In cultures of Escherichia coli strain K12, the sequence of physiological events at the time of growth cessation in populations limited by availability of carbon source (glucose), nitrogen source (ammonium sulfate), and oxygen was observed to differ in several respects. These included differences in the relationship between mass synthesis and cell division, the utilization of nonlimiting nutrients, and the nature of changes in energy metabolism. These observations are discussed with reference to problems involved in the experimental control of variables in growth studies.

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The relationship observed between maximal attainable growth in a microbial population and initial concentration of limiting nutrient depends upon the parameter selected for measurement of growth. A linear relationship is found only when cell mass is the parameter. When population is used as the measure of growth, the relationship is: Nmax = KtCo8, where Nmax is the maximal attainable population, Co is the initial concentration of limiting nutrient and K' and s are constants. The use of this expression in demonstrating subtle differences between strains is shown. I This paper is based on a dissertation submitted by the senior author to the faculty of Iowa State University in partial fulfillment of the requirements for the Ph.D. degree.

MEWork with the drug is continuing, and, if our present results are confirmed, chlorthenoxazin should prove a most useful addition to the present range of mild analgesics.We are grateful to Messrs. For 41 men the coefficient of correlation between the two instruments was 0.81, and for 23 women 0.62. These can be compared with the coefficient of 0.86 obtained by Higgins (1957). The mean values of various categories for the two instruments are shown in the

Escherichia coli was grown in continuous culture at various rates in a defined medium with either the carbon or the nitrogen source as rate-limiting substrate. Average cell size and the cellular content of ribonucleic acid, protein, and free amino acids varied both with growth rate and with the identity of the limiting nutrient, being greatest at high growth rates, and greater for nitrogen-limited than for carbon-limited populations. Immunoelectrophoresis of cell extracts revealed antigenic components associated uniquely with growth rate, and others associated with the identity of the limiting nutrient on which cells had been grown. Cell composition thus appears to reflect the physiological adjustment of a microbial population to its total environment.