Bacteriologists have often wondered why the so-called "logarithmic growth phase," i.e., the phase of truly exponential multiplication, is so short in our culture media. With the rapidly growing aerobes and facultative anaerobes which are the common organisms for general growth studies, this phase where the cells multiply at a constant rate does not last more than 2 to 4 hours at or near the optimal temperature. As a rule, the logarithmic multiplication curve begins to deviate from the straight

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... ine when the cell population has reached approximately ten million bacteria per ml. The facts mentioned in chapters II and III of this treatise (J. Bact., 41, pp. 226 & 234, 1941) give a simple explanation of this short period of constant multiplication rate. It had been shown there that with most oxygen-consuming bacteria, the supply of oxygen dissolved in the medium is exhausted by the time these bacteria have reached 2 to 10 million cells per ml. Further multiplication will proceed at the same rate only on the surface, where oxygen from the air is continually renewed, but not at the bottom where anaerobic decomposition may still provide energy for growth, but at a much slower rate. The deviation of the logarithmic multiplication curve from the straight line indicates therefore, that the dissolved oxygen in the medium is exhausted. This can be proved very simply by comparing the same organism in an ordinary flask culture and in an aerated culture. Figure 1 shows this for Pseudomonas fluorescens in nutrient broth. While the ordinary culture in an Erlenmeyer-flask had a constant rate of multiplication only from 2 to 6 hours, the aerated culture, although delayed at first, multiplied at a constant rate from 4 to 12 hours. The initial delay by aeration which is a regular occurrence will be explained in chapter VI. Other examples showing that aeration greatly extends the period of a constant multiplication rate will be found in figures 3 to 6. In order to test these assumptions in a different way, and to make the argument more convincing, a calculation has been made of the depth to which the oxygen of the air will penetrate into a growing culture with more than 10 million bacteria per ml. Bacillus mesentericus was grown in air-saturated peptone solution in four cultures; one was continuously aerated; one was a 200 ml. culture in a 2 liter Erleumeyer-flask; the other two were kept under oil, one with methylene blue to indicate the moment of complete oxygen exhaustion, the other without the dye to see whether methylene blue would influence the growth or death rate. The results are shown in table 1. All data are averages of triplicate plates. 321 on May 9, 2020 by guest