Effects of paraquat on cultures of Escherichia coli: turbidity versus enumeration.

The dioxygen-dependent toxicity of paraquat has been studied both in terms of its effects on growth, monitored as increases in turbidity, and on viability, measured by plating and counting of colonies. In the absence of paraquat, turbidity and enumeration increased in parallel. However, in the presence of paraquat, turbidity increased for several hours even while enumeration indicated a marked decrease in viability. The basis for this apparent discrepancy is continued increase in size of individual cells, which have stopped dividing and are losing viability under the influence of paraquat. It can evidently be misleading to study the effects of paraquat on microorganisms in terms of changes in turbidity.