68 W. Howard Hughes 



Fig. 6 is already dead and lysis is occurring ; that at the top is 

 inhibited for division but not for growth and on transfer to 

 penicillinase-agar might recover; the group on the left is 

 dividing. By using the morphological changes and tests for 

 viability to assess the effect on the individual cell it is possible 

 to build up a kind of family tree of resistance and sensitivity. 

 A single cell is taken and allowed to divide to give a colony of 

 from 4 to 16 individuals. These are transferred separately to 

 penicillin-agar and their fate followed for a fixed period of time, 

 anything from 3-5 hours is suitable. At the end of this time all 

 cells which have not autolysed are transferred to penicillinase 



\ 



CCCC CCC* CC«C 9 9 9 9 

 5 6 7 



ccce#ccc ccc*cco ccc cccccc 



• Organism died C Organism f&rmed colony 



Fig. 8. Diagram showing fate of eight microcolonies derived from the same 



single cell when plated out on 7-5 u. penicillin/ml. agar. (Hughes, W. H. 



(19556), J. gen. Microbiol, 12, 269.) 



broth and their viability tested. Fig. 7 shows typical results. 

 Cell 3 gives among its descendants almost all variations. Cell 1, 

 on the other hand, gives a uniformly sensitive group. This 

 strain in the presence of penicillin gives daughter cells which 

 differ from one another in resistance (Hughes, 19556). Fig. 8 

 shows a complete population exposed and tested for viability. 

 The selection of sensitive populations as well as of resistant 

 ones from the small cell was first done on spontaneous charac- 

 ters in Esch. coli B (Hughes, 1955a). It was noticed that in 

 a strain repeatedly subcultured from single cells the growth 

 rate of microcolonies still varied. The difference between 

 slow and quick growing colonies was shown before there was 

 any depletion of nutrients in the medium. Fig. 9 shows the 



