CLONAL PHENOMENA 



out of work on micro-organismal mutation knows very well, 

 however, that, except for certain rather stereotyped mutations 

 like the phase change in Salmonellas, it is impossible to fore- 

 cast in detail the population changes which will follow a given 

 modification of the medium on which it is grown. 



Variability is an essential feature of all the phenomena in 

 this field, and having regard to the overall function of these 

 lectures it is probably best to discuss it in relation to the so- 

 called periodic selection, and to follow in general the work 

 of Ryan and his collaborators. 



{a) Periodic selection in bacterial cultures 



In the days before the use of freeze-dried cultures, when 

 bacterial cultures were maintained by regular subculture 

 and storage at room or refrigeration temperature, there was 

 one standard rule: to transfer on the same medium and to 

 re-isolate the culture from a single typical colony at relatively 

 short intervals. This procedure is sound population genetics. 

 If mutation is occurring with the infrequent production of 

 types with some survival advantage over the original, then 

 any culture that was initially pure will retain the original as 

 the dominant type in the population as long as any significant 

 proportion of the culture remains alive. Mass transfer to 

 another culture tube of the same medium will, however, give 

 a much increased opportunity for the mutant with survival 

 advantage to overgrow the original and, if this procedure is 

 continued, it will inevitably replace the original as the 

 dominant or sole form. Plating for single colonies between 

 stock transfers can be looked at from two angles. The simplest 

 situation is where our primary concern is to maintain some 

 quality in a stock culture which can be manifested in the 

 appearance of the colony, smoothness as against roughness, 

 colour in a chromogenic bacterium, capacity to ferment 

 a given sugar on a suitable indicator medium. As long as the 

 original form is present to the extent of a few per cent, 

 a typical colony can be selected, checked for purity, if neces- 



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