INTRODUCTION. 183 



of the two associated organisms at the same time. The effect will be the 

 same if first the one and later the other develops, and even after the first 

 organism is killed or removed, its effect upon the pure culture of the second 

 will still be noticed. This does not occur in the case of symbiosis. 



One species can favor the development of another by other means than 

 food provision or preparation. Certain bacteria cannot live in acid media, 

 and molds or Mycodermcz destroying the acid will render possible the 

 growth of these bacteria though they do not provide them with food. 

 This is the case in the ripening of certain soft cheeses. Another example 

 is the production of heat by fermenting organisms in manure, hay, ensilage, 

 enabling the development of thermophile organisms. A very interesting 

 and important problem is the growth of strictly anaerobic bacteria near 

 the surface of liquids in association with some aerobic bacteria. How 

 this is really possible cannot be satisfactorily explained. Though the 

 aerobic bacteria continuously remove the oxygen from the water a certain 

 amount will remain, sufficient to prevent the growth of the anaerobic bac- 

 teria under ordinary conditions. There seems to be a certain protective 

 influence derived from the aerobic bacteria, the nature of which is un- 

 known. 



ANTIBIOSIS. 



The standard examples of antibiosis are the alcohol production by 

 yeast in sugar solutions and the acid production by lactic bacteria in milk. 

 Fresh cider contains a large number of bacteria, yeasts and molds; some 

 of these organisms cannot develop in the acid medium, but many will be- 

 gin to grow. Some of the bacteria will produce or destroy acid, others 

 may begin to work on the nitrogenous material of the cider, and the yeasts 

 produce alcohol and carbon dioxide. The carbon dioxide will soon satu- 

 rate the cider and begin to bubble up, thus removing the other gases. 

 The molds will stop growing if the oxygen is taken away, but some of the 

 bacteria may continue growing until the alcohol concentration checks their 

 further development. They first cease to grow, then cease to produce acid 

 and finally die, while the yeast is still continuing in the fermentation. 



In the lactic fermentation of milk, Bact. lactis acidi combats all other 

 organisms by a rapid production of lactic acid. Though it is present in 

 fresh milk only in very small numbers, its rapid growth and the forma- 

 tion of acid which will check and even kill most other bacteria soon makes 

 it the dominant organism in the flora of milk, and at the time of curdling, 



