BACTERIA IN RELATION TO MILK 
313 
in a dry or a cold place. Note the influence (a) of warmth, (6) of cold, 
on the rapidity of growth. 
7. To show that bacteria are in the air, expose the surface of the cul¬ 
ture plate for a few seconds. 
8. To show that flies distribute bacteria, let a fly walk across the 
surface of the agar-agar in the petri dish. 
If bacteria have an opportunity, they work on every¬ 
thing which is capable of decay, so we need to know how 
to prevent their working upon food and other things which 
we do not wish to “ spoil.” Several ways in common 
use are: (1) cold storage, where there is not warmth suffi¬ 
cient for the growth of bacteria; (2) the use of salt and 
, other chemicals to prevent their getting a start, as in the 
curing and smoking of meat; (3) drying fruit and meat, 
thus removing water, a necessary condition for growth; 
and (4) heating fruit, vegetables, milk, etc., and sealing them 
in cans or jars while hot, thus killing any bacteria the 
substances may contain and keeping all others out. Any¬ 
thing prepared in this way is preserved by being made 
sterile or aseptic (Greek : a, not; sepein, to make putrid). 
269. Bacteria in Relation to Milk. — (See also Part III.) 
Milk as it comes from the healthy cow is practically free 
from bacteria of any kind. The number of bacteria present, 
however, is not of so much importance as the kind. But 
if a large number of bacteria are allowed to get into the 
milk, some of them are sure to be harmful and may find 
conditions so favorable for their growth as to make trouble 
for the person using the milk. 
A high grade of milk will not contain more than 500 to 
1000 bacteria per cubic centimeter. Such milk has been 
well cared for and comes from healthy cows. Some cities 
permit milk to be sold that contains as many as 100,000 
bacteria per cubic centimeter and some even more. Such 
milk comes from unhealthy cows or dirty barns, or has 
been kept too long, or has “ changed hands ” too many times. 
