340 THE BACTEBIOPHAOE 



in surface structure would appear to be concerned, though it is not detectable by 

 the ordinary serological methods. 



The Variations Induced in Bacteria in Response to Phage Lysis. 



In an earlier section of this chapter, it was noted that a secondary bacterial 

 growth usually appears in a culture that has undergone phage lysis. If this growth 

 is further examined, it will be found to be composed of bacteria that are resistant 

 to the strain of phage that caused the lysis, though they will usually be sensitive 

 to other strains of phage. 



Sometimes these resistant strains show a well-marked antigenic difference from 

 the original sensitive strain ; for instance, the lysis of a smooth strain of a dysentery 

 bacillus by an anti-smooth phage is frequently followed by the appearance of 

 characteristically rough resistant variants. But, as we have seen, resistant strains 

 are not always of a recognizably different antigenic type from the original sensitive 

 strain ; and, in the great majority of cases, we have at present no evidence as to 

 whether the change in phage sensitivity has, or has not, been accompanied by a 

 change in antigenic structure. What we do know is that submission to the action 

 of a phage is one of the most potent methods of inducing bacterial variation (see 

 Arkwright 1924, Hadley 1927, 1928), that the variants so produced may be of many 

 different kinds, that they share the character of resistance to the phage that induced 

 the variation, and that some at least of them differ antigenically from the parent 

 strain. 



The kind of variation induced is not necessarily similar to variations which, 

 like those noted in Chapter 9, occur in other circumstances. These latter varia- 

 tions may be indeed inhibited by the presence of phage. For instance, Lewis 

 and Worley (1936) noted that spontaneous dissociation of B. megatherium occurred 

 much less promptly in the presence of phage than in its absence. 



It is clear that the existence of phage-resistant strains of bacteria, and the possi- 

 bility of producing them at will, afford a method of studying in greater detail the 

 phages acting on single bacterial species. 



Bail (1923) was the first to stress the importance of cross-resistance tests between 

 different phages and different bacterial strains as a method of phage differentiation ; and 

 many others have since employed this method in the study of phages acting on particular 

 groups or species of bacteria, such as the Salmonella group (Burnet 19296), the Shigella 

 group (Burnet and McKie 19306, Morison 1932), or the cholera vibrios (Asheshov et al. 

 1930, 19336, Morison 1932). Many of these studies, however, were concerned with differ- 

 ences in phage sensitivity between different bacterial species or types, as well as between 

 different variants derived by phage action from a single bacterial strain. It is with 

 differences of the latter type, and their application as a method of separating and identifying 

 different phages that are active agamst a single bacterial species or type, that we are here 

 concerned. This method has been very extensively developed by Asheshov and his 

 colleagues, and we may take an illustrative example of an experiment performed according 

 to their technique (Asheshov et al. 19336). 



Suppose that we have three strains of phage, all acting on the same strain of a given 

 bacterium, but which we suspect, for one reason or another, to belong to different types. 

 We allow each of the three phages, which we may label I, II and III, and a mixture of them 

 (I-II-III), to act on four separate cultiues of our sensitive bacterium growing in a fluid 

 medium. We allow lysis to occur, and a secondary growth of resistant bacteria to foUow 

 it. We take a large agar plate, and mark it off into 16 squares (4 X 4). Over each square 

 in the first column of 4 squares we make a thick seeding from the secondary growth 

 from our Tjrpe I phage, over each square in the second column we seed the secondary 



