CHAPTER XIII 

 BACTERIOLOGY OF WATER 



Unless water has been specially sterilized, and received and kept 

 in sterile vessels, it always contains some bacteria, the number 

 usually bearing a distinct relationship to the quantity of organic 

 matter present. 



The majority of the water bacteria are bacilli, and are as a rule 

 non-pathogenic. Wright,* in his examination of the bacteria of 

 the water from the Schuylkill River, found two species of micrococci, 

 two species of cladothrices, and forty-six species and two varieties 

 of bacilli. Pathogenic bacteria, such as the spirillum of Asiatic 

 cholera, the bacillus of typhoid fever, and the bacillus of dysentery 

 may occur in polluted water, but are exceptional. 



The method of determining the number of bacteria in water is very 

 simple, and can be accomplished with very' little apparatus. The 

 method depends upon the equal distribution of a measured quantity 



Fig. 83. — Wolfhiigel's apparatus for counting colonies of bacteria upon plates. 



of the water to be examined in some sterile liquefied medium, whose 

 subsequent solidification in a thin layer permits the colonies to be 

 counted. 



The .method originated with Koch, and may be performed with 

 plates, Petri dishes, or Esmarch rolls. It is always best to make a 

 number of cultures with different quantities of the water, using, for 

 example, o.oi, o.i, 0.5, and i.o cc, respectively, to a tube of liquefied 

 gelatin, agar-agar, or glycerin agar-agar. 



The details of the method depend upon the quality of the water to 

 be examined. If the number of bacteria per cubic centimeter be 

 small, large quantities may be used; but if there be millions of bac- 

 teria in every cubic centimeter, it may be necessary to dilute the water 

 to be examined in the proportion of i : 10 or i : 100 with sterile 

 water, mixing well, and making the plate cultures from the dilutions. 



* "Memoirs of the National Academy of Sciences," vol. vii, Third Memoir. 



237 



