100 Discussion 



certainly does not apply to the 5-^5 /B mutation; 5/B grows at exactly 

 the same rate as 5, although there is roughly the same relative difference 

 between these two strains as between 569 and 569 /H, in rate of peni- 

 cillinase formation. 



Lederberg: Is there a difference in growth rate between 569 in the 

 presence and in the absence of penicillin ? This would be another basis 

 for comparison of cells that are and are not making enzymes. It could be 

 used as a method of inducing penicillinase formation to see if that slows 

 down the growth rate, in an inducible strain. 



Pollock: No, there is not. 



Knox: How do you measure sensitivity to penicillin? 



Pollock : We plate either vegetative cells or spores in plates containing 

 different concentrations of penicillin and titre at the concentration which 

 will be necessary to prevent, on an average, 50 per cent of the spores 

 developing into colonies. 



Knox: What is the inoculum size? 



Pollock: Such that you can count individual colonies on a plate quite 

 easily. If j'ou had a lot you would destroy the penicillin. 



Stocker: Is it the case that strain 569 can be grown, for instance, under 

 conditions in which it is unable to respond to penicillin? One might then 

 find an enhanced difference in sensitivity to penicillin, for instance, by 

 plating preadapted and non-preadapted spores on penicillin agar at 44°, 

 or anaerobically. 



Knox: We followed up Pollock's observation about temperature, and 

 we found, as you would expect, that penicillin sensitivity did enormously 

 depend on the temperature at which the test was carried out (Knox, R., 

 and Collard, P. (1952), J. gen. Microbiol, 6, 369). At 42° the organisms 

 were highly sensitive, at 37° they appeared resistant. 



Stocker: Did it also depend on the previous history of the inoculum, as 

 to exposure to penicillin when tested at the high temperature ? 



Knox: It depended both on the size of the inoculum and on the pre- 

 vious history. At 42° adapted cells were able to grow in 10-100 times 

 greater concentrations of penicillin than unadapted cells. 



Pollock: It is very difficult to get dissociation between induction of 

 penicillinase formation and growth. 



Slonimski : Do you have a specific inhibitor for the induction? 



Pollock : All one gets is competition between two inducers with different 

 inducing powers. It is rather doubtful whether one can look on that as 

 specific inhibition of induction, but there does appear to be competition 

 for a specific site on the cells. 



Slonimski : The difficulty in testing the penicillin sensitivity of strain 

 569 lies in the fact that it forms an enzyme so quickly that you cannot 

 measure its sensitivity. What if you were to add to the culture of 569 

 the inhibitor of penicillinase formation? 



Pollock: But there is only a competitive interaction between two in- 

 ducers both of which are very active in inducing the enzyme, one more 

 than the other, and if you mix them together you get a competitive 

 interaction. 



Slonimski: It might be worth a trial. Would it not be of interest for 



