Physiology. 1 



ranging from 80-100 F. Parasitic forms living in the 

 animal or human body have a high optimum, usually ap- 

 proximating the blood temperature (98-100 F.). 



12. 3. Gaseous environment. To most forms of life, 

 atmospheric air is a necessity, in order to supply the 

 oxygen used in growth. With the bacteria, the -great 

 majority require the free access of air the same as other 

 living organisms, and if denied this, fail to grow. Such 

 bacteria are called aerobic. Toward some, however, oxy- 

 gen acts as a direct poison. Only when they are sur- 

 rounded with an atmosphere other than air, such as 

 hydrogen or nitrogen gas, can they grow. These forms 

 are called anaerobic. All bacteria are not divided sharply 

 into these two classes. While some of them grow strictly 

 under one condition or the other, hence are called obligate 

 aerobes or anaerobes, other forms are seemingly indiffer- 

 ent to their gaseous surroundings. To this class, the 

 name of facultative or optional aerobe or anaerobe is given, 

 depending upon the relation of the germ to the oxygen 

 supply (fig. 1, d, e) . 



Most milk bacteria belong to the aerobic class, but 

 there are quite a number that are able to grow without 

 free oxygen. 



13. 4. Moisture. On a dry medium, bacteria can not 

 grow, neither can the spores germinate. A certain amount 

 of moisture is, therefore, necessary before bacterial growth 

 can take place. Organic substances such as vegetable or 

 animal tissue contain sufficient water to permit growth. 



14. Rate of growth. The rate of growth of actively 

 vegetating bacteria is in many instances perfectly astound- 

 ing. With many species under favorable conditions, a 

 single cell will divide in twenty minutes, and each of the 

 daughter- cells will repeat this operation in the same 



