AUTO-ADAPTATION: A NEW PHENOMENON OBSERVED IN A BACTERIAL POPULATION

Abstract

A strain of Escherichia coli, with a partial requirement for glycine, grown in a liquid "minimal" medium, undergoes progressive, stepwise increases in growth rate until glycine -synthesis no longer limits growth (wild type). Growth curves are composed of sharply defined logarithmic phases, whether the cell density is followed optically or by means of viable count. A rate increase is attributed to a mass conversion of the population rather than to a selection mechanism, for a number of reasons. 2 of them are: (1) A culture of partially reverted cells growing at rate A can establish a higher growth rate, B, 10 times faster than would be expected if cells growing at rate B were selected in a culture consisting predominantly of cells growing at rate A. (2) By plating frequently on minimal agar during an increase in growth rate, the ratio of type A and type B cells can be followed. The over-all change in ratio of the 3 cell types takes place about 10 times faster than can be expected on the basis of selection of the faster growing B cells. Mathematical analysis of growth curves of a transforming culture shows that A cells undergo conversion to B cells at the high rate of 1.2 per generation of A cells. Thus, an originally homogeneous population adapts itself, or auto-adapts, whether in the general sense of the culture as a whole becoming more efficient at growing in a minimal medium, or, in the stricter sense implying a mechanism which may be analogous to enzymatic adaptations. A possible interpretation is that the level of a substrate (glycine precursor) builds up during the cells'' metabolism, and it is this periodic accumulation which results in stepwise increases of enzymatic activity. The cultures can be "de-adapted" by supplying excess glycine, the utilization of which presumably results in the removal or inhibition of the adaptive substrate and its enzyme.