Discussion 99 



know, but we have not succeeded in getting a test critical enough to 

 decide this. 



Alexander: The type of dilution technique which Prof. CavalU-Sforza 

 described this morning could presumably be used. 



Pollock: No, because it is not a genetically stable phenomenon. It is 

 only possible to test indirectly the penicillinase-forming ability of indi- 

 vidual cells. Theoretically that could be done by penicillin sensitivity, 

 but I am not sure that it is sufficiently sensitive to allow one to decide 

 whether the enzyme-forming property is homogeneously distributed or 

 whether it is confined to the original cells induced. 



Barber: The staphylococcus penicillinase-producing culture will quite 

 frequently throw off variants which produce practically no penicillinase. 

 Is that the case with your 569 strain? 



Pollock: No, we never have a complete negative. I should point out 

 that we could show, by a reconstruction test, that the low activity of 5 

 was not due to the presence of 5P and 5B mutants in the 5 culture. We 

 don't know how homogeneously the property forming small amounts of 

 penicillinase is distributed amongst individual clones of strain 5 ; but 

 5 cultures are never penicillinase-negative. 



Pontecorvo : You would not be able to identify negative strains if you 

 got them. Suppose a strain, instead of having 50 molecules per cell, had 

 none : the only way would be to test millions of single cell isolates. 



Pollock : You cannot completely eliminate the possibility of there being 

 some true negatives. But the sensitivity of the micro-assay method is 

 much more than 50 molecules per cell. You can get it down to 50 times 

 that sensitivity quite easily. There may not be a stable genetic distri- 

 bution of the property amongst individual cells, but you get a very 

 constant level of activity from different 5 colonies. If you go on isolating 

 from individual colonies you always find about the same amount of 

 enzyme, so I suspect it is fairly evenly distributed. 



Alexander: Would you get the same rate of penicillinase production 

 if you irradiated an enzyme so as to stop it from dividing ? 



Pollock: Yes, you can give doses of ultraviolet which will almost 

 completely stop cell division and cause 99 • 9 per cent reduction in viable 

 count, with very little effect on the enzyme-forming ability, at least for 

 an hour or so. 



Alexander: That would indicate that it is probably the ability to 

 produce penicillinase that is diluted out and that you have the same 

 number of penicillinase-producing bacteria at the beginning of the hour 

 as at the end of the hour, when there are very many more organisms. 



Pollock: I don't see how you can draw any conclusions about the 

 distribution of the enzyme-forming ability from that. 



Stocker: Is the slower gro^\i;h of the constitutive mutant true only of 

 569? 



Pollock: No, 569/H devotes between 1 and 2 per cent of its synthetic 

 powers to forming an apparently useless enzyme. One would like to 

 think that it makes it up by not growing so rapidly, but it does not fit in. 

 For one thing that would be a very small change in relative gro^\i:h rate, 

 whereas in fact there is a difference of about 20 per cent. Furthermore, it 



