Microflora of Gastrointestinal Tract
Initially sterile in newborns, gastrointestinal tract is rapidly colonized by microorganisms, uptaken with food. In breast-fed infants the intestinal microflora largely comprises lactobacilli, lactic acid streptococci and bifidobacteria.
In healthy adults the esophagus has only accidental transient microflora passing from oral cavity.
In the stomach the normal acidity of gastric juice (in the range of 1.5-3.5) greatly diminishes the total amount of microorganisms. Actually, the gastric juice demonstrates remarkable microbicidal properties, being an efficient barrier on the way of incoming microbial agents.
Nevertheless, the protective function of the gastric juice is flexible, depending on food habits and preferences, the volume of water consumed, and many other factors, including the state of gastric mucosa. In hypoacidic patients with chronic atrophic gastritis the defensive barrier of the gastric juice is seriously weakened.
In healthy individuals the medium concentration of microorganisms in gastric juice doesn’t exceed 103-105 cells per 1 g of gastric contents. Various groups of bacteria and fungi, such us Sarcina ventriculi, lactobacilli, sporeforming Bacillus subtilis, yeasts may be present there.
In the 1980s a causative agent of chronic gastritis and duodenum ulcer was discovered in gastric mucous layer and then isolated. This bacterium was named Helicobacter pylori according to its spiral form. It is motile microaerophil persisting in gastric mucosal membrane. The stomach of children is usually free of helicobacter but among adults almost 50% of humans are the carriers of Helicobacter pylori. About 30 species of Helicobacter are discovered to date, some of them may persist in humans.
In the duodenum and other parts of small intestine the pH of lumen contents becomes alkaline, thereby raising the opportunities for microbial propagation. However, the small intestine carries moderate amounts of microbes in the range of 104-108 cells per 1 g of contents with a gradual increase towards the large intestine. In upper parts of the intestine lactobacilli and enterococci are found, in cecum the fecal microflora prevails.
The large intestine is literally overwhelmed with bacteria. About one-third of the dry weight of feces is made up of microbial bodies.
In distal parts of the bowel (sigmoid colon and rectum) about 1011 microbial cells per 1 g of feces are determined.
Strict anaerobes dominate within the large intestine comprising 96-99% of total microbial mass.
Among them are non-sporeforming gram-negative anaerobic bacteria (genera Bacteroides, Prevotella, Bilophila, Porphyromonas, Fusobacterium), anaerobic sporeforming clostridia (Clostridium perfringens), anaerobic gram-positive peptostreptococci, anaerobic lactobacilli and bifidobacteria.
The minority of facultatively anaerobic bacteria comprises the strains of E. coli and other coliform bacteria, Enterococcus fecalis, candida fungi and some others.
Normal microflora of the large intestine supports many important physiological functions of the bowel.
For instance, bifidobacteria and lactobacilli are the natural antagonists of pathogenic enteric microflora like salmonellas and shigellae.
Non-sporeforming gram-negative anaerobic bacteria play a significant role in food digestion, transforming carbohydrates and other nutrients into short-chain fatty acids that are used by the host as the substantial source of energy. These bacteria also stimulate local intestinal immune response and support intestinal colonization resistance that hinders pathogenic bacteria to attach and colonize the intestinal wall.
Similarly, Clostridium perfringens produces a number of digestive enzymes (e.g., proteases and lipases); E. coli and some other species synthesize the essential vitamins (primarily, of the groups B and К).
However, in case of intestinal damage by trauma or inflammation all these bacteria cause a serious pathology of the human body. For instance, the members of genera Bacteroides (mainly, Bacteroides fragilis), Fusobacteria, Prevotella, or Bilophila as well as E. coli actively participate in many infalmmatory disorders. They are found in acute appendicitis, postoperative infectious complications within the peritoneal cavity (abscesses and peritonitis), inflammatory diseases of the gastrointestinal tract, and in the emergence of sepsis.
Likewise, a long indiscriminate use of antibiotics especially of broad spectrum of action suppresses normal gut microflora, resulting in dysbiosis of the intestine. In these cases candida fungi are most commonly registered. Serious complications after long-term antibiotic treatment followed by dysbiosis are provoked by Clostridium difficile that cause antibiotic-associated diarrhea and severe antibiotic-associated pseudomembranous colitis with the deep damage of the intestinal wall.