Further evidence for the regulation of bacterial populations in soil by protozoa.

After the addition to soil of large numbers of a cowpea Rhizobium strain, the population declined steadily until the numbers reached about 10(7)/g, and the protozoa rose to about 10(4)/g. When indigenous protozoa were suppressed by the addition of actidione to the soil, the density of the test rhizobium did not fall initially, but its abundance declined to about 10(7)/g when actidione-resistant protozoa arose in significant numbers. The addition to actidione-treated soil of an antibiotic-resistant strain of Paramecium led to a rapid decrease in the population of the rhizobium, the density reaching essentially the same value as in soil receiving neither the drug nor the paramecia. The same changes occurred with Xanthomonas campestris as test prey except that its numbers fell to about 10(5)/g of soil. These data provide further evidence for the key role of protozoa in controlling the abundance of populations of certain bacteria introduced into soil.