234 BACTERIOLOGICAL CHEMISTRY 



Whether or not an organism decarboxylates the acid 

 R.COOH to R.H depends on whether or not it possesses 

 the enzyme carboxylase. Esch. coli does possess it and 

 can therefore form indole from tryptophane, but Sal. 

 paratpyhi and Sal. schottmulleri have no carboxylase and 

 therefore produce no indole but stop at indole -carboxylic 

 acid. 



The following general scheme of amino-acid degrada- 

 tion (pp. 235, 236) was proposed by Raistrick, as a result 

 of his observation that histidine under the action of 

 organisms of the coli-typhoid group gave rise to the 

 unsaturated acid, urocanic acid (see p. 230). All the 

 other products of amino-acid degradation are readily 

 accounted for by reactions of the unsaturated acid, which 

 is formed as the primary intermediate. 



The Factors Influencing the Type of Breakdown. 



1. The Organism. — (a) Yeasts. — The yeasts usually 

 cause hydrolytic deamination followed by decarboxyla- 

 tion and reduction to give rise to the alcohols, as in the 

 production of fusel oil. 



(b) Moulds. — ^The moulds usually give hydrolytic de- 

 amination, but do not cause decarboxylation, the product 

 being the hydroxy-acid. Sometimes the hydroxy-acid 

 may be completely oxidised. 



(c) Bacteria. — The bacteria can bring about any of 

 the types of reaction described, with formation of a 

 correspondingly much greater variety of products. De- 

 carboxylation does not occur so frequently as with yeasts. 



2. The Condition of the Medium. — Anaerobic condi- 

 tions are naturally usually accompanied by reduction 

 with accumulation of the saturated acids and the " hydro- 

 carbons." Aerobic conditions favour the production of 

 other types of product. Generally speaking, the presence 

 of a readily available source of carbon inhibits the forma- 

 tion of indole, p()ssil)ly by (vhangino; the course of the 

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