Ecology of the Gut

Question 1

Explain the term ‘Human Supraorganism’

Answer 1

Combination of eukaryotic + prokaryotic cells in the body

Question 2

What is plaque?

What bacteria is in plaque?

Answer 2

Biofilm of many different species of bacteria that attaches to teeth and matured into an anaerobic community which causes caries + periodontitis

Organism: Streptococcus

• Produce adhesive polysaccharides in the presence of sucrose - sugars convert to lactic acid (attacks enamel plus proteolytic enzymes causing caries)

When sucrose is low, plaque produces a complex matrix which matures into tartar + the bacteria is harder to remove

Question 3

What is gingivitis?

Answer 3

Inflammation of gum tissues

Causes bleeding of the gums

Advanced stage of periodontal disease - leads to bone loss around tooth (main cause of tooth loss)

Some streptomyces species from the mouth cause blood clots + increase risk of stroke + cardiovascular disease

Antibiotics are given to heart disease patients as treatment prior to dental procedures to reduce chance of contaminating the blood with mouth bacteria

Question 4

How do you prevent gum disease?

Answer 4

Physical removal of plaque

• as biofilm protects bacteria from nearly everything (electrical toothbrush + use of interdental brushes)

Bleeding of the gums is not normal (refer to dentist/dental hygienist if persist to preserve teeth)

Using disclosing tablets (contains vegetable dye to stain plaque) to self-monitor brushing

Fluoride in toothpaste can help against caries

Mouthwash (ineffective against gingivitis, except chlorhexidine containing products)

• no mouthwash reach deep pockets

Chlorhexidine disrupts bacterial membrane helping gingivitis

• Long term use causes dark staining of mouth and affects taste sensation

Anionic compounds in toothpaste inactivates with chlorhexidine (use chlorhexidine 30 min after brushing)

Question 5

Discuss the ‘Human Gut Microbiota’

Answer 5

Normal digestive tract contains over 10¹⁴ bacteria weighing 1.5kg, majority in lower intestines

To reach intestines, microorganisms must survive stomach acid; and to replicate, they must be able to resist bile + low O₂ conditions

Rapid proliferation of the gut microbiota allows rapid evolution of bacterial strains (as well as ecosystem)

Question 6

Label the diagram

Answer 6

Question 7

How do we acquire our gut microbiota?

Answer 7

Acquired early in life + become stabilised after infancy

Acquired from parents / siblings

Sometimes picked up from birth canal

Question 8

What is the normal intestinal gut microbiota composed of?

Answer 8

1000 species, 7000 strains

>160 different species present in each individual

90% of microorganisms in intestines belong to 2 phyla:

• firmicutes (clostridium, enterococcus, lactobacillus, ruminococcus - gram +)
• bacteriodetes (bacteriodes, prevotella - gram -)

Smaller number of microorganisms. are found of these phyla:

• proteobacteria (escherchia, klebsiella, enterobacter - gram -)
• actinobacteria (bifidobacterium - gram +)
• methanobacteria (archaea)

Question 9

Name each classification of intestinal microorganisms according to effect on humans

Answer 9

Symbionts

Commensals

Pathobionts

Pathogens

Commensals are shrinking as we start knowing them better

Question 10

What are symbionts?

Answer 10

Mutually beneficial ecological relationship

Beneficial to human health

Question 11

What are commensals?

Answer 11

Benefits the commensal, no effect on the host

No effect on human health

Question 12

What are pathobionts?

Answer 12

Commensals that sometimes cause disease

Question 13

What are pathogens?

Answer 13

Microorganisms not normally resident in the intestine that cause disease when present

Question 14

What is the role of the normal intestinal microbiota?

Answer 14

Digest materials that human enzymes can’t handle and provide important nutrients

• Indigestible plant sugars converted into short chain fatty acids
• can adapt to diet
• Many bacteria produce compounds that are further processed by other bacteria
  
  • biotin e.g. coenzyme for carboxylase enzymes
  • panthotenic acid e.g. used in CoA synthesis
  • folic acid e.g. essential for nucleotide production
  • Vitamin K precursor e.g. required for blood clotting

Reduce pathogenic microorganisms

• remove nutrients + O₂

Influence brain development

Influence immune system

• improves response to pathogenic bacteria
• modulates T cell function

Question 15

What are M cells + how does their function link with the gut microbiota?

Answer 15

Microfold cells (M cells) are found in Peyer’s patches

M cells sample particles from the gut microbiota by endocytosis + pass on antigens to underlying immune cells.

This leads to secretion of IgA antibodies against bacteria + development of T cells.

Question 16

Why does the normal microbiota not cause inflammation of the intestine?

Answer 16

The unstirred layer + mucosa physically impede bacterial invasion

• This is the same barrier that impede drug transport with low solubility; they also separate the gut bacteria from the enterocytes
• Symbionts + commensal ‘behave’ + do not cross this barrier efficiently

The mucosa contains antimicrobial factors

• 3-5g of IgA antibodies against both resident + pathogenic bacteria are secreted into the gut everyday
• Antimicrobial peptides such as RegIIIϒ and α-defensins are present in the mucosa

Gut bacteria communicate with immune system

• Immune system is defective in the abscence of gut microbiota
• Some resident bacterial species activate T cell responses, while others reduce them
• Resident bacteria change their gene expression profiles in response to IgA and antimicrobial peptides

Question 17

What determines the composition of healthy gut microbiota?

Answer 17

1. Which microorganisms one is exposed to during birth and childhood
2. Diet mediates the selection of these organisms as well as in determining their relative numbers
3. The interplay with the immune system