PRODUCTS OF METABOLISM 1 75 



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 (H2Se04 and H2Te04) 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 (AsHs) 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 hrevicaule (page 52, the 

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

 (As H3) or to diethyl arsin, As H(C2H6)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 

 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 without results. It is not known that the pigment is of any 

 material advantage to bacteria; for it is possible to cultivate colorless 

 strains of pigment bacteria which grow apparently as well as the original 

 pigmented culture. Again, pigments cannot take the place of the 

 chlorophyl in plants except perhaps the bacteriopurpurin of the purple 

 bacteria. It does not even protect the cells against intense light) 

 because the pigmented organisms are not more resistant than the corre- 

 sponding colorless " sports." The only exception are the colored spores 

 of the molds, especially Penicillium and Aspergillus, which are very 

 resistant to light, while the spores of Oidium are killed just as easily as 



