Escherichia coliresistance to chlorine after successive chlorinations

Abstract
The resistance of Escherichia coli to chlorine in liquid medium was determined after repeated exposures without subculture. The chlorine concentration required to inactivate, in 5 minutes, 50% of a 107 bacteria per milliliter population increased from 0.01 mg.l‐1 to 0.55 mg.l‐1 after 7 exposures. Tests performed in parallel showed that after 11 chlorinations the bacteria were slimy and 10 times more resistant to chlorine than the controls.