1 66 GENERAL BIOLOGY OF MICRO-ORGANISMS 



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

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

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

 uncombined oxygen dissolved in the medium, or the partial 

 pressure 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 a few 

 bacteria and some 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 sometimes 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. bifidus, B. abortus, 

 Spirochceta rossii, Plasmodium falciparum) . 



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 milk, 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 o and 40 C. It is possible to recognize 

 a minimum, a maximum and an intermediate optimum tem- 

 perature for growth of each species. Ordinarily the optimum 

 temperature is only a few degrees below the maximum at which 

 growth will take place. The^following table from Marshall's 

 Microbiology illustrates the relation of these temperatures. 



