Relationship between growth of Escherichia coli and susceptibility to the lethal effect of paraquat.

Paraquat, which mediates increased O2- production within Escherichia coli, inhibits growth without causing cell death in a minimal medium, whereas it allows growth while decreasing viability in a rich medium. Agents or conditions that inhibit growth in the paraquat-containing rich medium prevent this loss of viability. Antibiotics, chelating agents, nutritional paucity, and excess paraquat all acted this way. NaCl added to 0.2 M to the plates used for enumeration after paraquat exposure did not significantly decrease viable cell counts, although it did so after exposure to a cationic detergent. DNA, rather than cell membranes, thus seems to be a more likely target for the oxygen-derived free radicals engendered by the cycles of enzymatic reduction and autoxidation of paraquat. Inhibition of growth decreases the likelihood that DNA replication will precede DNA repair. Investigators seeking to evaluate the bactericidal actions of paraquat under a variety of conditions will need to be cognizant of this need for growth.