THE ECOLOGY OF PHAGES 345 



fermentation tests or antigenic structure, while other groups that are identical in 

 regard to their fermentation reactions may be divided into different types on 

 the basis of serological tests, so one particular method of analysis may serve 

 to separate strains of phage that would be grouped together when tested by 

 another. But correlation of characters is as frequent among the phages as it is 

 among the bacteria, and leaves no reasonable doubt that, by making full use of the 

 methods now at our disposal, we can separate and identify phage types that have 

 just as much claim to be regarded as biological entities as have the various species 

 or types of bacteria. 



Variation and Adaptation in Phages. 



The literature contains very numerous records of phage variation and phage 

 adaptation. D'Herelle, as has been noted, regards the phage as a single virus, that 

 may become adapted to attack a wide variety of bacterial species. Many of the 

 earlier accounts of adaptation must be discounted, because the technique adopted 

 did not ensure the purity of the original filtrates. There are, on the other hand, 

 numerous observations which show quite clearly that adaptation occurs, though 

 its range is probably more limited than was at one time supposed. Here, again, 

 the evidence suggests that the phage behaves in the same way as other known 

 micro-organisms. 



We may note, for example, that d'Herelle and Kakieten (1935) adapted a 

 staphylococcus bacteriophage, initially sensitive to neutralization by antiphage 

 serum, to lyse cocci in the presence of high serum concentrations. Burnet and 

 Lush (1936) observed a mutant from a Staph, albus phage. The two varieties 

 were similar, both serologically and by cross-tests on resistant survivors (see pre- 

 ceding section), though one had less lytic power than the other. But in one case 

 the resistant strain was lysogenic and in the other, not ; and infection of a culture 

 with the weak variant protected it against lysis by the strong variant. 



The Ecology of Phages. 



The problem of phage ecology raises points of the greatest biological interest 

 and importance. We have already noted that phages acting on one or other of 

 the normal or pathogenic intestinal bacteria can almost always be isolated from 

 faeces, from sewage, or from polluted water supplies (see, for instance, Sonnenschein 

 1927, Stewart and Ghosal 1931, Gildemeister and Watanabe 1931, Schlossmann 

 1932, von Vagedes and Gildemeister 1934, De and Paul 1940, de Assumpgao and 

 Leite e Silva 1942, Guelin 1943). This clearly suggests that wherever particular 

 species of bacteria occur in nature, there also are likely to be found phages to 

 which these bacteria are sensitive. But that is merely the first, and least interesting 

 step in the problem. 



Just as it was tacitly assumed, in the earliest days of medical bacteriology, that 

 a pathogenic bacterium, when it became parasitic on its natural host would always, 

 or almost always, cause the specific disease of which it was the causal agent, so it 

 was assumed in the early days of bacteriophage studies that a phage, if it became 

 attached to, and multiplied in association with, a particular bacterium would cause 

 phage lysis. The conception of phage-carriers among bacteria, or of a normal phage 

 flora living symbiotically with certain bacterial species, came relatively late. 



The demonstration that a phage, active against a particular species of bacterium, 

 could sometimes be isolated from an old laboratory culture of that organism by repeated 

 filtration, and the addition of each successive filtrate to a fresh broth culture of the bacterium 



