Two methods were used to determine mean volumes of cells of Escherichia coli B/rA in both stationary- and exponential-phase cultures, i.e., electronic measurement with a Coulter Counter-Analyzer system and biophysical measurement of the total volume and number of cells in sedimented cell pellets. Within experimental errors, the methods gave the same mean cell volumes.

Synchronous cultures of Escherichia coli 15-THU and WP2s, which were selected by velocity sedimentation from exponential-phase cultures growing in an acetate-minimal salts medium, were shifted to richer media at various times during the cell cycle by the addition of glucose or nutrient broth. Cell numbers and mean cell volumes were measured electronically. The duration of the division cycle of the shifted generation was not altered significantly by the addition of either nutrient. Growth rates, measured as rates of cell volume increase, were constant throughout the cycle in unshifted acetate control cultures. When glucose was added, growth rates also remained unchanged during the remainder of the cell cycle and then increased abruptly at or after cell division. When nutrient broth was added, growth rates remained unchanged from periods of 0.2 to 0.4 generations and then increased abruptly to their final values. In all cases, the cell volume increase was linear both before and after the growth rate transition. The strongest support for a linear cell volume increase during the cell cycle of E. coli in slowly growing acetate cultures, however, was obtained in unshifted cultures, in complete agreement with earlier observations of cell volumes at much more rapid growth rates. Although cell growth and division are under the control of the synthesizing machinery in the cell responsible for RNA and protein synthesis, the results indicate that growth is also regulated by membrane-associated transport systems.Regulation of cell division in Escherichia coli and Salmonella typhimurium was first examined by shifting exponentially growing cultures to richer media (4, 5, 10, 13, 26). These observations indicated that when exponentially growing cultures were shifted to richer growth media (an upshift), the rates of cell division were unchanged for about an hour (4,5,10,13,26). This rate maintenance of the cell division rate for about 60 min led to the suggestion that the delay reflected a constant time between the initiation of a round of chromosome replication and the cell division following that round of replication (5, 13, 26).Cell division rates in synchronized cultures did not remain strictly constant; some perturbations occurred (5, 24). Cooper (5) has suggested that these minor changes in cell division rate might reflect correspondingly small alterations in the C and D periods of the cell growth cycle (during and after DNA synthesis) and, therefore, that the concept of rate maintenance could still apply.Although the effects of shifts between steady states upon cell division rate have been studied extensively, there have been no corresponding studies of cell mass or volume increase. In a related major study, Zaritsky et al. (32) determined cell lengths and diameters in a culture of E. coli that was shifted to a richer medium. They compared their results with predicitions of several different growth models and concluded that the best fit was given by a linear surface growth model. The results presented below, however, are ...

Increase in the mean cell mass of undivided cells was determined during the division cycle of Escherichia coli B/rA. Cell buoyant densities during the division cycle were determined after cells from an exponentially growing culture were separated by size. The buoyant densities of these cells were essentially independent of cell age, with a mean value of 1.094 g ml-'. Mean cell volume and buoyant density were also determined during synchronous growth in two different media, which provided doubling times of 40 and 25 min. Cell volume and mass increased linearly at both growth rates, as buoyant density did not vary significantly. The results are consistent with only one of the three major models of cell growth, linear growth, which specifies that the rate of increase in cell mass is constant throughout the division cycle.

Growth was studied in synchronous cultures of Escherichia coli, using three strains and several rates of cell division. Synchrony was obtained by the Mitchison-Vincent technique. Controls gave no discernible perturbation in growth or rate of cell division. In all cases, mean cell volumes increased linearly (rather than exponentially) during the cycle except possibly for a small period near the end of the cycle. Linear volume growth occurred in synchronous cultures established from cells of different sizes, and also for the first volume doubling of cells prevented from division by a shift up to a more rapid growth rate. As a model for linear kinetics, it is suggested that linear growth represents constant uptake of all major nutrient factors during the cycle, and that constant uptake in turn is established by the presence of a constant number of functional binding or accumulation sites for each growth factor during linear growth of the cell.

No abstract