GENERAL CHARACTERS OF BACTERIA. 19 
still be a source of danger. But it must also be remembered that 
freezing destroys a very large proportion of these germs, so that the 
danger from the ice is far less than from the water before it was 
frozen. 
Relation to Air. Nearly all living organisms require air, and it 
was formerly supposed that nothing could live without it. Certain 
types of bacteria, however, are able to live without air. Indeed, 
some species, while they grow readily if they have no contact with 
air, fail to grow at all when the slightest amount of air is present, 
growing only in the absence of oxygen. This type of bacteria is 
spoken of as anaerobic. At the other extreme, there is a long list of 
bacteria which can grow only in the presence of air, failing to grow 
if they do not have oxygen at their command. This is the type of 
aerobic bacteria. Between the two is an intermediate group 
capable of growing either in the air or out of contact with it, and 
these are spoken of as facultative anaerobic. 
Relation to Moisture. Bacteria will grow only in the presence of 
considerable quantities of moisture; indeed, they demand more 
moisture than most organisms. Some of them will hardly grow at 
all unless there is 30 per cent, of moisture in the material in which 
they are living, and even then the growth is slow. On the other 
hand, they flourish most luxuriantly in localities where the water is 
from 90 to 100 per cent. Hence, as materials dry, bacteria will 
cease to grow in them, and any substance that can be dried can be 
thoroughly protected from their action. This explains why dried 
fish and dried meat, fruits, dried milk, etc., will keep indefinitely. 
The drying, however, does not actually kill the bacteria, for al- 
though they do not grow when the water is extracted from them, 
they may remain alive for weeks, months, or even years. In other 
words, it is impossible to depend upon drying as a means of de- 
stroying bacteria, for, while many individuals will fail to live, many 
others do not seem to be injured at all by the drying, and are capable 
of resuming life again as soon as they find moisture. 
Yeasts are much like bacteria in respect to need for water, and 
will not grow unless the water content is high. But other fungi 
with which we are concerned, the molds and mushrooms, can get 
