ISOLATION OF ANTIBIOTIC-RESISTANT LACTOBACILLI

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Abstract
The Lederberg replica plating technique is useful for the direct isolation of streptomycin-resistant colonies from parent cultures of lactobacilli from feces of man. The Lactobacillus selective medium (LBS) of Rogosa et al. is used, and surface-inoculated petri dishes are incubated in an atmosphere containing 90% by volume of CO2. Repeated doubling of the antibiotic concentration with each culture transfer, or the Szybalski gradient plate technique is necessary for the selection of tetracycline-resistant mutants which develop in a stepwise manner. The growth of colonies of lactobacilli upon the surface of selective agar, after incubation in an atmosphere which contains approxi-mately 90% CO2 by volume, also makes the replica plating technique useful for the simultaneous testing of resistance toward various concen-trations of several antibiotics. Cultures of lactobacilli resistant to antibiotics grow less rapidly than their respective parent cultures. This is the only difference in characteristics other than antibiotic resistance which was observed between the respective mutant and parent cultures on the basis of the morphological, cultural, fermentation, and nutritional studies conducted.