PHYSIOLOGY OF MICRO-ORGANISMS 1 73 



which produce acid or alkali are usually arrested in their growth as 

 soon as a certain concentration is reached, and the medium may 

 then rapidly kill the micro-organisms. 



Oxygen.— Oxygen, either free as atmospheric oxygen or com- 

 bined as in water or organic compound^, is an essential constitu- 

 ent of the food of all micro-organisms. The concentration of 

 uncombined oxygen dissolved in the medium, or the partial pres^ 

 sure of atmospheric oxygen, is the factor ordinarily meant when 

 oxygen requirement is mentioned. Many micro-organisms grow 

 best in a medium freely exposed to the air. These are called 

 aerobes. Some which will grow only when there is free access of 

 oxygen are called obligate aerobes. There are numerous bacteria, 

 including spirochetes, which grow only in the absence of, or in 

 extremely weak concentration of oxygen. These are called 

 obligate anaerobes. Many of the bacteria grow well in various 

 concentrations of oxygen or in its absence. These are spoken 

 of as facultative anaerobes, or sometiihes as facultative aerobes if 

 they seem to prefer the anaerobic existence. Finally there are 

 a few organisms, some bacteria and spirochetes, and perhaps some 

 protozoa, which seem to require a fairly definite partial pressure 

 of oxygen, but are not adapted to growth in a medium freely 

 exposed to the atmosphere (B. hifidus, B. abortus, Spirochceta 

 rossii, Plasmodium falciparum). In relation to oxygen require- 

 ment, these are designated as microaerophilic organisms. 



Temperature. — ^Among the various micro-organisms are found 

 types which are adapted for growth at different temperatures 

 throughout a considerable range. There are some bacteria and 

 perhaps some molds capable of growth at a temperature of -0.5° 

 C.,' in food substances such as miUc, which are not frozen at this 

 temperature. A certain yeast is said to multiply even at -6° C, 

 in salted butter. Microbes which grow at very high temperatures, 

 even up to 80° C, occur in the soil, in ensilage and sometimes 

 in the intestine of animals. The great majority of micro-organ- 

 isms grow only between 0° and 40° C. It is possible to recognize 

 a minimum, a maximum and an intermediate optimum tem- 



