30 HUMAN AND ANIMAL WASTES 



example (997), that in a sewage sludge free to undergo normal diges- 

 tion, typhoid bacteria probably survive for less than 7 days. It was sug- 

 gested, therefore, that sludge held in a digestion tank for about 10 days 

 might be applied to the soil for fertilizer purposes without detriment to 

 public health. 



The gram-negative bacteria of the typhoid-dysentery group die out 

 rapidly in septic material j the typhoid bacteria survive for about 5 

 days, the Flexner type of dysentery for about 3 days, and the Shiga 

 bacillus dies out even in a shorter period. If decomposition in the tank 

 has not advanced far enough, as shown by low alkalinity, the organisms 

 may survive for a much longer period. The efficiency of ripe tank ef- 

 fluent to destroy bacteria is believed to be due to both the alkaline re- 

 action and the presence of antagonistic metabolic products. The destruc- 

 tion of typhoid and dysentery bacteria in the soil depends on a number 

 of factors, chief among which are the moisture content and reaction, 

 and the nature and abundance of the microbiological population. In 

 moist or dry soils, most of the pathogenic bacteria were found to die 

 within 10 days (488). 



Numerous other pathogenic agents, including those causing some of 

 the most deadly human and animal scourges — tuberculosis, leprosy, 

 diphtheria, pneumonia, bubonic plague, cholera, influenza, mastitis and 

 abortion in cattle, the many poxes — constantly find their way into the 

 soil in large numbers. They disappear sooner or later, and no one now 

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

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

 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} (e) formation by 

 these antagonists of specific toxic or antibiotic substances destructive to 

 the pathogens} (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 (49). 



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



