BACTERIOLYSIS 47 



As soon as the culture is well developed, as after 18 to 24 hours 

 of incubation, if .the centre of such a plaque is touched with a 

 platinum wire and this is immersed in a culture of Shiga bacilli 

 the bacteriophage develops in this suspension and the latter is 

 lysed after a few hours. The plaque, although sterile, is not 

 ultrasterile; it is in fact a colony of the bacteriophage. 



Furthermore, if a trace of the bacillary growth at the periphery 

 of a plaque is taken with a platinum wire and seeded on agar it 

 remains sterile and inoculation into a bacterial culture shows that 

 the bacteriophage is present there also. But when the bacillary 

 layer is taken, not at the immediate edge of the area, but at a 

 distance of two millimeters from it, for example, and planted, the 

 tubes show the growth of a normal culture. The bacteriophage 

 is not found. 



If the culture showing the plaques is returned to the incubator 

 and the tests are repeated three or four days later, that is, culturing 

 the bacillary growth at a distance of two millimeters from a 

 plaque onto agar and into a suspension it will be found that the 

 bacteriophage is there present at that time. The bacteriophage has, 

 therefore, gradually invaded the bacillary layer. This invasion is 

 always slow proceeding more and more slowly as time pro- 

 gresses so that the ring invaded, even after several months, 

 amounts to a zone but a few millimeters wide. Beyond the limits 

 of this zone the Shiga organisms remain cultivable just as long 

 as they do in a normal control culture without the bacteriophage. 



The question immediately arises as to why the bacteriophage 

 does not invade the entire layer of bacterial growth. For this 

 there are two reasons. The bacteriophage attacks the bacterial 

 cell most readily when the bacterium is young. When placed upon 

 agar the bacteriophagous organisms find themselves located 

 in the immediate vicinity of bacilli which reproduce actively as 

 soon as they are deposited upon a nutrient medium. They find 

 then, within their range, very young bacilli distributed in a very 

 thin layer over the agar. Lysis is thus possible and the apparent 

 sterility of the plaque results. But beyond this zone invaded by 

 the bacteriophage during the first few hours the bacilli develop 

 freely forming a layer of increasing thickness comprised of or- 

 ganisms of increasing age. In other words, a thicker and thicker 



