702 BACTERIOLOGY OF THE SOIL, WATER AND AIR 
method. Again, the normal difference in bacterial content between 
ground waters, surface waters and polluted waters makes an inter- 
pretation of the bacterial count somewdiat difficult. For example, 
100 bacteria per cc. in a deep well water might have greater sanitary 
significance than 500 bacteria per cc. in a surface water, where bacterial 
counts are almost invariably higher. 
Attempts have been made to establish arbitrary bacterial standards; 
thus, waters containing less than 100 bacteria per cc. were formerly 
regarded as safe waters; those containing from 100 to 500 organisms 
per cc. Avere regarded with suspicion, and those containing 1000 or 
more organisms were pronounced dangerous for domestic use. In the 
abstract these standards are fictitious; surface waters even in unin- 
habited districts may contain many hundreds of bacteria per cubic 
centimeter after rains, yet the bacterial count would convey but little 
information of the actual sanitary status of the water. Successive 
bacterial counts carried out over long periods of time, on the other 
hand, are frequently of very great value. ^ 
A direct examination of water for pathogenic bacteria, as the typhoid 
bacillus, if it were practicable, would be a most satisfactory method of 
evaluating domestic water supplies, for it is the presence of these 
organisms harmful to man which, in the last analysis, makes water 
containing them dangerous for human consumption. Unfortunately 
it is not practicable, as numerous observers have amply demonstrated, 
to isolate pathogenic organisms of this type directly from water, and 
there are but few authentic records of a successful cultivation of the 
typhoid bacillus from water supplies known to be infected, in spite 
of numerous attempts. 
The practical impossibility of isolating pathogenic bacteria from 
water has led to the development of methods for the detection of 
B. coH and organisms found practically constantly in human and 
animal excrement. B. coli is somewhat more tolerant of environ- 
mental conditions as they exist in water than B. typhosus, and its 
constant presence in fecal discharges makes it somewhat more eft'ec- 
tive as an indicator of excrementitious contamination than the frankly 
pathogenic organisms. The simplest and in many respects the best 
method for detecting B. coli in water is to add graduated amounts of 
the sample to be analyzed— beginning with 1 cc. and decreasing the 
amount one-tenth in sucessive cultures— to lactose fermentation tubes. ^ 
A production of gas within twenty-four or forty-eight hours is sug- 
gestive, but not conclusive evidence of the presence of the organism. 
If gas develops, some of the culture should be placed on Endo medium, 
and red colonies that develop are tested for their ability to produce 
acid and gas in glucose and lactose media, for indol production in sugar- 
free broth, for their action upon milk, and an absence of liquefaction 
1 For an excellent resume of the subject see Clemesha (Bacteriology of Surface Waters 
in the Tropics, London and Calcutta, 1912) and Prescott and Winslow (Elements of 
Water Bacteriology, New York, 1913). 
- Theobald Smith: Notes on Bacillus coli communis and Related Forms, Am. Jour. 
Med. Sci.. 1895, 110, 283. 
