The production of free sulphur from l-cystine by a soil bacterium.

IN the ordinary putrefaction of protein material the protein-sulphur is liberated mainly as hydrogen sulphide accompanied in a few cases by relatively small amounts of mercaptans. The present paper is an account of an organism which is capable of the aerobic degradation of l-cystine with the direct liberation of free sulphur. In earlier work on the bacterial decomposition of cystine the medium used has always contained other sources of carbon such as carbohydrates or peptone, owing apparently to the difficulty of obtaining an organism capable of developing on cystine alone. Thus Sasaki and Otsaki [1912], who found that nineteen out of twenty-one different pure cultures formed hydrogen sulphide from cystine, employed Frankel's medium containing asparagine, ammonium lactate and inorganic salts. Burger [1914] also used this medium or a cystine medium which contained meat extract and lead acetate. He examined twenty-three different species of bacteria and observed hydrogen sulphide production in every case but never detected mercaptans. The question of mercaptan formation from cystine is still not settled; Kondo [1923] in a review of the available evidence concludes that the mercaptan arises from the interaction of hydrogen sulphide with substances derived from carbohydrates or histidine, and that in the absence of these the sulphur is liberated as hydrogen sulphide. During the course of our work a somewhat different mode of attack was described by Tarr [1933] who, using washed cells of Proteus vulgaris, quantitatively investigated the decomposition of cystine under anaerobic conditions. In this case too the sulphur appeared almost quantitatively as hydrogen sulphide, the cystine molecule yielding two molecules each ofhydrogen sulphide, ammonia, acetic and formic acids. In order to maintain the conditions of our own experiments as simple as possible an attempt was made to find some organism capable of utilising cystine as the sole source of sulphur, nitrogen and carbon. From garden soil a bacterium was isolated which was found to be able to attack cystine somewhat slowly in the absence of all other compounds of carbon, nitrogen or sulphur. On examination of the products it was found that free sulphur and free ammonia were produced in approximately equimolecular proportions and this sulphur appears to arise from the cystine without the intermediate formation of detectable amounts of hydrogen sulphide. EXPERIMENTAL.