ioo BACTERIA AND THE NITROGEN CYCLE 



4. Fatty and aromatic acids, all non-nitrogenous and therefore having no part in the 

 circulation of nitrogen ; acetic, butyric, succinic, and valerianic acids. 



5. Inorganic end-products of putrefaction : free nitrogen, ammonia, free hydrogen, 

 methane, carbonic acid, methylmercaptan, sulphuretted hydrogen. It is probable also, 

 but not certain, that phosphuretted hydrogen is formed and is oxidized at once by the 

 free oxygen of the atmosphere. 



Most of these substances are formed also by the chemical decomposition of 

 proteids, but there is a sixth group which may be termed specific putrefac- 

 tive products. These are the so-called ptomaines or putrefactive alkaloids 

 (67). They are nitrogenous compounds (amine-bases), but although a large 

 number have been described they are, in consequence of the difficulty of 

 preparing them, still only imperfectly known. From putrid flesh (mammalian, 

 including human), fish, and gelatine Brieger isolated neuridine (C 5 H u N 2 ), 

 trimethylamine (C 3 H 9 N), cadaverine (pentamethylendiamine, C 5 H 14 N 2 ), and 

 pntrescine, a diamine of the methylene series (C 4 H ]2 N 2 ). These are all of 

 them either not poisonous or only poisonous in large doses. Other ptomaines, 

 derived from putrid foods of various kinds (sausages, cheese), are highly 

 toxic (Ptomatr opine, Tyrotoxine). The name toxine was formerly used for 

 the amine bases only, but it is now applied indiscriminately to all bacterial 

 poisons irrespectively of their chemical constitution. ' (See also Ch. XVII.) 



As far as the circulation of nitrogen is concerned, we have to consider 

 only the end-products of putrefaction, free nitrogen and ammonia. All 

 the more complex intermediate bodies are broken down finally to these. 

 Leucin, for instance, splits up into valeric acid, ammonia, carbonic acid, and 

 free hydrogen. Tyrosin yields by aerobic putrefaction hydroparacumaric acid, 

 paraoxyphenylacetic acid, paracresol, phenol, ammonia, and carbonic acid. 

 By anaerobic putrefaction it gives rise to indol, carbonic acid, and hydrogen. 



The phenomena of putrefaction are so complicated that we do not 

 know all of the compounds that arise during the process. The course it 

 runs is, however, greatly modified by the presence of oxygen (68). 



This list of putrefactive products is far from being complete, for even 

 the qualitative investigation of the processes is still unfinished ; quantitative 

 analyses are at present impossible. We do not know for instance what 

 determines the predominance of one or the other intermediate product. 

 The effects of the presence of oxygen are somewhat better understood (68). 

 If air have free access, putrefaction may go on without any odour at 

 all, the evil-smelling gases (NH 3 and SH 2 for example) being oxidized at 

 once to form nitrates and sulphates. Aerobic bacteria too, such as the 

 nitre and sulphur bacteria, bring about this mineralization of organic 

 nitrogen. Moreover, when air is circulating freely, there is no accumulation 

 of intermediate products such as skatol or indol. To this kind of aerobic 

 decomposition, proceeding without offensive smell, the term ' decay ' may 

 be applied as distinguished from ' putrefaction.' It occurs on the surface of 

 manure-heaps, on the outer surface of carcases, and in well-ventilated soil. 



