1948] 



ORIGIN OF BACTERIAL RESISTANCE TO ANTIBIOTICS 



65 



ORIGIN OF RESISTANCE 



The numerous experiments made in gathering the data for the survival curves 

 showed clearly that in large populations of bacteria there were always some indi- 

 viduals more resistant to the antibiotics than others. Since we used very small 



4 6 8 10 12 14 16 18 20f-40 50 

 CONCENTRATION' UNITS PER ML 



Figure 2. Survival curves for Staphylococcus aureus plated on nutrient agar containing 

 various concentrations of streptomycin. 



inocula (50 to 300 bacteria), the proportion of resistant bacteria was too small 

 to account for their presence by assuming that they came about through division 

 of one or more resistant individuals that may have been present in the inoculum. 

 Therefore the resistant individuals must have originated in the experimental 

 cultures. Two alternative possibilities were considered with respect to the 



8i 



