C4 



M. DEMEREC 



[vol. 56 



Tests of survivors confirmed the conclusion indicated by the survival curves. 

 Strains isolated from the colonies of plates having high concentrations of either 

 penicillin or streptomycin proved to be more resistant to the respective compound 

 and grew on concentrations at least as high as those of the plates from which they 



' .01 .02 .03 .04 .05 .06 .07 .08 .09 .10 .11 .12 .13 

 PENICILLIN CONCENTRATION' OXFORD UNITS PER ML 



Figure 1. Survival curves for Staphylococcus aureus plated on nutrient agar containing 

 various concentrations of penicillin. The six light curves represent results of six independ- 

 ent experiments, and the heavy curve represents the average of these experiments. 



had been isolated. It was found that resistance to one of these antibiotics is 

 independent of resistance to the other; that is to say, strains with increased re- 

 sistance to penicillin are still sensitive to streptomycin, and vice versa. It seems 

 unnecessary to discuss in detail the by now well-established fact that strains that 

 once become resistant as a rule continue so. 



8o 



