The adaptation of bacteria to mono-and bis-type quaternary ammonium compounds (QACs) was carried out and studied. The minimum inhibitory concentration (MIC) of bis-QACs against the bacteria in contact with bis-QACs was nearly the same as that against the control. However, the MIC of mono-QAC against the strains adapted to mono-QAC was higher than that against the control. Moreover, bis-QACs were effective against even the strains adapted to mono-QAC. As the cell surface of the strains adapted

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... mono-QAC changed to hydrophilic, the difference in the bacterial adaptation was thought to arise from a potent property of bis-QACs whose antimicrobial characteristics are independent of the change in the cell surface hydrophobicity.