INJURY TO BACTERIA BY CHEMICAL SUBSTANCES. 37 



ficial reports of Schluter (C. B. xi, 589), which were sub- 

 stantiated by subsequent publications, many bacteria bear 

 much higher proportions of acid; according to observa- 

 tions in our own institute, as high as 100 c.c. of normal 

 acid per liter. 



Nutrient media containing sugar usually favor the pro- 

 duction of acid, which, according to Hellstrom, soon be- 

 comes so abundant that the micro-organisms are killed. 



Acid nutrient media are to be used for yeasts and 

 molds and whenever it is wished to isolate a new bac- 

 terium from an acid nutrient substance. For counting 

 the germs in air, soil, water, milk, etc. , a neutral medium 

 is always employed. 



3. INJURY TO BACTERIA BY CHEMICAL 

 SUBSTANCES. 



We have already learned that too large a proportion of 

 either acid or alkali 1 interferes with growth, or, if still 

 stronger, produces death. Most varied chemicals, in certain 

 concentrations, operate similarly. Those which are strongly 

 active are called antiseptics or disinfectants. 



Usually, with Hiippe, the following grades of influence 

 are distinguished: 



1. Growth is not interfered with, 2 but the pathogenic 

 or zymogenic functions are weakened : Weakening, at- 

 tenuation. 



2. The organisms are unable to increase, but are not 

 killed : Asepsis. 



3. The vegetative forms of the micro-organisms are de- 

 stroyed, but not the resting forms: Antisepsis. 



4. Vegetative forms and spores are both killed: Steril- 

 ization or Disinfection. 



1 Fermi (C. B. xxm, 208) has published an extensive table regard- 

 ing the sensitiveness of various micro-organisms to acids, alkalis, and 

 various poisons. Unfortunately, he gives the number of drops of 

 solutions of various percentages which inhibit the growth of bacteria 

 in 5 c.c. of agar. 



2 At times there occurs a transitory or permanent interference with 

 growth; in other cases briefly acting antiseptics, also heat, cold, etc., 

 cause a retardation of subsequent growth without producing a weaken- 

 ing. 



