The Selection of Bacterial Food by Soil Amoebae, and the Toxic Effects of Bacterial Pigments and Other Products on Soil Protozoa

IT has been shown that soil protozoa, especially amoebae, do not feed indis- criminately on any bacteria (Singh, 1941a, b; 1942a, b).In a preliminary survey (Singh, 1942b) of the edibility of miscellaneous bacteria (including plant pathogens) mostly from soil a negative correlation was observed between edibility and pigment formation.A suggestiont was made (Singh, 1942b) that pigment formation by bacteria exerts a protective action against protozoal attacks.An attempt has been made in the present work outlined to find out what determines whether a bacterial strain is eaten by amoebae, and to what extent amoebae are sensitive to certain metabolic products of other organisms.A few scattered accounts of the toxic action on protozoa of the metabolic products of bacteria, and of pigments among them, are found in the literature.Thus Birch- Hirschfeld (1934), Chatton, E., and Chatton, M. (1927a, b), Cleveland (1928), Kidder and Stuart (1939) and others have carried out some inconclusive experiments to show that bacterial pigments and metabolic products of bacteria are toxic to protozoa.Chatton and Chatton (1927a, b) claim that the chromogenic bacteria Ps. fluorescens and Chr.prodigiosum exert a cytolytic effect on a number of ciliates.Although they noted that this effect was most marked in highly pigmented strains, they drew the conclusion that pigment itself was not toxic; it was a combination of glucoside and lipoid that exerted the toxic effect upon the ciliates.They also found that saline extracts from Ps. pyocyanea (agar cultures) exerted cytolytic action on different ciliates.Birch- Hirschfeld (1934) obtained similar results with alcoholic extracts.They did not try to separate the various metabolic products from P8. pyocyanea, and study their toxic action on protozoa.Kidder and Stuart (1939) on the other hand, working with a ciliate (Colpoda sp.), claim that it is the pigment of Chr.prodigiosum, Chromobacterium violaceum and Ps.pyocyanea that are toxic; their conclusions are in.agreementwith the work carried out by the writer, but their experiments are inconclusive.Thev did not extract the pigments from the bacteria; they claim that when healthy ciliates and their cysts are put into heavy suspensions of Chr.prodigiosum and Chr.violaceum, the protozoa take up the colour diffusing fronm the bacteria.Recently Schoental (1941) has isolated three antibacterial substances from Ps. pyocyanea cultures: (1) Pyocyanin, the blue pigment which appears in the young cultures during the maximal growth of bacteria.(2) A yellow pigment appearing in * A part of this work was made possible by a grant from the Agricultural Research Council, to whomn the author's thanks are due.t The author is very grateful to Dr. H. G. Thomton, F.R.S., for this suggestion, from which the work on toxic effects of bacterial pigments, etc., originated.