[Estimating the biological effect of detrimental substances on E. coli with flow microcalorimetry. II. Studies using 3 antibiotics].

The influence of three antibiotics (Penicillin G, Oxtetracycline, Polymyxin B), which show different modes of action against gram-negative bacteria, is tested on the growth of liquid E. coli cultures in a flow microcalorimeter. The antibiotics were added during the logarithmic growth phase of the culture when a heat flux of 40 mu W/ml was approached. A supply vessel and a reaction vessel were used for establishing of the baseline and for performing the test, respectively. When testing Oxytetracycline and Polymyxin B the germ content in the nutrient broth was determined, additionally. Some measurements of the optical density of the nutrient broth were carried out parallel to the microcalorimetric investigation on the effect of Polymyxin B. As was to be expected, Penicillin G shows an influence on E. coli at concentrations of about 50 microgram/ml medium, only. An influence of Oxytetracycline on the heat production can be observed at concentrations between 0.3 and 0.6 microgram/ml. Polymyxin B acts on the power-time-curve at 0.025 microgram/ml. The minimum doses of Oxytetracycline and Polymyxin B found by the microcalorimetric technique are distinctly lower than those found by bacterial count and by measuring the optical density. Microcalorimetric results are achieved within 2 hours after adding the test substance to the bacterial culture. Flow microcalorimetry seems to be a promising tool for testing antibacterial drugs.