The number of viable Escherichia coli in a young, actively growing culture is decreased approximately 99.9 per cent by a 30 second exposure to 25 ϕg. streptomycin/ml. The injury induced by the antibiotic is only potentially lethal, however, and may be reversed by subculture within 5 minutes into fresh culture medium, NH4NO3, NH4Cl, (NH4)2HPO4, NH4 citrate, and NH4 tartrate. Subculturing into water, glucose, or MgSO4 results in a more marked decrease in the number of viable organisms. In KNO3, NaNO3, K2HPO4, and Na2SO4 solutions reversal occurs first, followed by a rapid decrease in viability.

True reversal of the streptomycin injury takes place, as demonstrated by the rapid rate of recovery to the viable count of the original culture. Development of resistance has been eliminated as the cause of regrowth since the streptomycin sensitivity of recovered cultures remained the same as that of the original culture.

The use of water as diluent for viability determinations potentiates the lethal effect of streptomycin activity. Several compounds, at various dilutions, substituted for water as the diluent gave rise to four types of responses, group I, NH4NO3, NH4Cl, KNO3, NaNO3, Ca(NO3)2, showed complete reversal of the streptomycin injury at all levels of the salts tested, from 0.01 to 0.5 M concentrations. Group II, NaCl and K2HPO4 showed complete reversal at 0.03 and 0.1 M. Group III, glucose and urea allowed complete reversal at 0.5 M. Group IV, glycerol and glycerine showed no reversal at 0.5 M concentration.

The reversal of the streptomycin injury to young actively growing bacteria is suggested as a tool for studying the pathology of the injury to the cells.

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