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 



