SUMMARY: Cell mass, the average number of nuclei/cell and the content of RNA and DNA were studied in Salmonella typhimurium during balanced (steady state) growth in different media. These quantities could be described as exponential functions of the growth rates afforded by the various media a t a given temperature. The size and chemical composition characteristic of a given medium were not influenced by the temperature of cultivation. Thus, under conditions of balanced growth, this organism exists in one of a large number of possible stable physiological states.The variations in mass/cell are due to changes in the number of nuclei/cell as well as in mass/nucleus. An increase in the number of ribonucleoprotein particles at higher growth rates could, it appears, largely account for the increase in mass/nucleus. Calculations indicate that the rate of protein synthesis per unit RNA is nearly the same a t all growth rates.

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SUMMARY : When cultures of Salmonella typhimurium undergoing balanced growth are shifted from one medium to another, a definite pattern of rate changes is observed. Shifts from a low to a high growth rate result in a strict succession of events: RNA synthesis is immediately affected and its rate rapidly increases to that characteristic of the new medium; the increase in optical density shows a lag of a few minutes before the new rate is attained; DNA synthesis and cell division, on the other hand, continue at the old rate for appreciable periods of time and then abruptly shift to the new rates. The times at which these shifts take place are, at 37', invariably 20 and 70 min., regardless of the actual growth rates before and after the shift. This rate maintenance effect on DNA synthesis and cell division is discussed in terms of specific rate-controlling mechanisms.In the preceding paper (Schaechter, Maalnre & Kjeldgaard, 1959) we showed that the size and chemical composition of Salmonella typhimurium vary exponentially with the growth rate afforded by different media. The present paper describes the transition from one state of balanced growth to another. The terms 'balanced growth' and 'cell' as used by us, have been defined in the preceding paper. Two types of experiments were performed, both involving precisely timed changes of medium at constant temperatures; one going to a higher growth rate (shift up), and the other to a lower rate (shijt d m ) .The shift up, exemplified by going from a glucose salt medium to nutrient broth, is characterized by an orderly dissociation of the main synthetic activities. Upon addition of broth, the rates of synthesis of ribonucleic acid (RNA) and of total mass immediately increase, whereas deoxyribonucleic acid (DNA) synthesis and cell division continue at the old rate for considerable periods of time. The new state of balanced growth is reached when, after about 70 min. (at 37O), the rate of cell division abruptly changes from thepreshift value to the definitive broth rate. We interpret this 'rate maintenance effect ' as evidence for the existence of separate rate-controlling mechanisms.The shqt down is rapidly effected by filtration of, say, a nutrient broth culture and resuspension of the cells in simple defined medium. It is characterized by a period during which cell division and DNA synthesis continue in the absence of net synthesis of RNA and total mass. By the time RNA and mass begin to increase the cells have already approached the size and the composition typical of balanced growth in the simpler medium. This behaviour

SUMMARYThe timing of cell and nuclear division of certain enteric bacteria was determined under conditions of balanced growth. Organisms were grown in a high refractive index medium and photographed at frequent intervals with a phase-contrast microscope. This allowed an estimation of the time between successive divisions of nuclei and cells (interdivision times) and the growth rate of each individual. The interdivision times of cell and nuclear divisions had a similar degree of variation (coefficients of variation of about 20%). The interdivision times of sister cells and sister nuclei were positively correlated to a significant degree. The correlation between mothers and daughters was negative to a significant degree in some, but not all, experiments. The correlation between interdivision times of a cell and that of its corresponding nucleus was positive in most experiments.The rate of mass increase of individual cells was estimated by measuring the rate of elongation. Within the limitations of the method of observation, it could be concluded that cells grew exponentially between successive divisions. Different individuals grew with very nearly the same rate constant. The variation in size of cells at the time of nuclear and cell division was smaller (coefficients of variation of about 10 yo) than that of the interdivision times.Some observations on the morphological changes of nuclei during growth and division are presented.

SUMMARYA model for the statistics of cell division is proposed. The model assumes : (1) that growth a t the cellular level is deterministic; (2) that the mean size of a cell at division is under cellular and environmental control; (3) that the distribution of sizes of cells at division has a small coefficient of variation, and is independent of the size at previous divisions; (4) that the cell divides nearly into equal halves. The observed coefficient of variation of the life-length distribution f (7) results from a twofold smaller coefficient of variation in the distribution of cellular mass a t division, g (c). The magnification of the coefficient of variation results from the fact that the mass variable enters twice, once in determining the size of cell formed a t division, and, secondly, in determining the size of the cell when it in turn divides. In the expression for 7, these sizes enter as the logarithm of their ratio. These mathematical operations contribute to the increased coefficient of variation of the f (7) distribution over the g (c) distribution. The skewed nature of the f (7) distribution is attributable to a number of causes. We feel that an important source of the skewness is due to deviations from equipartition of cell constituents at division. In this respect the present model is in disagreement with previous models which presume that the skewness results from the statistics of a small number of molecular events taking place inside each cell. The experimentally observed positive sistersister life-length correlation and the negative mother-daughter life-length correlation are explained by the model. Deviations from the predicted values of +0.5 and -0.5, respectively may be explained in part by deviation from equal distribution of cell contents at division. Deviations of the correlations in excess of these, observed in some cases, imply the existence of the other biological processes. The size distribution of cells in balanced growth cultures based on this model is given.