WHAT ARE BACTERIA? I/ 



of bacteria by freezing, at least with any certainty, for no mat- 

 ter how low the degree of temperature used, the life of some of 

 these organisms seems to be totally resistant to cold. Experi- 

 ments have shown that bacteria cooled to the temperature of 

 liquid air, or even liquid hydrogen, are not killed, and after 

 being warmed again still retain their life. Although these ex- 

 tremes of temperature do not destroy all bacteria, the simple 

 matter of freezing and thawing will kill a great number of in- 

 dividuals. If water containing a large number of bacteria is 

 frozen and subsequently thawed, the bacteria will be found 

 much reduced in numbers, although they are not by any means 

 all killed. When, therefore, water is contaminated by sewage 

 containing typhoid bacteria, and ice is collected from it for 

 domestic purposes, the typhoid bacteria may still be left alive 

 in the ice. Such ice may still be a source of danger. But it 

 must also be remembered that the freezing destroys a very 

 large proportion of these germs, so that the danger from this 

 use of ice is far less than the use of water from the same 

 source. 



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. Be- 

 tween the two is an intermediate group capable of growing 

 either in the air or out of contact with it, and this type is 

 spoken of as facultative anaerobic. The most important of all 

 dairy bacteria belongs to this class, the common lactic acid 

 bacterium, Bact. lactis acidi. 



