54 GENERAL PHYSIOLOGY OF BACTERIA 



somewhat, but the process is on the whole an orderly and definite one. 

 The association of various kinds of bacteria in this process, where 

 each succeeding kind profits by the activities of the preceding kind, 

 is a symbiotic one; that is, the several types of organisms mutually 

 profit by their combined activities. 



It frequently happens that the products of symbiotic activity may 

 be greater than the sum of the products of the separate activities of 

 the organisms. 1 On the contrary, many instances are known in which 

 one kind of organism by its activity actually crowds out a preexisting 

 organism, as for example, the lactic acid bacteria which sour milk. 

 They produce sufficient lactic acid from the fermentation of the lactose 

 to kill the proteolytic forms. This substitution of one type of organism 

 by another is known as antibiosis: the latter organism profits wholly 

 at the expense of the first organism. 



It not infrequently happens that one type of bacterium profits by 

 the activity of another type of organism without benefiting the former 

 in return. If two types of bacteria are concerned, the process is known 

 as metabiosis; if the bacterium is living on a host, the relationship is 

 spoken of as parasitism. 



N. MEDIA COMPOSITION AND REACTION. 



Most bacteria grow best in a medium containing a large percentage 

 of moisture in which diffusible proteins or protein derivatives are 

 present as sources of nitrogen: these substances are better adapted to 

 the dietary needs of the majority of bacteria than are ammonium 

 salts or even simple amino acids. A very few bacteria (nitrifying 

 bacteria) cannot grow in media containing organic nitrogen compounds : 

 a few strictly pathogenic bacteria appear to require nitrogen as it 

 exists in the highly complex tissues of man or animals for their growth. 

 Many bacteria can utilize carbohydrates for their carbon, hydrogen, 

 and oxygen requirements. Some bacteria appear to be able to utilize 

 fats for their carbon requirement. 



A neutral or feebly alkaline reaction is best adapted to the develop- 

 ment of the vast majority of bacteria; a few types develop best in a 

 medium which is distinctly acid the aciduric bacteria. 2 Mineral 

 acids are germicidal; organic acids may be utilized by bacteria for 

 foods. 



1 Kendall, Jour. Am. Med. Assn.. 1911, Ivi, 1084. 



2 Kendall, Jour. Med. Research, 1910, xxii, 153. 



