Automatic Adjustment in Bacterial Cells 327 



tribution. In a repetition of previous experiments we have 

 found only continuous distributions of sizes on plates seeded 

 with samples taken from lagging cultures before growth 

 begins. After growth has actually started, bimodal distribu- 

 tions may sometimes appear. The reason for them, we 

 believe, is that adaptation, although it has progressed some 

 way, is demonstrably by no means complete at the moment 

 when division and growth become possible, but after active 

 growth sets in adaptation rapidly proceeds further. Thus, the 

 cells which have already started to grow may for a time run 

 far ahead of those which have not yet begun. Hence the 

 temporary separation of the culture into two types: cells 

 which have not yet undergone division and multiplication, 

 on the one hand, and on the other those which have. 



Arguments about mutants have also been based on the 

 formation of papillae on colonies. But we have argued (Dean 

 and Hinshelwood, 19575) that the problem of papilla forma- 

 tion is much the same whether the new growth consists of 

 mutants or of adapted cells. 



The fundamental problem whether adjustments occur at 

 the level of the individual cell, or by a shift among components 

 of a population, can be most directly answered by the obser- 

 vation of the individuals themselves. In this connexion we 

 should like to offer a comment on the beautiful experiments 

 of Hughes. 



The process of response to penicillin and chloramphenicol 

 is strikingly shown in the microfilms taken according to the 

 technique of Hughes (1957).* A few cells are seen in medium 

 containing the inhibitor at a concentration so chosen as to 

 slow down growth without inhibiting it entirely. In the 

 presence of penicillin the majority of them are seen to grow, 

 often with the production of elongated or abnormal forms. 

 Survivors from this first culture are taken and subjected to 

 slightly higher concentrations. In general, about half the 



* Film shown at the Ciba Foundation Symposium on Drug Resistance in 

 Micro-organisms; obtainable from the Photographic Department, St. Mary's 

 Hospital, London. 



