Studies on atmospheric requirements of bacteria. 1. Water vapor tension

Quantitative experiments on the relation between the growth of bacteria and the amount of water present have been limited to studies on the effect of water content of media, and are surprisingly meager in the amount and character of the work done. Such investigations are limited to those of Wolf 1 and Weigert, 2 who working with different concentrations of media found definite limits of growth for various organisms. No investigation as to the effect of water vapor tension on surface colonies on a solid medium has apparently ever been made.

The method used for obtaining preliminary data as to the influence of atmospheric moisture on surface growth consisted in using either two tops or two bottoms of ordinary petri dishes which are placed edge to edge and held together with adhesive tape around the entire circumference giving practically an air-tight capsule. In the upper half, media was poured to a depth of 2 mm.; in the lower half, solutions of various dehydrating agents were placed to a depth of 5 mm.; about 2 cm., as a rule separating the two surfaces.

The organisms investigated were B. coli, B. subtilis, Staphylococcus aureus, and Streptococcus hemolyticus. The medium used was glucose infusion agar, P H 7.0, and cultures were usually incubated 18 hours at 37.5°C. The dehydrating solutions used were glycerin, 50 per cent. glycerin, 50 per cent. calcium chloride (saturated) and 25 per cent. calcium chloride, giving initial relative humidities of 0, 25, 35, and 75 per cent. respectively at the start of the experiment. The rate of change of the water vapor tension with addition of water, of the glycerin solutions is, however, so much larger than that of the calcium chloride solutions, that with the conditions as above stated the drying action of the calcium chloride is usually somewhat more energetic than that of the glycerin.