Discussion 247 



general inhibitory effect on metabolism. Therefore, the very smallest 

 difference, say in lag phase of the individual cells, or small changes like 

 that, might determine whether the whole colony appeared or not. I am 

 quite sure that is the explanation in the case of B. cereus. 



Knox: It is complicated by the decay of the drug. We have made 

 biological estimates of the amount of drug in the semi-solid medium by 

 simply adding organisms on the surface later. It is not a very good 

 method but it does give us some idea; and we have also checked it by 

 polarographic methods. 



Eagle: There is a superficial analogy between this phenomenon, in 

 which Prof. Knox gets fewer pseudomutants with a large inoculum than 

 he does with a small one, and the fact that with penicillin and a variety 

 of bacterial species, the number of resistant mutants which show up in 

 a penicillin plate often decreases, rather than increases, with the size of 

 the inoculum. At a threshold concentration of penicillin, with an inocu- 

 luin of e.g. 10^ organisms one may get ten resistant colonies; at the same 

 penicillin concentration, with an inoculum of 10^ organisms, instead of 

 getting the expected thousand colonies, one may get none. The resem- 

 blance in the two phenomena is only superficial, however ; because in our 

 case these are penicillin-resistant colonies, while in Prof. Knox's case 

 they apparently are not. 



Pollock: Is there some inhibition by the non-mutated ones? 



Eagle: In the case of Staphylococcus and penicillin, we hoped that 

 this might be a case of transformation of the resistant organisms to 

 sensitive, induced by DNiV deriving from the enormous numbers of 

 sensitive bacteria which are being killed in the immediate environment 

 of each resistant organism. Unfortunately, however, DNAse had no 

 effect, and we have no idea as to what is happening. 



Pontecorvo: The suppression of something that should grow, by the 

 remaining mass of the population which should not grow, is a very wide- 

 spread fact; it goes under the jargon name of "Grigg effect". It is very 

 specific, some mutants will show it, others will not. In some cases it 

 can be a nuisance in genetic experiments. 



Davis: In your case, Dr. Eagle, where you failed to get mutants, 

 might it be that the density of the inoculum was so large that it ex- 

 hausted the medium through its metabolic activity ? 



Eagle: No, this was examined and the failure of the organisms to grow 

 with the large inoculum was not due to exhaustion of the medium. 

 When the plate was flooded with penicillinase at the end of the experi- 

 ment, a surface inoculum of a few organisms grew out readily. There is 

 something going from the sensitive to the resistant organisms in this 

 experiment which makes the latter sensitive. This can be stopped by the 

 interposition of a cellophane membrane in the penicillin-agar between 

 the mass of normal cells and a few resistant organisms. The latter then 

 grew out in normal fashion. 



Dean : Would it be possible to confirm the suppression of mutants by 

 non-mutants by means of reconstruction expermients, in which mutants 

 are tested in the presence and absence of a large number of sensitive 

 cells? 



