200 
PRINCIPLES OF BACTERIOLOGY 
ties, by osomosis it tends to extract the salts and minerals 
from the tissues. 
Most communities have a regular bacteriologic ex¬ 
amination made of the water supply; knowing the num¬ 
ber of bacteria present in the water under normal con¬ 
ditions, any deviation from this standard must mean 
danger. 
A sample of water to be tested may be taken from the 
faucet, first allowing the water to run for thirty min¬ 
utes. When it is not possible to obtain a specimen in 
this way it should be collected in sterile vessels, kept at 
a cool, even temperature and hastened to the laboratory. 
The number of bacteria in the water is determined by 
placing 1 c.c. of the sample in a sterile Petri dish, add to 
it a tubeful of melted agar or gelatin, mix and let stand 
for three or four days in a dark, moist atmosphere, at 
37° C. If the number of colonies are small, they may 
be counted very easily. The result may be taken as the 
number of individual bacteria contained in a quantity 
of water measured. If the number of colonies are large 
the original specimen of water must be diluted with 
sterile water or a large number of colonies may be 
counted with the aid of some mechanical device. The 
Wolffhuegel plate is generally used, although any black 
surface ruled in squares with white lines may be used. 
A certain number of these are counted, then the aver¬ 
age is obtained. 
The typhoid and cholera bacilli are difficult to isolate 
from contaminated water, or rather it is difficult to dif¬ 
ferentiate the typhoid from the colon bacillus, unless 
cultural tests are made (see chapter on Typhoid-Colon 
Group). The presence of colon bacilli usually indicates 
the close proximity of sewage pollution. 
