32. THE NUCLEIC ACIDS OF MICROORGANISMS 171 



type of the DXA) is occupied by the representatives of the families of 

 Brucellaceae and Bacteroidaceae which were united earlier in one family of 

 I > arvobact eriaceae . 



As to the family of Enterobacteriaceae as a whole (Table II), its repre- 

 sentatives generally possess the DXA of more or less intermediate type 

 which nevertheless trends to a slightly expressed GC-type. Within this 

 family the variations of the (A + T)/(G + C) ratio run from 0.7 to 1.0. 

 The exception to this is P. vulgaris in which this value is significantly higher 

 than 1 (AT-type of the DXA). Besides, another representative of the 

 Proteus genus, Proteus morganii, drastically differs in DXA composition 

 from P. vulgaris and actually is very similar to the bacteria of the Shigella- 

 Salmonella group in. respect to this feature. Two questions then arise: (1) 

 Is it actually correct to refer P. morganii to the same genus as P. vulgaris} 

 (2) How close is P. vulgaris to most representatives of the family of Entero- 

 bacteriaceae? Thus, if one does not take into consideration the exception 

 mentioned, the family of Enterobacteriaceae may be said to differ from all 

 the other gram-negative bacilli in a slightly expressed GC-type of DXA. 



Most species of closely related families of Brucellaceae and Bacteroidaceae 

 (Pasteurella, Bacteroides, Fusobacterium, Hemophilus) are, on the other 

 hand, characterized by a marked AT-type of DXA. The value of the 

 (A + T)/(G + C) ratio varies from 1.6 to 2.2. However, Brucella, as well 

 as Fusobacterium polymorphum are drastically different, possessing a marked 

 GC-type of DNA [(A + T)/(G + C) = 0.7]. This fact must also raise a 

 number of questions concerning the systematics, at any rate, with respect 

 to the genus Brucella. In any case, most of the different representatives of 

 the two families in question form a rather close group with respect to DXA 

 composition. 



The analysis of gram-positive bacilli (Table III) is also of interest. Here, 

 the representatives studied divide into groups in exact correspondence with 

 their systematic positions. The organisms having the GC-type of DXA 

 belong to the first group. They represent, as a rule, the species belonging to 

 the family of Corynebacteriaceae (the genus Corynebacterium) . The or- 

 ganisms with the AT-type of DXA in which the (A + T)/(G + C) ratio 

 does not exceed 2 belong to the second group. All belong to the genus 

 Bacillus, the family of Bacillaceae. Finally, the third group, following the 

 second, includes the species with the most marked AT-type of DXA. All 

 of them also belong to the family of Bacillaceae, but to another genus, 

 namely, Clostridium. Thus, the systematic division into groups coincides 

 with that obtained from their DXA composition and their DXA types. 



Turning now to the analysis of the cocci (Table IV), it will be seen that 

 the marked GC-type of DXA in the cocci is typical of the representatives 

 of the family of Micrococcaceae (Sarcina, Micrococcus lysodeikticus). A 

 radically different DXA composition, of the extreme AT-type, is found 



