SURVIVAL OF PATHOGENS IN SOIL AND WATER 



29 



ever raises the question concerning the role of the soil as the carrier of 

 these disease-producing agents or as the cause of epidemics. 



This rapid disappearance of disease-producing bacteria in the soil may 

 be due to a number of factors: (a) unfavorable environment j (b) lack 

 of sufficient or proper food supply j (c) destruction by predacious agents 

 such as protozoa and other animals j (d) destruction by various sapro- 

 phytic bacteria and fungi considered as antagonists j (e) formation by 

 these antagonists of specific toxic or antibiotic substances destructive to 

 the pathogens J (f ) in the case of some organisms at least, increase of the 

 bacteriophage content of the soil resulting in the lysis of some bacteria, 

 especially certain spore-formers (50). 



The course of survival of only a few disease-producing organisms 

 outside the host has been studied in detail. Sufficient information has 

 been accumulated, however, to justify certain general conclusions. 

 When E. colt is added to sterile soil, it multiplies at a rapid rate (Table 

 3, p. 10), but when added to fresh, nonsterile soil it tends to die out 

 quickly (Table 4). The rate of its disappearance is independent of re- 

 action of the soil and of incubation temperature. 



In order to illustrate the fate of certain important disease-producing 



TABLE 4. SURVIVAL OF BACTERIA ADDED TO SOIL AND THEIR EFFECT 

 UPON THE SOIL MICROBIOLOGICAL POPULATION 



From Waksman and Woodruff (980). 



* In thousands per gram of soil. 



t Washed suspension of E. coli cells added at start and after 5 days. 



% CaCOs added to soil. 



