PHYSIOLOGICAL ADAPTATION IN BACTERIA

Bacteria are small, unicellular organisms whose ecological niche comprises an incredibly large number of microcosms which are subject to frequent and cataclysmic changes in their physical and chemical properties (Stanier, 1953). Evolutionary survival of these organisms has depended not only upon their ability to grow as rapidly as possible under a wide variety of ambient conditions, but also upon their ability to adjust as rapidly as possible to frequent changes in these conditions. Thus it is not surprising to find that bacteria possess many regulatory devices which render their metabolic processes adaptively responsive to the environment. These devices permit adjustment of the cells ' internal affairs so as to allow a rapid, orderly synthesis of protoplasm in the face of inconstant ambient conditions. Two devices which seem to contribute to the adap-tive responsiveness of cellular metabolism will be dis-cussed in great detail at this Symposium: the adjust-ment of the rates of microbial biosynthetic reactions (those resulting in the synthesis of amino acids, pu-rines, and pyrimidines) by means of endproduct inhi-bition, and the adjustment of the cell's enzymic con-stitution by means of induction-repression. These two regulatory mechanisms make the cell responsive to the qualitative nature of its chemical environment. Thus, in the presence of amino acids x, y, and z rela-tively little of each enzyme concerned with the for-mation of these compounds will be made by the cell, and the activity of each pathway will be further re-duced by the inhibitory action of x, y, and z on the initial enzyme of each pathway. Both mechanisms re-sult in responses of the cell to specific chemical stimuli. This paper describes a third regulatory device, somewhat different in character from the foregoing two in that it permits a response of the microbial cell (adjustment of its RNA content) to a general prop-erty of its chemical environment (growth-supporting ability)