The Isolation from Soil of Spore-Forming Bacteria Which Produce Bactericidal Substances

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Abstract
A rapid method has been developed for isolating soil bacteria that are capable of producing bactericidal agents. Samples of soil were plated in low dilution in order to obtain a large number of microbial colonies per plate. The resultant crowding of the colonies allowed any potential antagonisms that might exist between the various microorganisms in the soil to become manifest as clear zones around the antagonists, i.e., as areas in which other microorganisms were prevented from developing, owing to the production of antibacterial substances. By this method 24 strains of aerobic sporulating bacilli were isolated from various soils. The supernatant fluid of 5-day-old broth cultures of 7 of the strains contained substances which were bactericidal for various gram-positive bacteria; cultures of the remaining strains were inactive. The bactericidal principles have been obtained in a purified state in the form of highly potent gray or tan powders which are effective against gram-positive and gram-negative bacteria.