Mechanism of the action of didecyldimethylammonium chloride (DDAC) against Escherichia coil and morphological changes of the cells

The antibacterial activity of a disinfectant with geminated twin long-chain alkyl groups, didecyldimethylammonium chloride (DDAC), was investigated. The minimum inhibitory concentration (MIC) value of DDAC against Eschrichia coil was revealed to be a small value, 1.3 mg/L, by using the specific growth rates, mu, obtained from the cultivation in a liquid nutrient broth (NB) medium. The relationship between the leakage of proteins or beta-galactosidase and the DDAC concentration showed that the leakage of intracellular macromolecules occurs at around 3 - 4 mg/L DDAC. Furthermore, the effect of DDAC on the enhancement of membrane fluidity was examined by using liposomes labeled with a fluorescent probe. It was shown that the phase transition occurs at around 3 mg/L DDAC. The bleb formation of E. coli cells in the presence of DDAC was also examined by use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). However, bleb formation was not observed at around 3 mg/L DDAC but at concentrations higher than 50 mg/L. These results suggested that the action of DDAC toward the cell membrane causes the leakage of the intracellular molecules and the subsequent death of the cells. Thus the bleb formation seemed to be a result of the action of the DDAC toward the cell membrane but not to be a reason for the death of the cells.