The Influence of Aeration and of Sodium Chloride upon the Growth Curve of Bacteria in Various Media

Abstract

Average growth curves were established for Escherichia coli cultured at 37[degree] C. in 4 basic media, with and without continuous aeration, and with various stimulating and toxic concentrations of NaCl. Counts were made by plating methods, at frequent intervals up to 1 or more days. Aeration with COvfree, NH3-free air had a profound effect on growth. In the richer media (standard lactose broth, lactose-peptone water, peptone water) the lag period was slightly prolonged and showed occasional toxic effects; in a medium of low nutrient value (Dolloff synthetic) the lag was prolonged for 1 or more days without onset of active growth. This is thought to be due to removal by the air current of CO2 essential to growth or of intermediate growth products of cell metabolism, whose accumulation has been assumed by previous workers to be essential to active population increase. On the other hand, aeration of the richer media greatly stimulated growth once it had commenced, with prolongation of the period of logarithmic increase an extra hour, acceleration of its velocity of increase, and elevation of the population at the age of 6-7 hrs. to a figure nearly 10 times as great as occurred in unaerated. test-tube cultivation. This increase must be due to removal of toxic waste products or to increased oxygena-tion, since no difference in available nutriment is involved. New light was obtained on salt effects by their consideration throughout the population cycle instead of after a 1- or 2-day period, as in most previous studies. A stimulating salt concentration (0.1 M NaCl) was toxic during the lag period, but caused definite acceleration in the period of logarithmic increase. A mildly toxic salt concentration (0.5 M NaCl) was yet more toxic in the lag period and more stimulating in the logarithmic, but the ultimate limiting population did not reach that in salt-free media. Stronger salt concentrations (1.5, 2.0 or 2.5 M NaCl) were less toxic during lag than the above, and showed a temporary inflection representing a sort of suppressed attempt at growth and finally a constant period of death.