344 



THE BACTERIOPHAGE 



These may be entire or eroded, sharp or bevelled. They may, or may not, be 

 surrounded by a halo in which the bacterial growth is altered in appearance, though 

 not completely lysed. Phage plaques, indeed, present variations in morphology 

 as distinctive as those shown by bacterial colonies, and have the same classificatory 



value. Asheshov (1924) (see also 

 Asheshov el al. 19336) has paid 

 particular attention to these diifer- 

 ences in plaque morphology, and 

 Fig. 55 illustrates the differences, 

 in size and form, that may be 

 observed among the plaques pro- 

 duced by different phages acting 

 on the same strain of a sensitive 

 bacterium. 



If, then, we seek to classify any 

 particular group of phage strains, 

 we can make use of the following 

 series of tests. 



(A) The determination of the 

 species of bacteria against which 

 each phage is active. This will 

 enable us to differentiate broad 

 groups — coli-phages, dysentery 

 phages, salmonella phages, staphyl- 

 ococcal phages, cholera phages, and 

 so on. 



(B) By more detailed tests of cross- resistance we can subdivide these broad 

 groups, each into a number of different types — so many different types of dysentery 

 phage, so many of cholera phage, and so on. 



(C) We can apply to the same problem of the subdivision of our broad groups 

 the method of antigenic analysis, using specific antiphage sera. 



(D) We can study the size and morphology of the plaques formed, and divide 

 our original groups of phages into large-plaque-forming strains, small -plaque-forming 

 strains, and so on. 



(E) We can also apply certain other biological tests, such as relative resistance 

 to heat, or to the photodynamic action of methylene blue, or to the presence of 

 citrate in a medium. 



When, in fact, we apply several of these tests, our confidence in each of them is 

 increased by finding that the results they give are highly correlated. The classifica- 

 tion derived from detailed resistance tests corresponds closely with that derived 

 from neutralization tests with antiphage sera. The phages producing a particular 

 type of plaque, when acting on a particular sensitive bacterium, are usually 

 found to fall into the same antigenic group. An admirable series of studies in 

 which this correlation is brought out very clearly have been recorded by Burnet 

 (1933a, h) ; and other evidence, all pointing in the same direction, will be found in 

 the studies of Asheshov and his colleagues (1930, 19336), in papers by Sertic 

 and Boulgakov (1935ff, b), and in many of the other records referred to above. The 

 correspondence is not, of course, absolute. Just as some groups of bacterial strains 

 would be divided into the same species or types, whether classified on the basis of 



Fig. 55. 



Different types of plaque produced by different 

 phages acting on the same bacterial culture. 



