Indigenous bacteria in the human GI tract perform several beneficial functions for their host. Animal studies have compared germ-free animals (lacking bacterial flora) to those with flora to demonstrate these benefits, the most important of which include:
1. Normal flora synthesize and excrete vitamins in excess of their own needs - which can be absorbed by the host. For example:
• Enteric bacteria secrete Vitamin K and Vitamin B12
• Lactic acid bacteria produce certain B-vitamins.
2. Normal flora prevent colonization by pathogenic bacteria by competing for attachment sites or essential nutrients. Seen in the oral cavity, intestines, skin, and vaginal epithelium.Experiments have shown that a handful of Salmonella bacteria can infect germ-free animals, compared to a million cells to infect an animal with normal flora.
3. Normal flora may antagonize other intestinal bacteriathrough the production of substances which inhibit or kill non-indigenous species. Such substances include nonspecific fatty acids, peroxides,and highly specific bacteriocins (toxins that inhibit growth of closely related bacterial strains).
4. Normal flora stimulates the production of cross-reactive antibodies. Normal flora act as antigens in an animal inducing an antibody-mediated immune (AMI) response.Low levels of antibodies produced against components of the normal flora are known to cross react with certain related pathogens, and thereby prevent infection or invasion.Antibodies produced against antigenic components of the normal flora are called "natural" antibodies, and are lacking in germ-free animals.
5. Normal flora stimulates the development of certain tissues. i.e., the caecum and certain lymphatic tissues (Peyer's patches) in the GI tract. (the caecum of germ-free animals is larger, thinner-walled, and fluid-filled compared to conventional animals) Also, intestinal lymphatic tissues of germ-free animals are less able to be immunologically stimulated.
Reference: Kenneth Todar, PhD.
Online Textbook
of BacteriologyIn the GI Tract. (page 4)
The intestinal microflora provide protection against a broad range of pathogens - including certain pathogenic forms of:
- Clostridia. Main 4 species responsible for disease are:
• C. botulinum - produces toxin in food/wound causing botulism; honey can contains spores of C. botulinum, which may cause infant botulism in humans one year old and younger, but Clostridia do not compete well with the other rapidly growing bacteria present in the adult GI tract;
• C. difficile - can overgrow other gut bacteria during antibiotic therapy causing pseudomembranous colitis;
• C. perfringens - causes food poisoning, gas gangrene, enterotoxemia ("overeating disease") in sheep and goats, and more;
• C. tetani - causative organism of tetanus;
- Escherichia coli - only pathogenic serotypes of E. coli (variations evolved through the natural biological process of mutation and horizontal gene transfer from normal, beneficial gut E. Coli of the Enterobacteriaceae family) can cause food poisoning or UTIs in humans;
- Salmonella, Shigella and Pseudomonas
- Yeasts such as Candida albicans
Reference
Kenneth Todar, PhD. Online Textbook of Bacteriology In the GI Tract
