THE PRODUCTION OF SULPHURETTED HYDROGEN 21 



The formation of sulphuretted hydrogen from cysteine is 

 given in the following equation : — 



CH2— S— H CH3 



CH . NH2 + H., — > CH . NH2 + H2S 



1 I 



COOH COOH 



or it may be indirectly formed through such intermediate 

 compounds as 



a-Thiolactic acid (CH3 . CH . SH . COOH), 

 ThioglycoUic acid (CH^ . SH . COOH), 

 and Methyl mercaptan (CH3 . SH). 



The presence of sulphuretted hydrogen in tube cultures is 

 easily demonstrated by means of filter paper, which has been 

 previously soaked in lead acetate solution. The paper thus 

 treated is placed inside a tube containing a growing culture, 

 and turns black from the formation of lead sulphide if 

 sulphuretted hydrogen be given off. Or, the gas may be re- 

 cognized by the formation of the black ferrous sulphide in 

 the presence of iron salts. The sulphur, when dissolved by 

 aceto-carmine, crystallizes outside the cells, where it forms 

 typical fiat octohedra (see Fig. 34^), or the globules may be 

 treated with hot potassium cyanide and ferric chloride, when 

 they turn a blood-red colour from the formation of ferric 

 thiocyanate Fe(CNS)3. The chemical tests, however, are nor- 

 mally not necessary because the appearance of the sulphur 

 globules in these cells is so characteristic that they can be 

 identified from their general appearance. 



There are means of effecting the liberation of hydrogen 

 sulphide other than from an amino-acid split off from a protein 

 molecule. Thus the decomposition of protein matter by some 

 of the saprophytic bacteria is attended by the formation of 

 a scum of sulphur on the surface of the culture fluid. Such 

 sulphur must have been derived from albuminous matter. 

 Its union with hydrogen to form sulphuretted hydrogen is 

 described later. 



