Microflora of the Gastrointestinal Tract: A Review

Abstract

The mucosal surface of the human gastrointestinal (GI) tract is about 200-300 m2 and is colonized by 1013-14 bacteria of 400 different species and subspecies. Savage has defined and categorized the gastrointestinal microflora into two types, autochthonous flora (indigenous flora) and allochthonous flora (transient flora). Autochthonous microorganisms colonize particular habitats, i.e., physical spaces in the GI tract, whereas allochthonous microorganisms cannot colonize particular habitats except under abnormal conditions. Most pathogens are allochthonous microorganisms; nevertheless, some pathogens can be autochthonous to the ecosystem and normally live in harmony with the host, except when the system is disturbed. The prevalence of bacteria in different parts of the GI tract appears to be dependent on several factors, such as pH, peristalsis, redox potential, bacterial adhesion, bacterial cooperation, mucin secretion, nutrient availability, diet, and bacterial antagonism. Because of the low pH of the stomach and the relatively swift peristalsis through the stomach and the small bowel, the stomach and the upper two-thirds of the small intestine (duodenum and jejunum) contain only low numbers of microorganisms, which range from 103 to 104 bacteria/mL of the gastric or intestinal contents, mainly acid-tolerant lactobacilli and streptococci. In the distal small intestine (ileum), the microflora begin to resemble those of the colon, with around 107-108 bacteria/mL of the intestinal contents. With decreased peristalsis, acidity, and lower oxidation-reduction potentials, the ileum maintains a more diverse microflora and a higher bacterial population. Probably because of slow intestinal motility and very low oxidation-reduction potentials, the colon is the primary site of microbial colonization in humans. The colon harbors tremendous numbers and species of bacteria. However, 99.9% of colonic microflora are obligate anaerobes.

Histoplanimetrical Study on the Relationship between Cellular Kinetics of Epithelial Cells and Proliferation of Indigenous Bacteria in the Rat Colon

“…In general, the tendency of indigenous bacteria to increase toward the lower alimentary tract has been confirmed in various animal species; healthy humans [6,19,27,36], dogs [12], cattle [31], mice [43] and rats [25,44]. To the contrary, the migration speed of villous columnar epithelial cells decreases toward the ileum in rat small intestine [21,35].…”

“…In healthy men, the relative amount of indigenous bacteria increases toward the distal alimentary tract and finally reaches 10 10 -10 12 CFU in the colon [6,19,27,36]. This tendency is the same in other mammalian species (cattle [31], dogs [12], mice [43], rats [25]).…”

Bacteria, phages and pigs: the effects of in-feed antibiotics on the microbiome at different gut locations

“…Resident bacteria benefit from nutrient-rich digesta while assisting the host via nutrient digestion, vitamin synthesis, pathogen displacement and immune system maturation (Green et al, 2006;Bird et al, 2010). Factors such as nutrient availability, pH, redox potential and peristalsis strongly influence the composition of bacterial communities in the intestinal tract (Hao and Lee, 2004). The nutrient and bacterial distribution is additionally influenced by the spatial organization of the intestine, which can be thought to exist along two axes, a longitudinal axis (proximal to distal) and a radial axis (lumen to the mucosa) (Takahashi and Sakaguchi, 2006).…”

Measurement of oro-caecal transit time by magnetic resonance imaging

“…This difference was within the temporal resolution of measurement acquisition (15 minutes) and would normally be considered clinically irrelevant; however, it is of interest because it reflects a systematic difference between the two investigations. The >10ppm increase in breath hydrogen that occurred in 31% of subjects before MRI documented the arrival of the lactulose fluid bolus in the cecum indicates that bacterial colonization was present in the small bowel in a proportion of subjects as previously suggested (35,36). Detailed examination of the LHBT and MRI results revealed that in all cases the "early" increase in breath hydrogen occurred during 15 to maximum 30 minutes stasis of the lactulose fluid bolus in the terminal ileum.…”