172 GENERAL PHYSIOLOGY 



the same nutrient medium, a great variety of products is 

 produced by a particular organism. It very frequently hap- 

 pens, however, that one is more prominent than the others, 

 and certain bacteria have come to be known as bacteria which 

 produce certain characteristic by-products, as, for instance, 

 the acetic acid, the lactic acid bacteria ; but in all of these 

 cases other products are formed as well. These are only the 

 most striking by-products. The products produced by the 

 bacteria usually inhibit their growth if they are allowed to 

 accumulate. Sometimes the by-products are of such a na- 

 ture that they actually destroy the cells which produce them 

 after a considerable amount of the substance has been pro- 

 duced. The amount of substance which different bacteria are 

 able to tolerate varies with different species. For example, 

 the lactic acid bacterium has its growth inhibited when 0.8 per 

 cent of acid is formed. The acetic acid bacterium can produce 

 10 per cent of acetic acid before its growth is prevented, and 

 yeast can produce alcohol to the extent of 15 per cent before 

 its further development is inhibited. By removing the sol- 

 uble products it is possible for the bacteria to continue their 

 activity. This can be done by neutralizing them with chem- 

 ical substances, or in some cases by growing the organism in 

 a permeable sack, such as one made of collodion. The by- 

 products here are removed by dialysis, permitting the further 

 growth of the organism, which may continue until all the food 

 substances are used up. In considering the products of 

 metabolism, attention should be called to the difference 



