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Updated: May 15, 2026

Antimicrobial Synergy Testing by the Inkjet Printer-assisted Automated Checkerboard Array and the Manual Time-kill Method
Published on: April 18, 2019
Synergistic bactericidal effects of a sublethal concentration of didecyldimethylammonium chloride (DDAC) and low
Mitsuhiro Gomi1, Yukihiko Osaki, Miho Mori
1Research and Development Center, KOBAYASHI Pharmaceutical Co., Ltd., Nara, Japan.
Abstract:
DDAC is an effective disinfectant used in the medical and food industries and the environmental field. However, skin irritation in humans occurs at high DDAC concentrations. In this study, we analyzed the combined effect of a low concentration (0.3 ppm) of DDAC and low concentrations (6, 8, and 10 ppm) of 37 products of nonionic surfactants on the bactericidal activity against S. aureus. No bactericidal activity was found at 0.3 ppm DDAC alone. Results showed that a combination of a low concentration of DDAC (0.3 ppm) and some nonionic surfactants tested (synergistic effect of five products ≥ 2.0) improved the bactericidal activity of DDAC. Synergistic effects of DDAC and some nonionic surfactants are desirable and were suggested to occur as follows. Test surfactants acted against the cell walls of S. aureus, which allowed DDAC to act easily on the lipid double membrane in the cell wall, thereby increasing the bactericidal activity of DDAC. In the present study, synergistic effects of a low concentration of DDAC and some nonionic surfactants were observed, a phenomenon that may be considerable value in future developments.
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