PRODUCTS OF M1CROBIAL ACTIVITIES 245 



These "true denitrifiers " are found in soil and in old manure. Their 

 reducing process is as follows: 



Ca(NO 3 ) 2 - 5O = CaO + 2N. 



Nitrates are reduced through the efforts of the organism to secure a 

 supply of oxygen. The denitrifying bacteria have strong oxidizing 

 properties; they take oxygen from all sources possible. If cultures of 

 denitrifying bacteria are well aerated, as in soils with a proper mois- 

 ture content, they scarcely attack the nitrates, while they will reduce 

 them in ordinary liquid cultures so fast that the escaping nitrogen 

 gas forms a froth on top of the nitrate solution. Denitrifying bacteria 

 need the oxygen to oxidize organic matter. They cannot live without 

 organic food. 



Sulphates are reduced in a very similar way to hydrogen sulphide 



H 2 SO 4 - 40 = H 2 S. 



Tap-water, containing calcium sulphates, often forms hydrogen sulphide 

 if shut off from the air for some time. 



While only a few bacteria reduce sulphates, many reduce sulphites or 

 sulphur to hydrogen sulphide. The potassium and sodium salts of 

 selenic and telluric acid (H 2 SeO 4 and H 2 TeO 4 ) are reduced by certain 

 organisms and not by others. The reduction results in a colored 

 precipitate; this reaction has been suggested as a diagnostic means to 

 distinguish different species. The reduction of arsenious oxide to 

 arsin (AsH 3 ) is used as a very delicate test for arsenic; it is applied in 

 the detection of arsenical poisoning. The material to be tested is 

 sterilized and inoculated with Penicillium brevicauk (page 53, the 

 "arsenic mold"). This will reduce most arsenious compounds to arsin 

 (AsH 3 ) or to diethyl arsin, AsH(C 2 H 5 )2, both of which are easily 

 recognized by their very pronounced garlic odor. 



UNKNOWN PRODUCTS OF PHYSIOLOGICAL SIGNIFICANCE 



Among the products of microbial action, there are certain substances 

 which must be mentioned because of their importance, though their 

 quantity is insignificant compared with the ordinary products of fermen- 

 tation. These substances can be divided into four groups: pigments, 

 aromatic compounds, enzymes, and toxins. The chemical structure of 

 pigments and of many aromatic substances is scarcely known; and as 



