Adaptation of Bacteria to Antibiotics 



253 



The suspension was incubated in an ice-box at 4° C. At 

 appropriate time intervals the bacterial suspension was 

 examined for sensitivity by the plate assay method. Repre- 

 sentative results are shown in Table II. 



Table II 



Effect of cooling and of streptomycin on drug resistance of Esch. 



coli POPULATION (number of viable units giving visible colonies on agar- 



medium containing various concentrations of the antibiotic). 



Almost all cells now became resistant to 10 (jig./ml.; the 

 number of cells resistant to 25 [jLg./ml. increased tenfold and 

 more. The resistance to 50 [xg./ml. (the highest concentra- 

 tion tested) remained without appreciable change. On agar 

 plates containing 50 [xg. of streptomycin per ml., with the 

 exception of a few normal resistant colonies there were 

 observed very small colonies appearing much later (after 

 5-6 days of incubation), consisting of morphologically changed 

 rods of variable resistance. These small colonies developed 

 from microcolonies, which could be seen on the surface of the 

 medium after 24-48 hours of incubation. We are at present 

 studying the nature of these changed forms and their signi- 

 ficance. 



The resistant clones, obtained on the plates in the form of 

 visible colonies, were stable in many subcultures. The pre- 

 sumably genie nature of these stable variants is now being 

 studied in recombination experiments. 



Some discrepancies between the results of Akiba and of 

 Szybalski and those presented here may be due to differences 



