Abstract
A reciprocal replacement of CO2 and oleic acid exists in the nutrition of the minute streptococci and 2 Lactobacillus leichmannii strains. This mutual exchange of CO2 and oleic acid was not demonstrated among other bacteria known to require one or the other of these substances. High concentrations of pyruvate partially eliminate the need by the test microorganisms for either CO2 or oleic acid. Yeast extract contains a nonlipoidal, dialyzable substance which also replaces the CO2 or oleic requirements of the test microorganisms. This substance does not occur in any appreciable quantity in other common, commercial medium constituents.