PHAGE TAXONOMY 429 



investigated. As far as present knowledge goes, the phenomenon 

 of photoreactivation of ultraviolet-inactivated phages is not 

 likely to be useful in phage taxonomy because it seems to be a 

 general property of all phages so far examined. 



The phenomenon of lysis inhibition (Doermann, 1948a) and 

 the r+ to r mutation (Hershey, 1946a) is an example of a physi- 

 ological property which has been demonstrated only in the T2 

 species of phages although it has not been looked for outside of 

 the enteric group. This property may be of value only in identi- 

 fying members of the one species, which seems to be unique in a 

 number of respects. 



It seems probable that further research into the physiological 

 properties of phages will turn up other characteristic properties of 

 taxonomic value. One may anticipate that a pattern of be- 

 havior with regard to several physiological properties will be more 

 useful than any single property for taxonomic purposes. 



g. Results of Mixed Infection Experiments 



All of the properties listed above may be used to demonstrate 

 resemblances and differences between any two phage strains. 

 Suppose that we find that the two phage strains are indistin- 

 guishable in morphology, serologically related, and similar with 

 respect to several physiological properties. How do we decide 

 on the degree of relationship between them ; how do we allocate 

 them to their proper positions in the taxonomic hierarchy? 

 Are they derivatives of the same strain differing in a few mutated 

 genes; are they two varieties within the same species; are they 

 representatives of two different species in the same genus; or 

 should they perhaps be placed in different genera? This is often 

 a difficult problem with higher organisms and in the field of 

 microbiology there are as yet no satisfactory criteria for making 

 such decisions. However, this need not deter us from proposing 

 criteria which then can be put to the test. Such a criterion is the 

 result of the mixed infection experiment. If the two phage 

 strains under comparison have a common host strain of bacteria, 

 it is a simple matter to arrange conditions so that a majority of 



