VITAL PHEXOMEXA OF BACTERIA 97 



extraneous albumins of non-bacterial origin. That every extraneous 

 al bun li nous substance is harmful to the organism which seeks to resist 

 it> action is shown by those specific precipitating ferments, the pre- 

 cipitins, which are produced in the organisms after the introduction 

 of every extraneous albumin. 



Sulphuretted Hydrogen. Sulphuretted hydrogen is a very common 

 bacterial product. Its presence is determined by pasting a piece of paper 

 moistened with lead acetate inside the neck of the flask containing the 

 culture, closing the mouth with a cotton-wool stopper, and over this 

 again an India-rubber cap (black rubber free from sulphur). The 

 paper is colored at first brownish and later black; repeated observation 

 is necessary, as the color sometimes disappears toward the end of the 

 reaction. Apparently negative results should not be rashly accepted 

 as conclusive. 



Sulphuretted hydrogen may be formed: 



1. From albuminous substances. This power, according to Petri 

 and Maassen, of forming sulphuretted hydrogen, particularly in liquid 

 culture media containing much peptone (5 to 10 per cent.) is pos- 

 sessed, though in different degree, by all bacteria examined by them; 

 only a few bacteria form H 2 S in bouillon in the absence of peptone, 

 while about 50 per cent, in media containing 1 per cent, peptone. 



2. From powdered sulphur. All bacteria produce in culture media 

 to which pure powdered sulphur is added considerably more H 2 S than 

 without this addition. Petri and Maassen suggest that this is due to 

 the nascent hydrogen produced by the bacteria. 



3. From thiosulphates and sulphites. Studied particularly in yeast, 

 but demonstrated also by Petri and Maassen in several bacteria. 



The presence of sugar in the culture does not affect the production 

 of H 2 S by bacteria, but saltpetre reduces it, nitrites being formed. The 

 absence of oxygen favors the production of H 2 S. Light diminishes 

 the development of H 2 S by facultative anaerobes, sulphates being 

 formed instead. 



Reduction Processes. All bacteria, as we have seen, possess the 

 property of converting sulphur into sulphuretted hydrogen, for which 

 purpose is required the presence of nascent hydrogen. The following 

 processes depend also in part upon the action of nascent hydrogen: 



1. The reduction of blue litmus pigments, methylene blue, and 

 indigo to colorless substances. The superficial layer of cultures in con- 

 tact with the air shows often no reduction, only the deeper layers being 

 affected. By agitation with access of air the colors may be again restored, 

 but, at the same time, if acid has been formed, the litmus pigment is 

 turned red. According to Cohn, the property of reducing litmus 

 belongs to all liquefying bacteria, but some non-liquefying species also 

 possess it. 



2. The reduction of nitrates to nitrites and ammonia. The first of 

 these properties seems to pertain to a great many bacteria at least 

 Petri and Maassen found in six species, grown in bouillon containing 

 2.5 to 5 per cent, peptone and 0.5 per cent, nitrate, that almost all pro- 



