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[VOL. 56 



vary in degree of resistance from only slightly more resistant than the original 

 strain to very resistant. In fact, there is no difference in degree of resistance 

 between the most resistant strains of the first step and those of the second, third, 



20 30 40 50 60 70 80 90 150 200 



CONCENTRATION' UNITS PER ML 



Figure 4. Survival curves of first-step strains of S. aureus resistant to streptomycin, 

 showing the great variability in degree of resistance. Each strain was isolated from a 

 colony of the stock culture growing on the concentration of streptomycin indicated by the 



and higher steps. Consequently, strains that are highly resistant to strep- 

 tomycin may be obtained either in one step, by selection of survivors of very high 

 concentrations, or in several steps, by repeated selection of survivors from 

 bacteria grown on increasingly higher concentrations. 



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