Multiple Mechanisms of Drug Resistance 145 



the growth of the sensitive cells. That the phenomenon did 

 not occur with Esch. coli may be due to the difTerences in 

 the culture medium. With Bad. lactis aerogenes, this was 

 an inorganic medium which contained glucose, whilst with 

 Esch. coli the medium was peptone broth only. 



It is, however, possible to obtain with Esch. coli a small 

 increase in resistance in growing cultures if small amounts of 

 proflavine, giving a final concentration of not more than the 

 threshold inhibitory concentration of 5 [xg./ml., are added at 



Table I 



Increase in resistance in culture of Escherichia coli 36 by 



ADDITION OF PROFLAVINE IN LOGARITHMIC PHASE 



After 270 min. 

 Proflavine Control 



Increase in resistance 

 Gradient plates 

 Counts 6-6 fig. I jn\. 



10 Atg./ml. 

 20 /tig./ml. 



3 



3-2x102 



and 

 2x105 (s) 

 1-5x102 

 1-5x102 



1-3 

 1X102 



3X10 

 2-8x10 



(s) small colonies 



longer intervals of 30 minutes. The increase in resistance 

 occurred in up to 1 per cent of the cells (Table I). It was 

 somewhat less in degree than that of the first-step mutants 

 (see below). It occurred only when the additions of drug 

 were made in the logarithmic phase, and not when they were 

 made in the lag or stationary phases (Fig. 3). The cells show- 

 ing the increased resistance to proflavine were not cross- 

 resistant to chloramphenicol, aureomycin, erythromycin or 

 terramycin, drugs to which proflavine-resistant mutants show 

 cross-resistance. They lost their resistance to proflavine 

 when grown in the absence of the drug. For these two 



