The large intestine is divided into four segments: cecum, colon, rectum, and anal canal. Each species has a large intestine adapted to their particular diet. The overall function of the large intestine is the completion of absorption, the manufacture of certain vitamins, the formation of faeces, and the expulsion of faeces from the body.

Metabolism by gut wall and microflora

Bacterial flora in the gut and the environmental pH can contribute to the process and the bacterial biotransformation is greatest in the colon (large intestine). Factors that influence the microflora are: individual differences, subspecies differences, presence of foreign compounds such as antimicrobials, changes in intestinal transit time and defecation frequency, coprophagy in rats (not in rabbits, probably due to low pH throughout the stomach), diet (in humans conflicting effects, in animal species the host’s diet has a significant influence on the numbers and types of bacteria found), disease, and age. Metabolism of foreign compounds and nutrients by the microflora in the gut lumen is small in comparison with metabolism by the gut mucosa and liver. However, the intestinal microflora may play an important role in the first-pass metabolism of compounds which are poorly or incompletely absorbed by the gut mucosa (azo dyes are completely reduced by gut bacteria). Bacterial metabolism is largely degradative, hydrolytic and reductive with a great potential for both metabolic activation and detoxification of xenobiotics. The intestinal microflora also play an important role in the enterohepatic recirculation of xenobiotics (e.g. digoxin, ouabain, oral contraceptives, penicillin) and endogenous compounds (e.g. steroid hormones, bile acids, folic acid and cholesterol) which are well absorbed and subsequently re enter the gut via the bile.

Large intestine of the rat

Table 1: Activity of microflora enzymes of rats (Ilett et al., 1990).

  β-galactosidase* β-glucosidase* β-glucuronidase* α –galactosidase* α–glucosidase* azoreductase**
Enterobacteria 42 5.8 25 13 5.9 0.4
Enterococci 9 193 2.9 21 14 0.9
Lactobacilli 92 26 1.6 98 27 13
Clostridia 14 22 11 53 30 27
Bacteroides 51 35 6.0 24 9.8 0.2
Bifidobacteria 39 29 1.9 28 21 0

* activity expressed as μmol of the appropriate nitrophenyl substrate metabolised per 108 cells per hour 
** activity expressed as pmol of sulfasalazine metabolised per 108 cells per hour 

The human large intestine

There have been relatively few studies done on pre- and postnatal maturation of intestinal motility (Heimann, 1980). In infants, intestinal motor activity occurs less frequently than in adults, with a different pattern of rhythmic peristaltic activity (Radde, 1985). In general, intestinal peristalsis is irregular and partially dependent on food intake and feeding habits (Heimann, 1980). 
Another important difference between neonates and older individuals is the type and degree of bacterial colonisation of the gut. At birth the intestine is virtually sterile and a rapid colonisation occurs with a flora that is different in breast-fed and formula-fed infants (Raddle, 1985). A further change in bacterial flora occurs at the time of weaning (4-6 months) which is important for the hydrolysis of compounds that are conjugated and secreted in the bile so that unconjugated compound can be absorbed by the intestinal epithelium (e.g. conjugated bile acids).