SUMMARY: Variants whose growth was either dependent upon or enhanced by streptomycin appeared spontaneously during a sensitivity titration of a strain of isolated from a nose and throat culture. In addition to these variants, multiple single colony strains from the original culture exhibited a wide range of susceptibility to streptomycin. The variability after exposure to streptomycin in the laboratory was no greater than that exhibited by the original isolates.

The mechanism by which these variants utilize streptomycin is not yet known. Streptomycin inactivated by semicarbazide or -eysteine did not support or enhance growth. The acquisition of the ability to utilize streptomycin had no measurable effect on the other biochemical characteristics investigated.

When large inocula of the dependent variant were planted on streptomyein-free agar, a few colonies appeared after prolonged incubation. The cells of such colonies underwent a profound morphologic change not unlike the production of ‘large bodies’ which occurs in several species of bacilli. The progeny of such colonies were of normal morphology and either resistant or susceptible to streptomycin but were no longer dependent on it.

Survival of the variants in chick embryos, except in those instances where mutation occurred, was dependent upon the presence of streptomycin. These variants were mouse avirulent, even in the presence of hog mucin in addition to streptomycin.


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