ε-poly-L-lysine-modified orthodontic attachment resin with dual antimicrobial activity against Streptococcus mutans and Candida albicans: an in vitro study

White spot lesions (WSLs) are a common complication in clear aligner treatment. The colonization of Streptococcus mutans and Candida albicans plays a key role in the process. This study developed a ε-poly-L-lysine (ε-PL)-modified orthodontic resin attachment that combines mechanical stability and antimicrobial activity to prevent attachment-related WSLs. The minimum inhibitory concentration (MIC) and minimum bactericidal/fungicidal concentration (MBC/MFC) of ε-PL against S. mutans (UA159), C. albicans (SC5314), and C. albicans dental isolates were determined. To compare the phenotypes of the dental-derived clinical isolates with the reference strain SC5314, growth kinetics and a serial-dilution spot assay were performed. Then, ε-PL (0–3%) was added to resin, and the flexural strength, Vickers hardness, surface roughness, water contact angle, surface morphology, and cytocompatibility were evaluated. Antimicrobial efficacy was assessed against multispecies biofilms by live/dead staining and MTT assays, and against mono-species S. mutans and C. albicans biofilms (cultured separately) by CFU assays. ε-PL showed potent antimicrobial activity against S. mutans and C. albicans , and remained effective against clinical isolates (MIC range: 16–128 µg/mL). The 0.75% ε-PL group maintained flexural strength and Vickers hardness comparable to the control ( P > 0.05), and significantly inhibited multispecies biofilm formation and metabolism, reducing S. mutans and C. albicans colonization by 1–2 orders of magnitude ( P < 0.05). This novel resin attachment containing 0.75% ε-PL showed antimicrobial efficacy without exacerbating mechanical properties and biocompatibility, suggesting its potential clinical use in clear aligner therapy.