Oxygen Demand and Oxygen Supply

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Abstract
The dissolved O2 in a liquid culture is exhausted when the number of bacteria reaches 2 to 10 million per ml. From then on, strict aerobes multiply only in the surface layer of not more than 2 mm., while they die gradually at the lower strata. Facultative bacteria multiply more slowly below the supply line for O2. Aeration of a culture does not usually increase the rate of multiplication, but maintains a constant rate for a longer period, resulting in a much larger crop. Streptococcus lactis is not favored by aeration, and decreased by pure oxygen. Cultures cease to grow primarily from lack of oxygen, and the "autotoxin theory" is not necessary. The delayed start of aerated cultures is due to O2 entering resting cells with incomplete enzyme mechanisms. An old cell in a new environment establishes a zone of optimal conditions in its immediate neighborhood, probably at the expense of reserve compounds. If this is continuously disturbed, e.g., by convection currents, the cell will finally die from exhaustion. The good growth on agar may be due mostly to the prevention of convection currents.