Survival of Pathogens in the Soil 283 



very short periods. It is also important to cite the fact that typhoid 

 and dysentery bacteria, which are known' to contaminate watersheds 

 and water suppHes, sooner or later disappear. No one now raises 

 the question concerning the role of the soil as the carrier of these 

 disease-producing agents or as the cause of severe or of even minor 

 epidemics. This rapid disappearance of disease-producing bacteria 

 mav be due to several factors, such as unfavorable environment, 

 lack of sufficient or proper food supply, destruction by predaceous 



Fig. 112. Eflect ot soil organisms against parasitism b\' P. vohitiuu on Agrostis 



(from van Luijk). 



agents, such as protozoa and other animals, and destruction by vari- 

 ous saprophytic bacteria and fungi considered antagonists. 



Jordan and his associates found that Eberthella typhosa survived 

 in sterilized tap water for 15-25 days, as against 4-7 days in fresh 

 water; it died off even more rapidly (in 1-4 days) in raw river or 

 canal water. The degree of survival of this organism in water was 

 found to be in inverse ratio to the degree of contamination of the 

 water, the saprophytic bacteria being directly responsible for the 

 destruction of the pathogen. Freshly isolated organisms survived 

 a shorter time than laboratory cultures, and higher temperatures were 

 more destructive than lower ones. 



The presence of certain bacteria in water is often found to hinder 

 the survival of E. typhosa. When Tseudomonas aeruginosa, on the 

 other hand, is present in drinking water, it may not be accompanied 

 by any other bacteria. Media inoculated with this organism and 

 with Escherichia coli gave, after 13 days' cultivation, cultures of 



