There is good evidence that the complex microbial flora present in the gastrointestinal tract of all warm-blooded animals is effective in providing resistance to disease. However, the composition of this protective flora can be altered by dietary and environmental influences, making the host animal susceptible to disease and/or reducing its efficiency of food utilization. What we are doing with the probiotic treatments is re-establishing the natural condition which exists in the wild animal but which has been disrupted by modern trends in conditions used for rearing young animals, including human babies, and in modern approaches to nutrition and disease therapy. These are all areas where the gut flora can be altered for the worse and where, by the administration of probiotics, the natural balance of the gut microflora can be restored and the animal returned to its normal nutrition, growth and health status.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2672.1989.tb05105.x

Probiotics in man and animals

Trillions of microbes inhabit the human intestine, forming a complex ecological community that influences normal physiology and susceptibility to disease through its collective metabolic activities and host interactions. Understanding the factors that underlie changes in the composition and function of the gut microbiota will aid in the design of therapies that target it. This goal is formidable. The gut microbiota is immensely diverse, varies between individuals and can fluctuate over time — especially during disease and early development. Viewing the microbiota from an ecological perspective could provide insight into how to promote health by targeting this microbial community in clinical treatments.

/pdf/diversity-stability-and-resilience-of-the-human-gut-2sktyw2s6e.pdf

Diversity, stability and resilience of the human gut microbiota

http://www.umassmed.edu/globalassets/immunology-and-microbiology/documents/papers-for-bbs821-2015/nov-24-reviewfor-general-info.pdf

Role of the Microbiota in Immunity and Inflammation

http://envismadrasuniv.org/Physiology/pdf/Gut%20flora%20in%20health%20and%20disease.pdf

Gut flora in health and disease

The effect of oral administration of antibiotics on the intestinal flora of conventional mice and their resistance to colonization by orally introduced Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa was studied. Colonization resistance (CR) was expressed as the log of the oral bacterial dose followed by a persistent take in 50% of the contaminated animals. The intestinal flora was virtually eliminated by the antibiotics and this elimination was accompanied by a precipitous fall of CR. CR gradually returned to normal values during the period of repopulation of the intestinal tract by the organisms surviving the treatment. Antibiotic treatment resulted in the disappearance of Enterobacteriaceae, enterococci, staphylococci and yeasts and, under appropriate housing conditions, the animals remained free of these organisms indefinitely. Germ-free mice contaminated with the intestinal flora of an antibiotic-treated animal and their offspring housed in a germ-free isolator showed high values of CR. Their intestinal flora consisted of anaerobic bacteria only. Apparently, these anaerobes are responsible for CR in these and in conventional mice.

/pdf/colonization-resistance-of-the-digestive-tract-in-2xyk2clgo1.pdf

Colonization resistance of the digestive tract in conventional and antibiotic-treated mice.

Conventional CBA mice subjected to lethal whole-body irradiation and allogeneic bone-marrow transplantation developed delayed secondary disease, which caused 95% mortality within 100 days. Symptoms of secondary disease as well as mortality were virtually absent in similarly treated mice kept in the germfree state or given a colonization-resistant (CR) flora. Conventionalization of these mice as early as 40 days after transplantation did not induce a significant degree of secondary disease except in 1 group of CR mice derived from conventional mice by antibiotic treatment. The acute form of secondary disease occurring after transplantation of allogeneic spleen cells was much less influenced by the gnotobiotic conditions, which confirmed the concept that the mortality was caused primarily by severe graft-versus-host reactions. The implications of these findings for the treatment of patients receiving bone-marrow grafts are discussed.-J Natl Cancer Inst 52: 401-404,1974.

Mitigation of Secondary Disease of Allogeneic Mouse Radiation Chimeras by Modification of the Intestinal Microflora

The bacterial flora in the human colon, although extremely diverse, has a relatively stable composition and non-infectious anaerobic bacteria are dominant. The flora forms a pool of numerous different antigens separated from mucosal immunocompetent cells by just a single layer of epithelial cells. Despite this thin barrier, however, the colonic mucosa is physiologically only mildly inflamed. This study looked at the mucosal humoral immune response against faecal anaerobes. By flow cytometric analysis the in vivo immunoglobulin coating of anaerobic bacteria in faecal samples of 22 healthy human volunteers was determined. In a previous study flow cytometric analysis of faecal bacteria has been found to be a very sensitive method to detect immunoglobulins on faecal bacteria. This technique showed that in vivo many bacteria are coated with IgA (24-74%) and less with IgG and IgM. The presence of many bacteria coated with IgA implies that IgA coating does not result in permanent removal of the species from the colon. The absence of immunoglobulin coating suggests that there is immunological unresponsiveness for anaerobic bacterial antigens. It is concluded that both immunological unresponsiveness and preferential coating with IgA are responsible for the relative absence of colonic mucosal inflammation.

https://gut.bmj.com/content/gutjnl/38/3/348.full.pdf

In vivo IgA coating of anaerobic bacteria in human faeces.

Colonization resistance of the digestive tract: clinical consequences and implications

Article de synthese. Etude des divers aspects de l'ecologie microbienne de l'intestin dans des modeles animaux. Developpement de la flore intestinale apres la naissance. Facteurs ecologiques associes a la colonisation de l'intestin par Clostridium difficile

D. van der Waaij

Papers

Colonization resistance of the digestive tract in conventional and antibiotic-treated mice.

Mitigation of Secondary Disease of Allogeneic Mouse Radiation Chimeras by Modification of the Intestinal Microflora

In vivo IgA coating of anaerobic bacteria in human faeces.

Colonization resistance of the digestive tract: clinical consequences and implications

The Ecology of the Human Intestine and its Consequences for Overgrowth by Pathogens such as Clostridium Difficile