INTERPRETATIONS OF BACTERIOLOGY 55 



mission of the water supply, so that foul gas or liquid is sucked back 

 into the house-pipe. 



A further point has relation to bacterially polluted water when it 

 has gained entrance to the body. It has been known for some time 

 past that not all waters polluted with disease germs produce disease. 

 As we have before said, this depends upon the infective agent, its 

 quantity and quality, and upon the human body. The body is able 

 in many cases to resist a small dose of poison. It is, however, 

 necessary to infection, especially in water-borne disease, that the 

 tissues shall be in some degree disordered, weakened, or injured. For 

 instance, the perverted action of the stomach influences the acid 

 secretion of the gastric juice, through which bacilli might then pass 

 uninjured. Particularly must this be so in the bacillus of cholera, 

 which is readily killed by the normal acid reaction of the stomach. 

 Hence, in this disease at least, it is the opinion of bacteriologists that 

 the condition of the mucous membrane of the stomach is of primary 

 importance. Metchnikoff has indeed demonstrated the presence of 

 the bacillus of cholera in the intestinal excretion of apparently healthy 

 persons, which shows that they were protected by the resistance of 

 their tissues to the bacilli. Further light has been thrown on this 

 question by the researches of MacFadyen, who has pointed out that 

 suspensions of cholera bacilli in water passed through the stomach 

 untouched, and were thus able to exert their evil influence in other 

 parts of the alimentary canal. When, however, cholera bacilli were 

 suspended in milk, none appeared to escape the germicidal action 

 of the gastric juice. The explanation of this is probably the simple 

 one that the stomach reacted with its secretion of gastric juice only 

 to food (milk), but passed the water on into the lower and more 

 absorptive parts of the alimentary canal. Such a condition of affairs 

 clearly increases the danger due to water-borne germs. 



The Interpretation of the Findings of Bacteriology 



Bacteriology is the most direct and delicate test of the safety of a 

 water for driuking purposes. By it we obtain exact information not 

 alone as to the constitution of a water, but as to its potentiality to 

 cause disease. It is also a more dehcate test than a chemical 

 examination.* Klein and others have shown that by bacteriological 

 methods it is possible to detect smaller degrees of sewage pollution 

 than by chemistry. On the other hand, it is useless to expect to 

 learn of the exact chemical constitution of a water by bacteriological 

 methods. Bacteriology must be interpreted by what it can 



* Clark and Gage state that polluted waters which might become unfit for 

 drinking purposes are more plainly indicated by a single chemical analysis than by 

 a single determination of B. coli. 



