Microbial Symbioses with Humans - Gastrointestinal Microbiota

10 important questions on Microbial Symbioses with Humans - Gastrointestinal Microbiota

What are the three different general enterotypes? What are they enriched in?

  • Enterotype 1: enriched in bacteroides
  • Enterotype 2: enriched in prevotella
  • Enterotype 3: enriched in ruminococcus

What are the functions of the gastro intestinal microbiota?

  • Metabolism of xenobiotics
  • Intestine functioning is stimulated
  • Development and activity of the immune system

What are the products of the intestinal microbiota?

  • Vitamins (thiamine, riboflavine, pyridoxine, B12, K)
  • Gas (CO2, CH4, H2)
  • Odor (H2S, NH3, amines, indole)
  • Organic acids (acetic, propionic, butyric acids)
  • Enzymes (glycosidase reactions)
  • Steroid metabolites (esterified, dehydroxylated, oxidzied, or reduced)
  • Neurotransmitters (tryptamine, 4-ethylphenylsulphate)
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Describe the microbial composition of the stomach and the factors influencing the diversity of bacteria in the gastric environment.

The stomach hosts a vibrant bacterial community with low pH. Despite preventing bacterial overgrowth, the stomach holds a core microbiome dominated by Bacteroidetes, Firmicutes, Actinobacteria, Fusobacteria, and Proteobacteria. Factors such as low pH, distribution in the gastric lumen and wall mucus layer, and acid-tolerant bacteria like Helicobacter pylori influence microbial diversity.

Explain the changes in bacterial numbers and microbial composition from the stomach to the small intestine.

Bacterial numbers increase from about 10^4/g (stomach) to 10^8/g (small intestine). The small intestine, consisting of the duodenum, jejunum, and ileum, starts acidic and gradually becomes less acidic. Microbial composition in the small intestine adapts to "feast or famine" conditions as bacteria compete with the host for small carbohydrates.

Highlight the characteristics of the large intestine's microbiota, focusing on the role of obligate anaerobes and bacterial diversity.

he large intestine is a fermentation vessel with anoxic conditions. Obligate anaerobes like Clostridium and Bacteroides predominate, comprising over 99% of prokaryotic cells. Bacterial diversity is extensive, with Firmicutes, Bacteroidetes, and Actinobacteria dominating.

Discuss the microbial diversity in the human colon and the concept of enterotypes.

The human colon exhibits vast microbial diversity, with Firmicutes (Lachnospiraceae, Ruminococcaceae) and Bacteroidetes playing key roles. Enterotypes are functionally distinct gut communities enriched in Bacteroides (Enterotype 1), Prevotella (Enterotype 2), and Ruminococcus (Enterotype 3). These enterotypes influence vitamin and metabolite production, responses to diet and drugs, and health outcomes.

Elaborate on the products of gut microbial metabolism and their significance in host physiology.

Gut microbial metabolism produces volatile fatty acids, gases (H2, CO2, CH4), vitamins B12 and K, essential amino acids, and bioactive steroid compounds. Metabolites like short-chain fatty acids from resistant starch fermentation have health benefits. Gut microbes also influence the gut–brain axis, suggesting a connection between the microbiome and the nervous system.

Examine the role of the gut microbiome in "educating" the immune system and the consequences of insufficient microbial exposure.

The gut microbiome stimulates immune system development, and early exposure is vital for immune tolerance. Excessive hygiene in infancy may lead to a poorly trained immune system, potentially promoting autoimmune conditions like allergies, asthma, and inflammatory bowel diseases.

Give examples on faculatative aerobes, obligate anaerobes, and other thypes of bacteria, that can be found in the colon

Facultative aerobes like Escherichia coli, Enterococcus faecalis can be found in the colon. Obligate anaerobes such as Bacteroides fragilis, Clostridium species are also present. Other bacteria in the colon include Firmicutes (e.g. Faecalibacterium prausnitzii), Actinobacteria (e.g. Bifidobacterium), and Proteobacteria.

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