PHAGE TAXONOMY 433 



point of view seems to be that they require knowledge of the 

 genetics of the host bacterium, which is not generally available. 



6. Relationship of Bacteriophages to Other Viruses 



In the classification of Holmes (Bergey's Manual) the bacterio- 

 phages, plant viruses, and animal viruses are classified as sub- 

 orders in the order Virales. In the modern concept of system- 

 atics this implies a common ancestry for these three groups of 

 microorganisms. There are strong resemblances among these 

 three groups, but these can all be attributed to the fact that the 

 viruses are obligate, intracellular parasites with no detectable 

 metabolic activity of their own. These resemblances may be 

 superficial rather than basic and the result of adaptation to a 

 specific environment, much like the superficial resemblance be- 

 tween penguins and seals. It seems probable that each of the 

 three main groups of viruses originated and evolved independ- 

 ently of the other two groups, and it seems possible that dif- 

 ferent groups of phages have evolved independently of each other. 

 The morphology of the phages as demonstrated in the electron 

 microscope is diff'erent from that of all other viruses, which do 

 not possess morphologically distinct head and tail structures. 

 This structural diflferentiation is correlated with antigenic and 

 functional differentiation. From the chemical point of view, no 

 phage is yet known to contain ribonucleic acid, whereas the 

 latter is present, alone or with deoxyribonucleic acid, in quite a 

 few animal and plant viruses. Also the mode of attack by phage 

 on its host cell differs from the attack of other viruses so far in- 

 vestigated. The injection of phage nucleic acid into the host 

 cell, leaving the phage membrane on the cell surface, seems to be 

 unique. 



Perhaps the most compelling evidence for an independent 

 phylogeny for the bacterial viruses may come from studies with 

 temperate phages (Chapter XIX). It has been proposed that 

 there is a homology between the genetic material of the phage 

 and a portion of the chromosome of the host bacterium. The 

 lysogenic state may be the result of crossing over between the 



