402 Microbes and You 



has been developed that is specific for certain strains of typhoid 

 organisms. 



Phage is probably responsible for the destruction of large num- 

 bers of bacteria in water polluted with human and animal sewage, 



Fig. 75. Bacteriophage typing of Salmonella typhosa. Each spot 

 has been inoculated with the same specific type of bacteria and with a 

 loopful of a difi^erent phage. Clear areas indicate phage activity. 

 {Courtesy of P. R. Edwards and the Enteric Bacteriology Unit, Labora- 

 tory Branch, Communicable Disease Center, U.S. Public Health Service, 

 Atlanta, Georgia.) 



and thus this lytic agent aids in the self-improvement of bodies of 

 water with the passage of time. It was d'Herelle's belief that 

 phage might explain the recovery of people from disease— a bac- 

 teriophage theory of immunity. He postulated that when the 

 phage level becomes high enough, the bacteria causing the disease 



