Physiology and Microbiology of the GI Tract

Physiology

The human GIT is the most appropriate environment to conduct studies on the human GI microbiota but for practical reasons animal models are used extensively for these types of studies. The wide range of similarities between the animal and human GIT makes it possible to draw reasonable parallels between these two hosts, however, results from studies on the human microbiota in animals may not entirely reflect processes occurring in the human GIT. The reason for this is that there are also many differences between human and animal gut physiology, diets, and behavior. Rodents are the most extensively used animals in the research of human GI microbiota. The differences between the human and rodent GIT may be important in interpreting any research findings.

When considering the differences between the human and rat GITs, the issue of size is certainly obvious. This difference has impact on transit time of GI contents. Also, the rate of passage can vary between the type of diet, the particle size of digesta and morphological characteristics of the GITs. In rats the transit time is 12-35 hours depending on the type of diet and transit markers used (1,2). In humans, native Africans, consuming a traditional diet, have an average GI transit time of 33 hours, which is approximately half of the transit time that has been observed in Europeans or Africans on a Western diet.

Many more subtle and potentially important morphological and physiological differences exist between the human and rat GITs. An example of this exists in the fact that the adult human appendix, known to be the undeveloped caecum, does not correspond in function to the developed, functioning rodent ceacum. The adult human GIT is roughly divided into three major regions, namely, the stomach, small intestine and the large intestine (colon). In the human fetus the caecum commences deveploment as a conical diversion. As the rest of the intestine grows, caecal growth is arrested and a vermiform appendix remains. In adult humans, the colon, which is haustred throughout its entire length, takes the shape and function of the caecum which is found in many other animals (Fig. 1B) (3).

The mouse and rat GIT is divided into four major regions, namely the stomach, small intestine, caecum, and colon (Fig. 1A). In contrast to the human stomach, the

Rat Grid Prevention Coprophagy

Figure 1 The mouse (A) and human (B) gastrointestinal tracts.

Figure 1 The mouse (A) and human (B) gastrointestinal tracts.

stomach of rats and mice have a large area of nonsecreting epithelium that expands considerably as the animals are eating. In rodents, microbial fermentation is mainly occurring in the caecum. The colon of these animals is not haustred and is less important for microbial fermentation, compared to the caecum. The large intestine of these animals is important for re-absorption of water and formation of fecal pellets.

Microbiota

The physiological properties of the human GIT, with its many unique features provide a vast number of microbial niches. Host factors such as enzymes, mucins, proteases, bile acids dietary factors and regimes contribute to this diversity. The result is a complex microbial community composed of several hundred microbial species (4) that collectively form the GI microbiota. The mammalian GI microbiota forms dense microbial populations, particularly in the posterior part of the intestine (5). The composition of both human and rodent microbiota has been extensively investigated and discussed in several comprehensive studies and reviews (6-8). The microbial profiles of rodents such as rats and mice are in many ways similar to that of other mammals, including humans (5,9). In such rodents, lactobacilli are present in levels of 109 colony forming units (CFU) per gram of feces, (5) whereas in humans, the average levels of fecal lactobacilli are usually 104-106 CFU per gram of feces (10). As described by Finegold et al. (10), diet has impact on population levels of lactobacillus and other microbial groups in humans. Bifidobacteria may be detected in both human (10) and rodent feces (11), however commercial rodent feed may not support GIT colonization by bifidobacteria as much as some other diets (Fig. 2). This suggests that the type of diet should be considered carefully to ensure that the diet used supports the colonization of important microbial groups. The effect of feed composition is further discussed in the section "Conventional Animals."

There are also behavioral differences between various animal species that may contribute to the resulting GI microbiology of these animals. Rodents are known as coprophages, and unless coprophagy is prevented, it is possible that the GIT of these rodents are continuously re-inoculated with their own fecal microorganisms. This behavior, which could possibly affect the microbial profile, may be inhibited by fitting a tail cup which makes the fecal pellets unavailable to the animals (13). Other techniques have been attempted, such as keeping animals on a grid to allow fecal pellets to fall through and become inaccessible, however coprophagic animals, including rats, usually

HAS Normal

Type of feed

Figure 2 Fecal bifidobacteria of mice (Balb/C) fed high amylomaize starch (HAS) diet, containing 40% starch [AIN 76 (12)] and a commercial rodent feed (Normal). Results presented are the average + SDV of six animals per group.

HAS Normal

Type of feed

Figure 2 Fecal bifidobacteria of mice (Balb/C) fed high amylomaize starch (HAS) diet, containing 40% starch [AIN 76 (12)] and a commercial rodent feed (Normal). Results presented are the average + SDV of six animals per group.

collect fecal pellets as they are extruded from anus (14), making such a grid less efficient in preventing coprophagy.

The relative importance of coprophagy, and specifically the rate of microbal re-inoculation, has been investigated in a number of studies. The rat may consume 35-50 percent of the total output of feces, or an even larger proportion if the rat is on a vitamin depleted diet (14). It has been reported that prevention of coprophagy has reduced weight gains in rats and also caused major changes in caecal and fecal lactobacilli, enterococci, and coliforms (15). In another report, prevention of coprophagy made no change in GI microbial profiles, apart from a minor decrease in lactobacilli of the stomach and the lactobacilli of the small intestine (16). A study conducted by Smith (5) indicated that coprophagy has no, or minor effects on gastric microbial populations. These studies, whilst showing dramatically varying conclusions, possibly resulting from varying feed and housing conditions, indicate that coprophagic behavior should remain an important consideration.

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