Stability of antibiotics under growth conditions for thermophilic anaerobes

It was shown that the inhibitory effect of kanamycin and streptomycin in a growing culture of Clostridium thermohydrosulfuricum JW 102 is of limited duration. To screen a large number of antibiotics, their stability during incubation under the growth conditions of thermophilic clostridia was determined at 72 and 50 degrees C by using a 0.2% yeast extract-amended prereduced mineral medium with a pH of 7.3 or 5.0. Half-lives were determined in a modified MIC test with Escherichia coli, Staphylococcus aureus, and Bacillus megaterium as indicator strains. All compounds tested were similar at the two temperatures or more stable at 50 than at 72 degrees C. The half-life (t1/2) at pH 7.3 and 72 degrees C ranged from 3.3 h (k = 7.26 day-1, where k [degradation constant] = 1/t1/2) for ampicillin to no detectable loss of activity for kanamycin, neomycin, and other antibiotics. Apparently some compounds (e.g., lasalocid and neomycin) became more potent during incubation (k greater than 0). A change to pH 5.0 caused some compounds to become more labile (e.g., kanamycin) and others (e.g., streptomycin) to become more stable than at pH 7.3.