SUMMARY: A given race of phage grows in a relatively limited range of bacteria. A coli phage, for instance, will not lyse a staphylococcus or a corynebacterium. Within these limits, however, some phages have a much wider host-range than others: some attack only one or a few bacterial strains; some a whole species; and some can lyse members of several species which on other grounds are considered to be not too distantly related. For instance, some pasteurella phages also attack strains of Salmonella and Shigella (Lazarus & Gunnison, 1947). The phage-sensitivity of a strain as a basis for bacterial classification can be interpreted in two ways, just as there are two levels at which bacterial classification can itself be regarded. That is to say, either as just another phenotypic character which the two strains may have in common; or at the level of the genetic material, the nucleic acid, so that, if two bacterial strains interact with the same phage at the genetic level, each of the strains is manifesting some degree of genetic compatibility with the phage, and thus with each other.


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