The relationship between specific growth rate of Escherichia coli and the concentration of limiting nutrient (glucose or phosphate or tryptophan) has been determined for populations in a steady state. At high concentrations the specific growth rate is independent of the concentration of nutrient, but at low concentrations the specific growth rate is a strong function of the nutrient concentration. Such a relationship was predicted by Monod; however, Monod's equation does not predict the relationship over the entire range of nutrient concentration. If parameters of the equation are estimated from the results obtained at low concentrations, then at high concentrations of nutrient, the specific growth rate is significantly higher than that predicted by Monod's equation. These results were interpreted on the basis that the rate of growth is controlled by at least two parallel reactions and that the affinities of the enzymes catalyzing these reactions are different. The relationship between specific growth rate and mean cell volume was also measured, and the results indicate that mean cell volume depends not only on the specific growth rate but also on the nature of the limiting nutrient. There are different mean cell volumes at the same specific growth rate established by different limiting nutrients. Therefore, the mean cell volume is not uniquely determined by the specific growth rate. vide a test of the relationship inferred by Monod 210 on July 4, 2020 by guest

The distributions of cell volumes of steady-state Escherichia coli ML30G cultures at various temperatures were measured. For cultures in a minimal medium, the distributions were indistinguishable at several temperatures between 15 and 30 C; at higher temperatures the cells were slightly smaller, and at lower temperatures they were slightly larger. For cultures in a complex medium, the cells were slightly larger at both high and low temperatures of growth. An abrupt change of temperature within the middle range led to a transient change in the distribution of cell volume, suggesting that the size of dividing cells is well regulated. No synchrony of division was induced by a change in temperature.

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Forty isolates from 97 raw milk samples (heated to 80 C for 10 min and stored at 4 to 7 C for 3 to 4 weeks) were sporeforming, aerobic, gram-positive or gramvariable, rod-shaped bacteria. Fifteen isolates that were identified had characteristics similar to species of Bacillus, except that they had lower growth temperature ranges, were gram-variable, and were somewhat different in sugar fermentations. Four isolates grew well within 2 weeks at 0 C, but they grew faster at 20 to 25 C. These psychrophilic sporeforming bacteria, the importance of which is discussed, are considered to be variant strains of mesophilic bacilli adapted to low temperatures. Most psychrophilic bacteria isolated from natural sources are heat-sensitive, gram-negative, nonsporeforming rods (3, 5, 10, 19, 22). However, Larkin and Stokes (11) and Sinclair and Stokes (15) isolated psychrophilic species of Bacillus and Clostridium from soil, mud, and water, and Marshall and Ohye (12) isolated a gram-negative psychrophilic Bacillus from sub

Helmstetter and Cummings devised a technique of synchronization in which cells are implanted on a membrane filter and the membrane is subjected to reverse flow of liquid medium. The cells in the effluent stream have predominantly the characteristics of newborn cells. The advantage of this technique is that the population experiences a minimum of physiological stress; hence, the behavior of the synchronous culture should reflect the normal divisional cycle. The disadvantage is that strains other than Escherichia coli B/r cannot be synchronized. We have found that a modification of the method makes it possible to synchronize several strains of E. coli, including both male and female strains, as well as Salmonella typhimurium LT2. The principal difference in technique is a prolonged period (>400 doublings) of cultivation in glucose minimal medium at 30 C and at low density (<5 x I06 cells/ ml) prior to implantation. This precaution was taken to insure that the bacterial growth population is in a steady state of balanced growth. From the resulting synchronous growth, the distribution of interdivision times has been computed; these distributions have coefficients of variation in the range 0.18 to 0.22 and are not appreciably skewed.