142 M. J. Thornley, J. Sinai and J. Yudkin 



and on the number of subcultures in the absence of drug. But 

 complete reversion rarely occurred even after one subculture 

 in fairly low drug concentration followed by many subcul- 

 tures in the absence of drug. Conversely, when several sub- 

 cultures were made in the presence of drug, considerable 

 reversion still occurred and to about the same extent after 

 12 subcultures in the presence of drug and after 84. 



These results are not in conformity with the suggestions 

 of Hinshelwood. First, we do not find that the degree of 

 resistance achieved corresponds to the concentration of drug 

 in which a culture is grown. Second, we find that reversion 

 is usually incomplete even after short contact with the drug, 

 and yet it still occurs after very prolonged contact with drug. 

 We conclude that this approach does not promise to reveal 

 much as to the nature of the origin of drug resistance. 



Phenotypic adaptation 



Baskett (1952) has claimed a rapid increase in resistance to 

 proflavine in a growing culture of Bacterium lactis aerogenes 

 to which the drug was added at short intervals. We carried 

 out this type of experiment with Esch. coli but growth ceased 

 soon after the additions of proflavine were begun. 



However, since Baskett's work seemed to be a conclusive 

 proof of rapid phenotypic adaptation, we repeated his experi- 

 ments with the same strain of Bad. lactis aerogenes, kindly 

 supplied by Sir Cyril Hinshelwood. We confirmed Baskett's 

 observation that additions of proflavine at intervals of ten 

 minutes to growing cultures of this organism only slightly 

 decreased the rate of growth, even though a high concentra- 

 tion of proflavine of up to 100 [xg./ml. was finally achieved. 

 The same concentration of proflavine added at once inhibited 

 growth completely. We found, however, that the cells them- 

 selves showed only a small increase in resistance. On the other 

 hand, the presence of filtrate from a culture grown in the 

 absence of proflavine allowed growth of an inoculum when a 

 high concentration of proflavine was added (Fig. 1). We 

 therefore looked for a factor produced in a culture during 



