THE NATURE OF THE BACTERIOPHAGE 327 



the occurrence of a vitreous, or granular, degeneration in colonies that 

 have already attained a relatively advanced stage of growth. 



(3) The lytic agent may be propagated indefinitely in association with 

 growing cultures of the bacteria on which it acts. A few drops of a filtrate 

 from the first culture in which lysis has occurred may be added to a second 

 young, growing culture of the bacterium ; this may be filtered after lysis 

 and the filtrate added to a third freshly inoculated culture, and so on in 

 series. A phage filtrate may be active in very high dilution (1 X 10~^ or 

 higher), and in successive passages, carried out as above, the original titre 

 is often increased during the earlier transfers, and is then maintained. It 

 is clear, therefore, that the lytic agent, whatever it may be, is actively 

 reproduced during the lytic process. There are few instances of its repro- 

 duction in the absence of bacterial cells, or in the presence of dead bacterial 

 cells (see Krueger and Baldwin 1937). Under certain conditions, however, 

 it can be produced in the presence of bacterial cells that, though living, 

 are not growing or dividing. There has been some controversy on this 

 point (see Otto and Munter 1923, Twort 1925, 1926, Gratia and Rhodes 

 1926, Gohs and Jacobsohn 1927, Bronfenbrenner and Muckenfuss 1927), 

 but recently Krueger and his colleagues have defined conditions in which 

 phage appears to be produced without growth of the bacterial cell 

 (Scribner and Krueger 1937, Krueger and Fong 1937, Northrop 1939). 



(4) Any given lytic agent will be found to be most active against one 

 bacterial species or type, or against a few species or types that are known 

 to be related to one another. Against unrelated species, or types, there is 

 usually no action. The phage is, then, a highly specific agent. 



(5) Phage lysis has been observed in a large number of bacterial species. It 

 has also been reported in yeasts and actinomyces (Wiebolsand Wieringa 1936). 



(6) A secondary result of phage action is the appearance, in the bacterial 

 culture that is undergoing lysis, of variants that are resistant to the action 

 of the particular phage concerned. These variants are usually susceptible 

 to the action of other phages. 



(7) A large number of bacteria are known to carry a lytic agent to 

 which they are themselves resistant. These phage-resistant carrier strains 

 are called lysogenic. Lysogenic strains are indistinguishable from others 

 except for the fact that they release an agent which is actively lytic for 

 phage-sensitive bacteria. A lysogenic strain, and the bacteria sensitive to 

 the lytic agent it carries, usually belong to the same species or group of 

 micro-organisms. 



(8) One of the commonest natural habitats of the phage is the intestinal 

 tract of man and animals, and filtrates active against one or more bacteria 

 can almost always be obtained from any specimen of faeces, or from material 

 that has been subjected to faecal pollution. 



The Nature of the Bacteriophage. 



As we have seen, d'Herelle has consistently maintained the view that the 

 phage is a filtrable virus, parasitic on bacteria. This hypothesis has always been 

 in accord with many of the most striking features of phage behaviour (see Flu 

 1923, Reichert 1924, Schuurman 1925). There can be little doubt that it would 

 have gained early and general acceptance but for the fact that certain recorded 

 observations seemed almost irreconcilable with it. 



