BACTERIAL DIGESTION IN THE INTESTINE 535 



Bacterial Digestion of Protein 



From a pathological standpoint, the most important action of bacteria 

 is that which takes place on protein. Under anaerobic conditions the 

 intestinal bacteria have in general the power of splitting off the amino 

 group whereas under aerobic conditions they split off the carboxyl 

 group. This splitting off of the carboxyl group as carbon dioxide is per- 

 formed by the so-called carboxylase bacteria, and it may take place either 

 before or after deamidization (see page 649). If it happens after this 

 process, the products are not highly toxic and include phenol, cresoL 

 indole ar^ ftfcat.nlA, whir*Vi a re partly absorbed into the blood and partly 

 excreted with the feces. 



The fractions of those substances that are absorbed into the blood 

 have their toxicity removed by conjugation mainly with sulphuric acid 

 to form the so-called ethereal sulphates. A part is also combined with 

 p'lycuronic acid (see page 665). In the case of phenol and cresol this 

 conjugation occurs immediately after absorption, but in the case of 

 indole and skatole it is preceded by an oxidative process, converting 

 these substances into indoxyl and skatoxyl respectively. The_jleiaxica- 

 tion process occurs in the liver, as has been shown by experiments in 

 which this organ was artificially perfused outside the body. They are 

 then removed from the blood by the kidneys and excreted in the urine. 

 The proportion of ethereal sulphates in this fluid therefore gives an esti- 

 mate of the extent of intestinal putrefaction of protein (see page 665). 

 The indican, being readily detectable by the well-known color reaction 

 of Jaffe, serves as an indicator of excessive intestinal putrefaction. 

 The indole and skatole which are not thus absorbed and detoxicated are 

 excreted with the feces, to which they give the characteristic odor. 



The source of the phenol is tyrosine and that of the indole is trypto- 

 phane. The chemical processes involved are shown in the following 

 equations, in which the by-products of the reactions are in brackets. 



COH COH 



HC CH HC CH 



HC CH HC CH 



\/ \x 



C CH 

 CH 3 (CO 2 



! (NH.) | (C0 2 +H 2 0) | (C0 2 ) 



CHNH 2 CH 2 COOH 



I I 



COOH COOH 



(tyrosine) (p-oxyphenyl- (p-oxyphenyl- (paracresol) (phenol) 



propionic acid) acetic acid) 



