15 Commensals

Commensals

  • Definition: Refers to partner species living in host species.

  • Habitat: Commensals (microbes) live on or in the bodies of humans, animals, or microorganisms.

  • Commensalism: Association of two organisms of different species living together, sharing food resources.

Where Commensals Associate with Hosts

  1. Exposed Topical Body Surfaces

    • Skin

    • Hair and nails

  2. Internal Environments

    • Upper respiratory tract

    • Digestive system

    • Excretory system

  3. Hostile Environments

    • Rare in stomach due to low pH.

    • Rare in immunologically aggressive environments like bronchioles.

    • Absent in internal organs (brain, heart, kidney, liver).

Types of Commensals

  • Indigenous (Autochthonous):

    • Able to grow and multiply on the host's surface.

    • Example: Staphylococcus epidermidis.

  • Transient (Allochthonous):

    • Cannot grow or multiply on the host's surface.

    • Example: Coliforms.

Microflora and Microbiota Examples

  • Skin:

    • 10^12 bacteria per 2 m²

    • Includes bacteria: Staphylococci, Streptococci, Cutibacterium acnes, Bacillus

    • Yeast: Candida.

  • Oral Cavity:

    • Houses over 700 bacterial species.

    • Includes genera like Staphylococcus, Fusobacterium, Streptococcus, Actinomyces

    • Protozoa: Entamoeba gingivalis

    • Yeast: Candida.

  • Stomach:

    • Low bacterial count (<10 bacteria/ml).

    • Includes Staphylococci, Streptococci, Peptostreptococcus, Lactobacillus, and Helicobacter pylori (can cause ulcers or stomach cancer).

Factors Affecting Microbiota

  • Antibiotic Treatment: Reduces microbial competition, allowing opportunistic pathogens to grow.

  • Dietary Changes: Fatty acids in skin secretions alter microbiota diversity.

  • Decreased Immune Function: Increased dandruff and other conditions due to stress and malnutrition.

Human Microbiome Composition

  • Up to 60% of feces dry mass consists of bacteria.

  • Bacteroidetes:

    • Examples: Bacteroides fragilis, anaerobic, gram-negative, non-sporing, motile/non-motile rods.

  • Firmicutes:

    • Includes Clostridium difficile, Bacillus, Lactobacillus, and Streptococcus.

  • Opportunistic Pathogens:

    • Members of microbiota become pathogenic due to breaks in skin or tissue.

    • Clostridium difficile is associated with hospital-acquired infections, especially after antibiotic use.

Hygiene Hypothesis

  • Increased Incidence: Industrialized countries see higher rates of diseases caused by immune dysregulation due to lack of contact with infectious agents in childhood.

  • Microbiota and Immune System:

    • Lack of microbial exposure may result in poor immune regulation.

  • Helminthic Therapy:

    • Investigated for its potential to improve immune responses associated with allergies and inflammatory bowel disease.

Role of Commensals in Immune System Development

  • Old Friends: Organisms causing little to no harm that co-evolved with humans.

  • Microbial Influence on Immunity:

    • Research indicates that polysaccharides from Bacteroides fragilis can positively affect immune development in mice.

  • Importance of Exposure: Reduced encounters with these 'old friends' may lead to increased immunological disorders.

Benefits of Gut Microbiota

  • Utilization of Plant Polysaccharides: Microbiota like Bacteroides strains break down non-digestible substances.

  • Short-Chain Fatty Acids (SCFAs):

    • Produced during fermentation; they are absorbed and contribute to the host's caloric intake (10% of calories in Western diets).

  • Link to Obesity:

    • Studies indicate an imbalance in the Bacteroides to Firmicutes ratio in obese versus lean individuals.

Rumen Microbiota in Ruminants

  • Rumen: Upper digestive chamber rich in microorganisms, allows cows and other ruminants to digest cellulose.

  • Digestive Process: Involves stages of chewing, microbial fermentation, and nutrient absorption.

    1. Chewing and swallowing into the rumen.

    2. Microbial digestion of cellulose-rich food.

    3. Regurgitation and re-chewing of cud.

    4. Water absorption in the omasum.

    5. Final digestion in the abomasum.

Microbes in the Caecum and Probiotics

  • Microbial Activity:

    • Breakdown of complex polymers (e.g., cellulose) in non-ruminants like horses and rabbits.

  • Probiotics: Defined by WHO as live microorganisms that confer health benefits when administered in adequate amounts.

  • Role of Probiotics: Aimed at restoring balance to microbiota post-disturbance (e.g., after diarrhea or antibiotic use).

Probiotic Applications and Effects

  • Helpful strains include Lactobacillus, Streptococcus, and Bifidobacterium.

  • Examples include probiotic cultures used for colonization post-diarrhea, especially effective against relapsing C. difficile.

  • Quality Control:

    • Probiotic products should aim for specific needs for both human and animal health.