University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

86

Pages:

3
Author:

Mariam A. A.

NORMAL GUT FLORA Normal flora (commensals) • • • • • Most areas of the body in contact with the outside environment harbour resident microorganisms o Large intestine has the highest numbers of bacteria Do not cause disease under normal conditions Internal organs & tissues & fluids are microbe free Defence mechanisms prevent invasion Normal flora prevent overgrowth of pathogenic microorganisms by: o Outcompeting for food o Producing inhibitory compounds/conditions GIT • • • Has a mucosal surface of 200-300m2 Provides microbes with: o Constant nutrient flow o Stable temperature o Special niches for growth ▪ Aerobic vs. anaerobic Up to 10-100 trillion bacteria g-1 of faeces o >1000 different kinds colonise the GIT Human microbiome project • National Institute of Health (NIH) initiative aiming to: o Determine if there is a core human microbiome o Understand if changes in the human microbiome can be correlated with changes in human health Pathogenic microorganisms • • Opportunistic pathogens: cause disease when the host’s defences are compromised or when they grow in part of the body that is not natural to them (access to normally sterile site e.g. Staphylococcus epidermis from skin entering the bloodstream via a central line) True pathogens: capable of causing disease in healthy individuals with normal defence mechanisms Relationships between organisms • Symbiosis o Commensalism: one organism benefits, other is unaffected e.g. Staphylococcus epidermidis on skin o Mutualism: both organisms benefit e.g. Escherichia coli in colon synthesises vitamin K o o Parasitism: one organism benefits, other is negatively affected e.g. Hookworm Opportunism Factors affecting bacterial distribution in the gut • • • • • pH Intestinal motility o Stomach (low pH) and upper small bowel (rapid peristalsis) have few bacteria: 103-104 mL-1 Low oxidation-reduction (redox) potential and anaerobic conditions Ileum: decreased peristalsis + increased pH → many diverse bacteria: 107-108 mL-1 Large bowel: decreased peristalsis → 1011-1012 mL-1 (35-50% of the volume of the colon contents) o Many different species: 99.9% are strict anaerobes Relationship between normal flora and host • • • • • • Normal flora are in equilibrium with each other Colonic bacteria produce nutrients and vitamins → folate and vitamin K Dietary sugars converted to fatty acids which can be absorbed as an energy source Metabolise some drugs Prevent colonisation with pathogenic bacteria and stimulation of the immune system by food antigens Colonising bacteria do not activate an immune response o Unable to penetrate the intestinal epithelial cells o Local immune mechanisms prevent invasion Germ-free mice • • • Thin small intestinal villi with shortened crypts Reduced mucosal regeneration rate Decreased number of cells involved in immune response