DISTRIBUTION AND HABITAT OF THE BACTERIA. 715 



It is always very difficult to pick out the pathogenic Demonstra- 



i * n i ^ e turn of the 



bacteria from among the large number of saprophytes, presence of 

 And in attempting to ascertain their presence we have 

 also to remember that they do not as a rule live long in 

 well water ; that as they never multiply in the water, 

 every withdrawal of water, and every addition of pure 

 ground water diminishes their numbers ; and it is only 

 in those cases where there is repeated contamination by 

 pathogenic bacteria that the chances of demonstrating 

 them by cultivation are at all favourable. It is probably 

 owing to these difficulties that pathogenic bacteria have 

 never as yet been demonstrated with absolute certainty 

 in any water. 



In addition to the ground water which is chiefly 

 employed for drinking and household purposes, the means of 

 water which flows on the surface of the ground often 

 serves as a means of transport of saprophytic, and at 

 times of pathogenic bacteria. In fact the water in 

 gutters, streams, and rivers is particularly dangerous, 

 because it not unfrequently serves the double purpose 

 of taking up and removing waste water of the most 

 various kinds, and at the same time of supplying water 

 for household purposes. 



Further, stagnant and superficial collections of water, Banks of 

 the muddy banks of rivers, and fields which are at times 

 submerged, are probably of special importance for the 

 etiology of many infective diseases. These act not only bacteria. 

 as means of transport for all sorts of disease germs, but 

 they also in all probability facilitate the growth and 

 multiplication of the facultative parasites. It has been 

 shown that anthrax, typhoid, and cholera bacilli can 

 grow well on moist, dead portions of plants, such as 

 often occur in enormous quantities on the banks of 

 rivers, in regions which are flooded, &c. In these places 

 the bacteria mentioned find a favourable temperature, as 

 well as the necessary moisture and nourishment, during 

 a great part of the year, and if a saprophytic existence 

 of parasites is possible anywhere in the natural sur- 

 roundings of man, it must be here. Such a saprophytic 

 growth is most likely to occur in tropical climates ; it 



