Incidence of coliform bacteria in the intestinal tract of Gambusia affinis holbrooki (Girard) and in their habitat water.

were measured on nutrient agar plates. In all three species, 0.1, 0.5, and 1.0% lithium chloride progressively increased the colony sizes. On the other hand, 2.0, 3.5, and 5.0% LiCl progressively reduced the colony diameters, resulting in the isolation of predominantly small-colony variants (Table 1). It is probable that the higher concentrations of LiCl selected small-colony variants from the original population, since there was a marked reduction in the total population when the cells were grown in 2% LiCl or higher concentration. At the lower concentrations of LiCl, selection may still have been involved; however, probably other mechanisms were involved that stimulated the growth of the colonies. Evidence is presented for the existence of diphasic effects of LiCl on the colony size of shigellae. A wide variety of diphasic effects have been recorded with enzyme poisons (Clark, General Pharmacology, "Handbuch der Experimentellen Pharmakologie," vol. 4, Verlag von Julius Springer, Berlin, 1937). To our knowledge this is the first report of diphasic colony variation caused by a toxic agent.