A study of bacterial populations in which 



nuclear and cellular divisions are induced 



by means of temperature shifts 



by 



OLE MAAL0E and KARL G. LARK* 



Statens Seruminstitut, Kobenhavn 



A growing population of micro-organisms is a mixture of cells representing all 

 phases of the division cycle. Studies on such a population, therefore, cannot lead to 

 identification of the successive physiological and cytological states of a dividing cell, 

 and very little can be learned about these states by observing individual cells under- 

 going division. Many problems concerning cell multiplication, which cannot be 

 studied on single cells or on conventional cultures, might be solved by studying 

 samples from a culture in which the cells were induced to divide more or less 

 simultaneously. General considerations of this kind lead us to investigate possible 

 means of producing the desirable 'phasing' or 'synchronization' of cell division in 

 bacterial cultures. 



Our choice of means was greatly influenced by the recent work of Scherbaum and 

 Zeuthen (1954) and of Hotchkiss (1954), who have used temperature changes to 

 obtain phasing of cultures with respect to certain physiological characters. Our 

 approach is closer to that of Hotchkiss than to that of Zeuthen; and, since Zeuthen's 

 studies are presented elsewhere in this symposium (Zeuthen and Scherbaum, 1954) 

 we may at this point draw attention particularly to Hotchkiss's work (1954). In his 

 experiments, periodic changes in rate of division, as well as in susceptibility to 

 change into a streptomycin-resistant form, were induced by exposing the cultures 

 to a temperature considerably below the normal, and optimal, growth temperature. 

 Hotchkiss mainly used shifts between 37 and 25 G, and the same interval was 

 adopted for our experiments with Salmonella typhimurium. 



The strain we have employed has properties of particular value for the present 

 investigation: firstly, young cultures of this organism consist of well-isolated cells 

 and cell pairs with no tendency to clumping; secondly, the strain interacts with a 

 certain bacteriophage thereby becoming changed in a characteristic and easily 

 recognizable manner (Boyd, 1950; Lwoff, Kaplan and Ritz, 1954); and, finally, 

 abundant growth can be obtained even in media composed of inorganic salts plus 

 a simple carbon source such as glucose. The first of these properties has permitted 



* Fellow of the American Cancer Society, upon recommendation of the Committee on Growth, National 

 Research Council. Present address: Statens Seruminstitut, Copenhagen, Denmark. 



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