ability to inhibit planktonic bacteria and biofilms. The objectives of this study were to de novo design and synthesize a series of cationic, amphipathic alpha-helical AMPs that would be shorter, less cytotoxic, and more potent than existing AMPs against cariogenic bacteria. Design: Three short AMPs (GH8, GLLWHLLH-NH2; GH12, GLLWHLLHHLLH-NH2; and GH16, GLLWHLLHHLLHLLHH-NH2) were designed, synthesized and characterized structurally. Minimal inhibitory. concentration (MIC) and minimal bactericidal concentration (MBC) against eight major cariogenic bacteria were tested to select the most promising peptide. Scanning electron microscopy (SEM) was used to observe the bacterial membrane after treatment with selected peptides. The bactericidal kinetics, effects on biofilm and cytotoxity were further investigated. Results: Of the three AMPs, GH12 had the most balanced structural parameters and a high content of alpha-helical structure. GH12 had a MIC of 4.0-8.0,mu g/mL and MBC of 8.0-32.0,mu g/mL. The corresponding values for the other two AMPs were 2-to 64-fold higher. In time-kill assays, GH12 killed all bacterial strains within 60 min at 4-fold MBC. SEM observed lysis and pore formation of the cytomembrane after treatment with GH12. 8.0 mu g/mL GH12 inhibited Streptococcus mutans biofilm formation. Confocal laser scanning microscopy showed that GH12 effectively reduced the biomass of 1-day-old S. mutans biofilm. Cytotoxicity assays indicated that GH12 showed little toxic effect on the viability of human gingival fibroblasts. Conclusion: These results indicate that GH12 shows antimicrobial activity against cariogenic bacteria and biofilms in vitro. (C) 2017 Elsevier Ltd. All rights reserved.