Since 5-Chloro-2-methyl-4-isothiazolin-3-one (CMI) has relatively wide antibacterial spectrum, and low toxicity to human, it is widely used as a bactericide and fungicide, or a slime control agent for a cooling tower. The minimum inhibitory concentration of CMI to E. coli K-12 was 1.6mg·l-1. Short time (-45min) exposure of E. coli K-12 to CMI solution of 20mg·l-1 prolonged lag phase of the cell growth. On the other hand, normal logarithmic growth phase was observed after the lag phase. When E. coli K-12 was exposed to CMI of 1.0mg·l-1 for 25 hours, the sterilization fraction reached to 99%. CMI concentration of more than 10mg·l-1 and more than 5 hours exposure time were needed for attaining 99.9% sterilizing fraction. The antibacterial activity of CMI to E. coli K-12 cells embedded in 0.6% agar was high in the upper layer of the biofilm and low in the bottom layer of the biofilm. When activated sludge was supplied with artificial sewage with CMI of 20mg·l-1, bacteria resistant to CMI became dominant in the bacterial consortia.
