PHAGE TAXONOMY 431 



elusion the two strains do not belong in the same species. If the 

 lesult is mixed growth without genetic recombination the cri- 

 terion is not applicable and other criteria must be used. If no 

 common host is available the test cannot be used unless by this 

 means each strain can be related to a third strain. 



In cases in which the criterion of mixed infection is not appli- 

 cable one must make an arbitrary decision as to whether two 

 strains are to be placed in the same species or not. It is sug- 

 gested that if two phage strains are either morphologically indis- 

 tinguishable or serologically related and also share a number of 

 physiological properties in common they should be tentatively 

 allocated to the same species. If they are serologically related 

 and morphologically indistinguishable they are almost certain to 

 belong in the same species. 



4. Possible Taxonomic Criteria at Levels Above the Species 



At the present stage of development of phage taxonomy it is 

 impossible to define the limits of the genus or of higher categories. 

 However, one can make tentative suggestions for further explo- 

 ration in this field. One possible basis for defining the genus 

 would be morphological resemblance. For instance, phage 

 strains which were morphologically similar but serologically and 

 physiologically dissimilar might be placed in different species in 

 the same genus. Another possibility is the use of host range. 

 The host range pattern is a valuable identifying characteristic 

 for individual phage strains, but the fact that this can change by 

 single step mutations limits its value at the species level. The 

 total range of hosts available to various phage strains within a 

 single species is still unknown but it is already evident that among 

 cne enteric phages the host range of a phage species may cover at 

 least four genera, Escherichia, Serratia, Salmonella, and Shigella, in 

 the family Enterobacteriaceae (Adams and Wade, 1 954) . How 

 much broader than this the host range of a species may be no one 

 knows but it is quite possible that the hosts for a phage species 

 may be limited to the various bacterial strains within one family. 

 In fact it is possible that extensive study of the host ranges of bac- 



