Influence of the Culture Medium 43 



food products is sugar. As is well known, this is em- 

 ployed in the manufacture of condensed milk, as well 

 as in the making of preserves. It seems that the concen- 

 tration of the material to which large amounts of sugar 

 are added precludes the development of the bacteria. 



Still another instance of the depressing effect of 

 excessive concentration may be found in the relation 

 of the nitrifying bacteria to large amounts of soluble 

 organic matter. The development of these bacteria may 

 be entirely suspended by quantities of soluble organic 

 matter not in the least injurious to other bacteria. Thus 

 the growth of nitrifying bacteria in manure heaps does 

 not begin until the litter and animal excreta have been 

 largely decomposed by the great hosts of decay bacteria 

 and until the soluble organic materials have been changed 

 into insoluble modifications. ^ 



The reaction of the culture medium. — The reaction of 

 the culture medium, that is, the amount of free acid or 

 base present in it, is of the utmost importance for bac- 

 terial growth. The same condition, or acidity, of a 

 culture medium may be remedied by the addition of a 

 base, that is, of a substance which has properties oppo- 

 site to those of an acid. Lime, soda, or potash may be 

 given as examples of bases. All of these may be employed 

 for the neutralization of acidity. 



A culture medium which is neither acid nor basic 

 (alkaline) is said to be neutral, and is best adapted to 

 the growth of many species. There are numerous or- 

 ganisms that prefer a faintly acid medium, among 

 them some of the common inhabitants of milk. There 

 are others that prefer a slightly alkaline medium. In 



