SUMMARY: The main action of streptomycin on is bactericidal. In an untrained population the number of cells able to survive in streptomycin diminishes with increasing concentration of the antibiotic. If the surviving cells are mutants, many mutant types must be assumed to explain the many degrees of resistance. Ability to survive does not necessarily imply ability to grow in streptomyein. Survivors sometimes have a long lag before growth occurs in liquid or solid media, which suggests that modification of cell protoplasm may be taking place. Colonies picked from high-concentration streptomycin plates often contain so many non-resistant cells that their origin from a single resistant mutant is unlikely. Training to grow in streptomycin may involve a mixed process of selection (by killing) and adaptation. There is close correspondence between the degree of resistance and the training concentration. Partial dependence on streptomycin is acquired. An attempt to ‘train’ a culture by a method designed to exclude selection was unsuccessful.

Morphological changes were found in untrained cultures grown in streptomycin.

Different samples from one culture frequently showed considerable variation in the number of apparently resistant cells present, depending at least partly on the dilution of the sample and the conditions of spacing on the plate. Resistance is a conditional property; in view of this it is dangerous to construct mutational theories on the evidence of variability between different cultures.


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