Two hundred strains of recently isolated from soil were tested for an antagonistic effect on a phage-resistant strain of the same species. Fifty-one strains gave a zone of inhibition on a lawn of the indicator strain. This effect was increased by a small dose of ultraviolet radiation which also caused 41 other strains to show an antagonistic effect. Sixteen strains which were completely lysed after u.v. irradiation also produced confluent lysis of the indicator strain. When grown in liquid media, these strains gave irregular growth curves owing to partial lysis during the exponential phase. Lysis was almost complete when such cultures were incubated after exposure to small doses of u.v. radiation. Lysis was always associated with accumulation of the antibacterial principal previously named ‘megacin’. Irregular growth in liquid media and production of megacin were strictly correlated. However, only 2 megacinogenic strains were lysogenic and liberated phage. Populations of megacinogenic strains were examined by replicaplating for the presence of non-megacinogenic mutants. Spore suspensions of 4 megacinogenic strains contained non-megacinogenic mutants in various proportions depending on the strain and the particular sample. Non-megacinogenic mutants could not be made to lyse by exposure to u.v. radiation; did not produce any antibacterial effects; and all showed normal patterns of growth in various liquid media. It is supposed that megacinogeny and aberrant growth, as well as inducibility by u.v. radiation, of some strains are governed by a single hereditary unit (gene). It cannot be decided at present whether or not this unit is a highly defective prophage.


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