134 LABORATORY EXERCISES IN BACTERIOLOGY. 



For less exacting examination the mere exposure of sterile media, as potatoes, 

 gelatine, agar, or other substance, in open Petri dishes is usually sufficient ; in such 

 case, however, one should guard against the convection of organisms to the nutrient 

 substance by flies and similar agents. 



Examination of Water. Depending largely upon the amount of organic im- 

 purities (there are some bacteria capable of living in filtered water, depending on 

 the minute impurities and carbonic oxide absorbed from the air), the temperature, 

 the exposure of the water to sunlight (inimical to the bacteria), and the aeration brought 

 about by rapid and dashing currents, and other factors, the number and kind of 

 bacteria found in a sample of water vary within a wide range. Artesian water, from 

 its filtration through great distances of the ground, and underground water generally, 

 must for mechanical reasons, if for no other, represent the purest types in nature. 

 No definite numerical limit may be set for purity of water from a standpoint of pota- 

 bility, so many of the common forms of water bacteria being innocuous when taken 

 in the human system, and a very few pathogenic germs, on the other hand, rendering 

 a sample unfit for drinking purposes. In the most general sense, however, a water 

 containing no more than one hundred bacteria to the cubic centimeter is regarded 

 as a pure water; above one hundred to five hundred to the cubic centimeter, as doubt- 

 ful; and above five hundred, as unfit for drinking use. Sewage water during summer 

 temperatures may sometimes be found to contain 25,000,000 or more organisms to 

 the cubic centimeter. In studying any sample of water, experience has shown that 

 the really important bacteria those of diseases like typhoid fever or Asiatic cholera, 

 which are commonly believed to be transmitted through drinking-water are very 

 likely not to be found even though their presence is suspected, the conditions of collec- 

 tion and cultivation usually adopted not favoring their development in the cultures 

 and consequent recognition. They are apt to be few in the water compared with the 

 ordinary water bacteria; and the temperature of the room usually employed for cul- 

 tures and the overwhelming growth of the water bacteria probably interfere with their 

 best development. Should these or other bacteria, as the colon bacillus or the putre- 

 factive bacteria, be recognized, however, even in the smallest numbers, the water- 

 supply from which they have been obtained should without further question be con- 

 demned as unfit for use, and corrective measures insisted upon. Therefore, in the 

 examination of a sample, even though it be impracticable to isolate and identify every 

 one of the numerous forms of microbes likely to be encountered, every bacteriologic 

 analysis should occupy at least these two phases of inquiry : the number of organisms 

 in a definite amount of the sample (one cubic centimeter) and the presence of patho- 

 genic species or such as indicate excrementitious or putrefactive contamination. More- 

 over, such biologic examination should be accompanied by the usual chemical deter- 

 mination of organic impurities, the number of bacteria becoming the more significant 

 as the chemical analysis indicates a small amount of such contamination. It is not 

 the inert organic matter present which is of direct moment, but rather the active 

 life it supports in the water ; not so much the number of these living things, but essen- 

 tially their character, which must finally indicate the value or the noxious qualities 

 of the supply. As far as the mere presence of the water bacteria is concerned, in 

 one sense they must be regarded as coadjutors of man, in that they destroy the organic 

 matter in water and aid in its purification a fact easily realized by one who has 

 been accustomed to the use of cistern water during warm seasons. After a rain, 

 when there are carried quantities of organic refuse from the roof into the cistern from 

 rotting shingles and from the surface dust blown on the roof by the wind, in a favoring 



