Effects of injured and dead cells of Escherichia coli on the colony-forming rate of live cells.

Osmotic stress-induced injured cells of Escherichia coli were prepared by sorting live cells onto tryptic soy agar (TSA) containing 10-50% sucrose. The time course of colony-forming rate (CFR%) was analyzed. A time delay in colony formation indicated a sublethal effect. The final CFR level at 24 h indicated the relative number of culturable cells irrespective of injury. A value of (100-CFR)% at 24 h indicated a lethal effect. When cells were grown on TSA containing 10% sucrose, the time delay was 4 h and the lethal effect was 4%. However, dead cells inhibited the growth of live cells. Physical contact with insoluble matter derived from dead cells or dead cells themselves might have caused growth inhibition. These findings highlight a novel perspective on colony count methods in practical situations, such as when sampling foods containing a high concentration of sucrose.