14 A. C. R. Dean and Sir Cyril Hinshelwood 



was in evidence also showed the reduction in lag but since 

 the increase in mass and the onset of cell division took place 

 almost at the same time as the plate lag began to fall it was 

 not possible to draw any definite conclusions from them. Of 

 a series of eighteen experiments nine were of the first two types 

 and nine were of the third type. 



A technique somewhat similar to that of Baskett has been 

 used by Akiba (1955) and by Szybalski (1954-55). They have 

 shown in certain cases that cells exposed to streptomycin 

 were, at the end of a definite period, if they had survived at 

 all, fully resistant to the drug. Akiba and Szybalski, however, 

 used a medium lacking the materials essential for division 

 while Baskett and Hinshelwood used a medium which would 

 support growth and division. 



Dean (unpublished) has carried out experiments of the 

 Akiba-Szybalski type with Bad. coli mutabile and lactose and 

 with Bad. ladis aerogenes and D-arabinose by omitting a 

 nitrogen source from the medium. Out of four experiments 

 two gave definite positive results as regards the plate lag 

 whilst in the other two very little adaptation took place. In 

 another experiment which involved Bad. ladis aerogenes and 

 both D-arabinose and streptomycin, the plate lag on D-arabi- 

 nose and D-arabinose-streptomycin agar dropped progressively 

 in the usual manner while the survival on streptomycin plates 

 containing either glucose or D-arabinose as sole carbon sources 

 gradually increased to 100 per cent. In a final experiment 

 involving Bad. coli mutabile and chloramphenicol the survival 

 on chloramphenicol-agar gradually increased to 100 per cent. 



Although this experiment was continued until the viable 

 population had fallen from 10^/ml. to 400 cells/ml. there was 

 no evidence of lysis, a fact which excludes the possibility of a 

 multiplication of mutants on the debris from other cells. 



Graded Response 



Sometimes the degree of resistance of bacteria to a drug is 

 continuously graded to conform to the exact concentration 



