PRODUCTS OF METABOLISM. 119 



nitrates can be reduced by the denitrifying bacteria only if they can use the 

 oxygen to advantage; that is, if they can obtain by the oxidation of organic 

 bodies more energy than they expend in the reduction of the nitrates. 

 The same rule holds true with other reductions. 



Sulphates are reduced in a very similar way to hydrogen sulphide 



H 2 SO 4 - 4 O = H 2 S. 



Tap-water, containing calcium sulphate, 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 contents of the stomach are sterilized and inoculated 

 with Penicillium brevicaule (p. 22), the "arsenic mold." This will re- 

 duce most arsenious compounds to arsin (As H 3 ) or to diethyl arsin, 

 As H(C 2 H 5 ) 2 , both of which are easily recognized by their very pro- 

 nounced 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 

 far as enzymes and toxins are concerned, it is not even determined 

 whether or not they are of protein nature. The last two groups are 

 known only by their actions, while the pigments are very conspicuous 

 and cannot possibly be overlooked. 



PIGMENTS have naturally attracted the attention of microbiologists 

 ever since pure cultures were known, and many investigators have tried 

 to explain the nature and the meaning of pigments. All experiments 

 concerning the purpose of pigment-formation by microorganisms have been 



