134 AGBICULTUEAL AND INDUSTEIAL BACTERIOLOGY 



moderate changes in hydrogen ion concentration do not 

 markedly change the rate at which bacteria grow except 

 near the limits. Practically the optimum is a relatively 

 wide range. It has previously been emphasized that in the 

 preparation of certain types of media great care is neces- 

 sary to insure the correct hydrogen ion concentration; 

 otherwise growth will not occur. 



Effect of Oxygen upon Rapidity of Growth. — ^Appar- 

 ently for each kind of organism under a definite set of 

 conditions or in a particular culture medium, there exists a 

 certain concentration of oxygen which is most favorable to 

 growth. This may be termed the optimum. For most bac- 

 teria there likewise exists a point in diminished oxygen 

 pressure where the concentration is not sufficient to allow 

 of growth. This may be termed the minimum. For certain 

 bacteria there likewise exists a maximum concentration, and 

 the organisms will not grow in a medium containing more 

 than this amount. Bacteria which require a relatively high 

 concentration of oxygen, usually about that of the atmos- 

 phere, are said to be aerobic. For such bacteria the opti- 

 mum and the minimum are particularly significant. These 

 bacteria will grow usually only on the surface of media or 

 in the surface layers. "When inoculated into a fermentation 

 tube, for example, they will grow only in the open arm 

 near the surface of the medium. At the other extreme there 

 are bacteria whose optimum condition apparently is 

 approximately complete absence of free atmospheric oxy- 

 gen. Many of these will not develop at all in an atmos- 

 pheric concentration. Such organisms are termed obligate 

 anaerobes. For these anaerobic bacteria the optimum and 

 the maximum concentrations of oxygen are most important. 

 Lying between these two extremes are certain bacteria 

 termed micro-aerophiles, which grow best in a concentration 

 of oxygen something less than that of the atmosphere, but 



