Viruses 401 



to be a contagious fluid, a transmissible enzyme, or nucleoprotein 

 molecule, but today it is accepted as a virus— a living organism 

 which has adapted itself to a completely parasitic, intracellular 

 existence. 



Phage exhibits cell specificity. That is, a phage which is cap- 

 able of lysing one species of organism does not generally attack 

 another species of microbe. Many of the cross-reactions reported 

 in the early literature were undoubtedly due to impure cultures 

 of the lytic agent. 



To demonstrate the presence of phage, two methods are com- 

 monly employed. Using the broth culture technic of d'Herelle, 

 young (from six to eight hours) cultures of the specific bacteria 

 showing visible turbidity in broth are seeded with filtrates con- 

 taining specific phage. Within a few hours the turbidit)^ of the 

 broth culture will disappear, and microscopic examination of the 

 broth will usually reveal the absence of the host bacteria. Should 

 the bacterial culture contain phage-resistant cells, a secondary 

 growth and turbidity of the tube will occur after the initial lytic 

 reaction. The strength (titer) of the phage (or the number of 

 phage particles) can be determined by quantitative dilution 

 procedures. 



Another satisfactory method of demonstrating phage is to streak 

 an agar culture plate with phage-susceptible bacteria, and on top 

 of this the specific phage is streaked. Upon incubation of the 

 plate, clear areas, called plaques, will appear in the surface growth 

 of the bacteria on the plate. Some colonies may appear "moth- 

 eaten" where the virus has attacked the bacterial cells. Phage 

 plaques might be considered to be the reverse in appearance of 

 bacterial colonies. That is, bacterial colonies are visible concen- 

 trations of growth on the plate, while phage plaques are areas of 

 no growth on heavily seeded culture plates. 



The specificit)^ of phage in its action permits what is called 

 PHAGE TYPING to detect Specific strains of given species of bacteria. 

 An interesting epidemiological application of phage typing is in 

 tracing the source of such disease outbreaks as typhoid fever. 

 Not only is the phage specific for Salmonella typhosa, but virus 



