THE DEGRADATION OF THE ALBUMINOIDS 225 



and hydroparacumaric acid, of which tyrosine may be regarded as the amine 

 (alanine), is formed 



/OH /OH 



C 6 H 4 < yields C 6 H 4 / 



X CH 2 CH.NH 2 COOH \CH 2 CH 2 COOH. 



Tyrosine. Hydroparacuuiaric acid. 



When air is excluded, the results are, however, very different, indole, together 

 with carbon dioxide and hydrogen, being produced. This reaction is approxi- 

 mately represented by the equation 



/OH / HN \ 



C H 4 ( = C 6 H 4 <; \CH+C0 2 + HoO + H 2 

 \CH 2 CH.NH 2 COOH \CH^ 



Tyrosine. Indole. 



The evolution of sulphuretted hydrogen is a frequent accompaniment of 

 putrefaction. A large number of bacteria are endowed with the power of 

 liberating this gas, the production of which depends, however, not solely on the 

 species of ferment, but also on the composition of the nutrient medium, a circum- 

 stance which explains the contradictory results obtained by different workers. 

 Thus, for example, STAGNITTA-BALISTRERI (I.) denied that Bacillus subtilis, 

 Bacillus tetragenus, the so-called Wurzel bacillus, and others could form sul- 

 phuretted hydrogen ; but PETKI and MAASSEN (III.) then showed this contention 

 to be incorrect, and that, in presence of peptone, the gas in question is produced 

 by these microbes. In other cases, again, this product may be masked, e.g. by 

 combination with ammonia formed at the same time. A good deal of the sul- 

 phur present in the nutrient medium is utilised by the bacteria themselves for 

 structural purposes, the amount so consumed having been found by M. RUBNER 

 (I.) to be equivalent to 23-40 per cent, of the total sulphur in the medium. The 

 sulphur in organic combination is first occluded, a circumstance harmonising 

 with the well-known fact that the sulphur in albuminoids is very easily removed. 

 The more delicate processes leading finally to the evolution of sulphuretted 

 hydrogen, still remain unelucidated. PETRI and MAASSEN (IV.) are of opinion 

 that the bacteria liberate hydrogen, which in the nascent state then extracts 

 sulphur from the sulphur compounds and combines with it. They found that 

 very little of the gas in question is produced when nitrates are present in the 

 medium, but that these latter are thereby reduced to nitrites. With reference 

 to the fact (put forward to refute this explanation) that sulphuretted hydrogen 

 is liberated by aerobic bacteria in well " roused " (aerated) cultures, Petri and 

 Maassen showed that hydrogen is also liberated under this treatment, and that 

 consequently the presence of air favours the reducing action. 



The faculty of producing sulphuretted hydrogen is very common among the 

 pathogenic bacteria, being absent in not a single one out of thirty-seven species 

 examined ; and in many of them e.g. the bacilli of swine erysipelas the inocu- 

 lated nutrient solution fairly bubbles from the quantity of gas liberated. A 

 convenient means of detecting and separating sulphuretted-hydrogen-generating 

 microbes from a mixture of bacteria by the aid of plate cultures is afforded by 

 the ferro-gelatin, recommended by A FROMME (I.) for this purpose ; i.e. a pepto- 

 nised meat-juice gelatin qualified by 3 per cent, of iron saccharate or tartrate. 

 In such nutrient media each colony of the sulphuretted hydrogen bacteria will 

 become surrounded by a black halo of FeS. 



The conversion of sulphates into sulphides by bacterial agency is also a deci- 

 sive indication of reducing power. The conditions of vitality of a particularly 

 active species of fission fungus were investigated by BEYERINCK (II.), who named 

 the organism Spirillum desulfuricans. This strictly anaerobic microbe is utilised 



