Growth studies on Tetrahymena geleii H.

l’ctmhymena g&ii II, a holotrichous protozoan, was isolated as a bacteria-free culture in 1931 by Dr. Alford Hetherington at Pacific Grove, California. The culture of the ciliate employed in the present studies was obtained in 1945 through the courtesy of Dr. George W. Kidder, who had received a culture from Dr. R. H. Hall in 1938 (2). Hetherington named this microorganism Colpidium campylum, but in 1940 Furgason (3) renamed it T. geleii, in honor of the eminent protozoologist, J. von Gelei. It was observed by Furgason that T. geleii and other similar organisms differed morphologically from Colpidium and related genera. Investigations on the nutrition and metabolism of T. geleii H were initiated by Hall (4-9) in 1935 and by Kidder (10) in 1939. During the past decade, Kidder and collaborators have studied the requirements of T. geleii H and other strains for amino acids (2, ll-15), vitamins (16-22), carbohydrates (2, 12), purines (23), pyrimidines (23), nucleosides (23), nucleotides (23), growth factors (10, 24-28), and other substances (11, 29). The present authors’ interest in Tetrahymena was stimulated by the report of Kidder and Dewey (2, 18, 24) in 1944 that certain amino acids were essential for the growth of these protozoa and that bound, as well as free, amino acids were utilized. It seemed appropriate, therefore, to study further the nutrition of Tetrahymena and to explore the potentialities of this organism for the determination of amino acids in unhydrolyzed proteins. Although microbiological assay, isotope dilution, amino acid decarboxylase, and calorimetric and other procedures were available for the determination of free amino acids, there was no general method for the determination of bound amino acids. In 1944, Regnery (30) determined leucine microbiologically in unhydrolyzed casein with the aid of