150 MICROBIOLOGY 



Apparently negative results should not be rashly accepted as 

 conclusive. 



Reduction power of bacteria. All bacteria seem to pos- 

 sess the property of converting sulphur into sulphuretted 

 hydrogen in the presence of nascent hydrogen. The following 

 processes depend also in part upon the action of nascent 

 hydrogen. The reduction of blue litmus pigments, methylene 

 blue and indigo to colorless substances frequently occurs in 

 the deeper layers of the culture. The surface layer in contact 

 with the air often shows no change. By agitation the access 

 of air restores the color but at the same time, if acid has been 

 formed, the litmus pigment is turned red. The ability to 

 reduce litmus seems to be more commonly present among the 

 liquefying bacteria. 



The reduction of nitrates to nitrites and ammonia. 1 The 

 first of these properties seems to pertain to a great many bac- 

 teria. In a number of bacteria studied by Rubner only one 

 failed to produce nitrite. Certain bacteria produce other sub- 

 stances as the result of their growth. As these are formed 

 during the decomposition of albumins they are present in the 

 digestive tract. The best known of these substances and 



1 The test for nitrites may be made as follows: Two bouillon 

 tubes containing nitrates are inoculated, and, along with two unin- 

 oculated tubes, are allowed to remain in the incubator several days; 

 then to the cultures and control test is added a small quantity of 

 colorless iodide of starch solution (thin starch paste containing 

 0.5 per cent potassium iodide) and a few drops of pure sulphuric 

 acid. The control tubes remain colorless or become gradually 

 slightly blue, while if nitrites are present a dark-blue or brown-red 

 coloration is produced. 



The demonstration of ammonia may be made by the addition of 

 Nessler's reagent to culture media free from sugar. In bouillon, if 

 ammonia is present, Nessler's reagent is almost immediately reduced 

 to black mercurous oxide. A strip of paper saturated with the re- 

 agent can also be suspended over the bouillon tube, or this can be 

 distilled at a low temperature and with the addition of magnesium 

 oxide and the distillate treated with Nessler's reagent. A yellow to- 

 red coloration indicates the presence of ammonia. Controls are 

 necessary. 



