Experimental Studies of Natural Purification in Polluted Waters: III. A Note on the Relation between Food Concentration in Liquid Media and Bacterial Growth
Results presented were obtained in a study of the standard medium previously described by the author. Bad. aero-genes growing in this medium when it contains 0.5-100 mgm. each of dextrose and peptone per liter (the 100-mgm. concentration, in terms of biological O demand, corresponds approximately to an undiluted normal domestic sewage) produces no change in the observed PH. In higher concentrations, changes in pH were observed, but they were not marked in media containing less than 5000 mgm. of dextrose and peptone per liter. A definite relationship was found to exist between limiting numbers of bacteria developing in liquid media and concentration of food. For Bact. aerogenes, the mathematical relationship which apparently exists is: Y = 2.29 X0.818 when Y = bacteria in millions per cc. and X= concentration of food in mgm. per liter. It was also observed that in a medium of a given concentration, the limiting numbers obtained with bacteria of different genera were not the same. Results presented suggest that: (1) in addition to other controlling factors, the limiting number of bacteria developing in a medium may be governed by the size of the individual cell (the smaller the cell the larger the number developing, and conversely), and (2) the extent of the depletion of dissolved O may be proportional to the total volume of cells produced rather than to the actual number of cells. Similarly, results obtained with 4 different organisms suggest that the minimum concentration of food required to stimulate growth may vary with size or surface area of individual cell. That is, the larger the cell the greater the concentration of food required to produce growth, and conversely.