Inheritance in Single Bacterial Cells 



65 



to the satisfaction of other workers who had a bias in favour 

 either of adaptation or of major mutation as the basis for 

 bacterial variation. 



If single cells are cultured and from the clone further single 

 cells are selected these will differ from one another in anti- 

 biotic sensitivity (Fig. 1). The differences here are so small 



0-05 0-5 5-0 



log penicillin concentration (u./ml.) 



Fig. 2. Results of tube titration of sensitivity of single-cell strains of staphylo- 

 cocci. Key to curves: , strain I; , strain Via; — O — » strain 



XXX« ; , strain XXXIa. The four subcultures show a similar distri- 

 bution of sensitivity, although the average sensitivity of each is at a different 

 level. Each curve on this is built up from 11 separate points, the only 3 points 

 not falling on their line are marked «. (Hughes, W. H. (1952), J. gen. Microbiol., 



6, 175.) 



that had the titration not been repeated on several occasions 

 with different batches of penicillin over intervals of many 

 months the author would not have been convinced that they 

 were not due purely to experimental error. 



If now from the tail of the population the more resistant 

 cells are picked, new populations with increased resistance are 

 obtained. Fig. 2 shows the stages of the investigation. It will 

 be seen that the increase is fairly steady. This impression 

 can be misleading since only the most resistant population is 



DRUG RES. — 3 



