12- Microbiology of humans Flashcards
Overview of human–microbial interactions
• Most microorganisms are commensals, few contribute to health (beneficial) and fewer pose direct threats to health (harmful).
• Normal microbiome
– Microorganisms usually found associated with human body tissue
– Humans are colonized by microorganisms at birth
– Normal microbiome changes over time and according to the condition of the host (diet, environment, age, sex, occupa@on, etc.)
Microbiome of the ski (epidermis, Sebaceous glands and apocrine sweat glands)
Barrier? What kind of environment? Duct? Normal microbiome secretion? Other bacteria?
• The epidermis is composed of an inner layer of living,
actively replicating cells and an outer layer of dead cells.
• Sebaceous glands, apocrine sweat glands: secretes salt
water, with various amounts of proteins, lipids, and sugars. Secretions are slightly acidic and have some antimicrobial effects (lipids and proteins).
- Dead cell layer forms a tough barrier that prevents microorganisms from penetrating deeper tissues.
- The skin is generally a dry, acid environment that does not support the growth of most microorganisms.
- The ducts that carry secretions to the surface are colonized by only a few well adapted species (Gram-positive).
- The normal microbiome secretes bacteriocin that kills incoming competitors and protects – to a certain extent – against colonization by harmful bacteria.
- The skin may also harbor anaerobes (aerotolerant): Propionobacterium acnes inhabits hair canals.
Oral cavity: habitat? Nutrient? Teeth?
- The oral cavity is a complex, heterogeneous microbial habitat. Both aerobic and anaerobic niches are available.
- High concentrations of nutrients near surfaces in the mouth promote localized microbial growth.
- The teeth consist of a mineral matrices (enamel) surrounding living tissue (dentin and pulp).
Dental plaque: how is it colonized? What is the species creating plaque? What do they do?
• Bacteria colonize tooth surfaces by first attaching to acidic glycoproteins deposited there by saliva.
• Extensive growth of oral microorganisms, especially streptococci, results in a thick bacterial layer (dental plaque).
In the presence of sucrose – from diet – Streptococcus mutans synthesizes extracellular polysaccharides (dextran) that aid its attachment to the tooth surface (form an extracellular matrix, ECM).
• As plaque continues to develop, anaerobic bacterial species begin to grow.
• The microorganisms in dental plaque – S.mutans and various lactobacilli – ferment sucrose and produce lac4c acid that demineralizes the tooth enamel (solubilizes
calcium).
What kind of microorganisms are found in the gastrointestinal tract?
Anaerobe and facultative aerobes
Gastrointestinal tract of humans: when does it begins? Acid?
Colonization begins at birth. Microbial populations in different areas of the GI tract are influenced by diet and the physical conditions in the area.
• The acidity of the stomach and the duodenum of the small intestine (~pH 2) prevents many organisms from colonizing the GI tract. Microorganisms in food particles might be protected.
Microorganisms in the human colon: Species? Digest what? Ferment what?
• Mostly strict anaerobes or facultative aerobes: Bacterioides, enteric bacteria (E.coli), yeasts, anaerobic protozoa (Entamoeba coli).
High variability in gut communities between different individuals
- Microorganisms digest complex carbohydrates (cellulose, starch, etc.) and produce VFAs.
- Intestinal microorganisms of the large intestine ferment VFAs and produce gas (CO2 and H2). Methanogens convert CO2 and H2 to CH4 (found in 1/3 of adults).
Microorganisms in the human colon: Sulfate reducers? metabolic reactions? Maturing?
• Sulfate reducers and organisms that can ferment cysteine and methionine produce H2S. Organosulfur compounds (methanethiol) are responsible for the smell.
• Intestinal microorganisms carry out a variety of essential metabolic reactions that produce various nutrients that benefit the host.
– Amino acids
– Vitamins (B12, K, thiamine, riboflavin, etc.)
• Microorganisms contribute to the ‘maturing’ of the gastrointestinal tract.
• Microbes in the gut affect early development, health, and predisposition to disease: microbiome for immune system
• Gut microorganisms may play a role in obesity:
Bacteroidetes, firmicutes, methanogens
In lean mice: High H2 retards fermentation
In obese mice: Low H2 promotes fermentation
Respiratory tract: organisms found in the nose? Lower respiratory tract…?
• Microbiome of the nose and nasopharynx is very similar to that of the mouth: staphylococci, streptococci, corynebacteria.
• Nasopharynx may harbor potential pathogens that are under control by the host immune system:
Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis. Healthy carrier.
• The lower respiratory tract, including trachea, bronchi and lungs was historically considered free of microorganisms in healthy
individuals (using the culture technique). Recent study shows that the lower respiratory tract also harbors a microbiome (low biomass).
Genitourinary tract: microorganisms? Consequence of altered conditions?
• The genitourinary tract of men and the urinary tract of woman are generally free
of microorganisms due in part to the flushing action of urine.
• Altered conditions (such as change in pH) can cause potential pathogens in the urethra to multiply and cause disease. A few of the normal microorganisms of the gastrointestinal tract can cause urinary tract infections (E. coli, Proteus mirabilis).
Vaginal tract:
- Variety and types of organisms present depend on age of the individual.
- Hormones control the glycogen concentration of the vaginal epithelium.
• Microorganisms degrade glycogen and produce lactic acid which reduces the pH of
the vaginal tract to 4.5.
- Changes the environement with age
What are the sterile zones? (7)
- Blood
- Spinal fluid
- Internal portion of urinary tract (bladder, kidney)
- Peritoneal cavity (gut cavity)
- Pleural cavity (lung cavity)
- Sinuses
- Interior region of other body @ssues (bones, muscle, …)
