172 A. N. BELOZERSKY AND A. S. SPIRIN 



however in the staphylococci. It is not ruled out that these two groups of 

 the Micrococcaceae are, indeed, phylogenetically much more remote from 

 each other than is usually thought. But the streptococci appear to be closely 

 related to the staphylococci in respect to the composition of their DNA 

 which shows a marked AT-type. Gram-negative cocci (family of Neisseria- 

 ceae) occupy, by their DNA composition, an intermediate and peculiar 

 position among the cocci. The Neisseria genus is characterized by equimo- 

 lar base proportions. 



Thus, the analysis of the DNA composition of the bacteria carried out 

 in respect to their classification in families confirms the viewpoint that the 

 DNA composition is closely related to the systematic position of the or- 

 ganism and its phylogenic origin. 



When one now considers the problem of the differences in the DNA composition in 

 species belonging to the same genus, i.e., in closely related species, certain conven- 

 tionality of the classification of the bacteria should be noted at once. We do not know 

 to what extent the bacterial species correspond to the conception of the species in 

 the systematics of higher organisms. At any rate, biological differences between the 

 bacterial species within a genus may prove to be of a very different order. This may 

 be just the reason for occasionally finding considerable differences in the DNA com- 

 position in species belonging to the same genus (for example, the genera of Pseudomo- 

 nas, 130 Azotobacter, 131 Pasteurella, 130 Bacteroides, 130 Corynebacterium, 130 and Bacillus 130 ), 

 whereas in other cases, species of the same genus are indistinguishable by their DNA 

 composition (for example, the genera of Neisseria, 130 Streptococcus, 130 and Salmonella 

 I30 ). In any event, it is nevertheless possible to define slight species differences in 

 some cases in which special statistical treatment of the data shows the complete 

 reliability of these differences though they are very small. 146 We succeeded in showing 

 the reliability of such very small differences in Salmonella typhosa and Salmonella 

 typhimurium, as well as their difference from the DNA of E. coli. i46 



The problem of strain differences in the DNA composition is not yet completely 

 solved. In the literature, there is mentioned repeatedly the possibility of the exist- 

 ence of small but statistically significant differences in DNA composition between 

 different strains of the same species. For example, significant differences in the DNA 

 composition are found between the original strain B. cereus var. alesti and its mutant 

 obtained by means of cultivation in an alkaline medium. 130 Difference of the DNA 

 composition is shown in a streptomycin-resistant strain of A. aerogenes when it is 

 compared to the normal strain. 181 In most cases, however, the authors stress the ab- 

 sence of any appreciable strain differences in DNA composition. 28 ' 85 ' 130 ' 131 ' 132 ' 144 



It is evident that in connection with the agreement shown here between 

 the DNA composition of the bacteria and their systematic position, the 

 analysis of the data on the DNA composition in other microorganisms is 

 of great interest. In Table V, the findings on the DNA composition in 

 mycobacteria and actinomycetes are summarized. The species shown in 

 the table are grouped in 3 families of the order Actinomycetales. All these 

 representatives are characterized by the extreme GC-type of DNA. The 

 mycobacteria, however, show a considerable difference in their DNA com- 

 161 A. S. Jones, G. E. Marsh, and S. B. H. Rizvi, /. Gen. Microbiol. 17, 586 (1957). 



