24 A. C. R. Dean and Sir Cyril Hinshelwood 



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DISCUSSION 



Lederberg: In 1954, Sir Cyril, you sent us a culture of your Bad. lactis 

 aerogenes ; we have been experimenting with it from time to time, and I 

 regret to say that we do not get the observational results that you have 

 indicated. According to your published results (Baskett and Hinshel- 

 wood, 1951, loc. cit.), the inoculation of this organism into tubes of 

 minimal medium containing D-arabinose as a sole carbon source is 

 followed by an interval in which there is an increase in total mass; 

 and during this interval there may be a homogeneous response during 

 which each of the cells in the suspension has become gradually (but 

 all of the cells uniformly) better adapted to this environment, so that 

 when they are plated they show a uniform decrease in the lag for the 

 time of colony development. Our own findings are that when samples 

 are taken from a tube inoculated — say with 10'' or 2 x 10' cells per ml. 

 — in such a medium, and these samples are plated out at half-daily or 

 daily intervals, there is a variable time of onset of visibly turbid growth. 

 That is invariably preceded by the appearance of a large colony-forming 

 type, and during the interval this increase in turbidity gives a very 

 clearcut distinction between small and large colonies, and the large 

 colonies simply increase. I have seen nothing in this system which is not 

 most readily explainable by the sporadic occurrence of a better adapted 

 mutant which will form large colonies. However, there are several 

 features in this system which make it less precise to work with than some 

 other selectable systems, and which may perhaps account for some of the 

 questions of increase in mass. It is quite clear that the original type is 

 capable of utilizing arabinose, but I think at a much slower rate than 

 the wild type. Even unadapted cells will form colonies, so that as these 

 colonies are replated they continue to form visible colonies ; it may take 

 a week before they reach a size of 1 mm. or so in diameter. Every once in 

 a while one finds a colony — one of a number of hundreds of colonies on a 

 plate — that has become quite large over a period of a day or two, and 

 this on replating always gives a large colony. In addition, there is a very 

 large stimulation of colonies by products from the adapted types, and I 

 suspect that is one of the reasons for the conclusion that there is a uni- 

 form response. If the colonies are plated too densely there is a marked 



