EFFECTS OF KADI A TlOX ON' BACTElllA 403 



Oil heterozygous strains, haxc shown fon\ inringl}' that individual cells 

 contain the genetic potentiahties of the two parent strains. 



C'ytologically, it is known that bacteria possess Feulgen-positi\'e bodies 

 which Robinow (1945) considers as chromosomes. Furthermore, the 

 number of such bodies is known to vary from one to four or more per cell 

 depending on the species, the stage of the growth cycle, and the stage in 

 the division cycle of the individual bacterium observed. Observations 

 indicating a genetic role for the so-called "chromatinic bodies" are just 

 beginning to appear. Lederberg et al. (1951) have made comparative 

 cytological studies of E. coli cells known to be haploid or diploid from 

 genetic evidence. Perhaps the strongest evidence indicating an actual 

 genetic function for these structures is that of Witkin (1951) who found a 

 correspondence between the average number of chromatinic bodies per 

 cell and the size of nonlactose-fermenting mutant sectors in colonies of 

 E. coli. Zelle and Lederberg (1951, and unpublished observations) have 

 genetic evidence that individual cells can be multinucleate, since a single 

 cell has been observed to divide and form a diploid and haploid cell, the 

 differentiation being based on subseciuent genetic behavior. The fact 

 that bacterial cells do not necessarily have a constant nuclear constitution 

 has an important bearing on the mechanism of bactericidal effects of radi- 

 ation as well as an obvious relation to genetic phenomena in bacteria. 



If it is granted that the basic unit of inheritance in bacteria is the gene 

 and that gene mutations are responsible for the observed heritable varia- 

 tions, bacteria become a very valuable experimental tool because of the 

 relative ease of making quantitative studies of both spontaneous and 

 induced mutations. Systems of bacterial mutations which are particu- 

 larly valuable in such quantitative studies include mutations to resist- 

 ance to certain antibacterial agents such as bacteriophages, antibiotics, 

 radiation, and metabolic inhibitors; reverse mutations of biochemical 

 mutations in which the ability to carrj^ out certain syntheses has been 

 lost; and mutations affecting the fermentation reactions. 



It has generally been assumed that the kinds of viable mutations 

 observed following irradiation do not differ from those which occur spon- 

 taneously, and that irradiation merely increases the rate of occurrence. 

 Bryson and Davidson (1951) have studied a series of independently occur- 

 ring, spontaneous, and ultraviolet-induced mutations to resistance to Tl 

 bacteriophage in E. coli B/r. The mutants were analyzed for resistance 

 to the other phages of the T series, for the tryptophane requirement 

 which Anderson (1946) has shown generally accompanies spontaneous 

 mutation to resistance to phage Tl, and for other biochemical require- 

 ments. Significant differences were observed in the proportions of Tl- 

 resistant and Tl- and T5-resistant mutations in the spontaneous and 

 irradiated series. Similarly, the frequency of tryptophane requirement 



