MICBOBIAL ASSOCIATIONS 243 



association of lactic bacteria with certain protein destroying organisms. 

 The lactic bacteria often develop much better if the protein bacteria 

 grow together with them or have grown previously in milk. Meta- 

 biosis does not require the growth 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 mycodermas 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 organ- 

 isms. 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 satisfac- 

 torily explained. Though the aerobic bacteria continuously remove the 

 oxygen from the water a certain amount will remain, sufficient to pre- 

 vent the growth of the anaerobic bacteria under ordinary conditions. 

 There seems to be a certain protective influence derived from the aerobic 

 bacteria, the nature of which is unknown. 



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 begin 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 saturate 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 

 fijst cease to grow, then cease to produce acid and finally die, while the 

 yeast is still continuing in the fermentation. 



