Microorganism-Host Relationships Quiz

Microorganism-Host Relationships

• Microorganisms, including bacteria, interact with host cells (monocytes and epithelial cells) through various mechanisms.
• FimH (bacterial adhesin) plays a role in bacterial adhesion.
• Pili are involved in the interaction.
• CD48 and Mannose are also involved.
• TLR4 is a key receptor on host cells.

References

• Bailey & Scott's Diagnostic Microbiology
• Kenneth Todar's Textbook of Bacteriology
• Brian Henderson Wiley's Cellular Microbiology

The Nature of Environment-Microorganism-Host Relationships

• The relationship between host, microorganism, and environment is complex and multifaceted.
• There are reciprocal influences between these three components.

Symbiosis

• Symbiosis is defined as "life together."
• It describes the association of two or more organisms.

Bacteria-Host Relationships in Humans: Mutualism

• Mutualism is a symbiotic relationship where both species benefit.
• Example: Lactic acid bacteria in the vaginal epithelium benefit the host.
• Beneficial effects include nutrient provision, stable environment, protection, transport, stimulation, and development of the immune system.

Commensalism

• Commensalism is a symbiotic relationship where one species benefits, and the other is unaffected.
• Example: S. epidermidis on human skin (if it doesn't cause illness).
• Hosts benefit from a stable environment, protection, and transport.

Commensalism (Details)

• Nutrients

• Stable environment

• Protection

• Transport

• If a commensal relationship is examined closely, parasitic or mutualistic features can sometimes emerge.

Parasitism

• Parasitism is a symbiotic relationship where one species benefits while the other is harmed.
• The parasite benefits from the resources of the host.
• Parasites can cause harm to hosts.
• Often, the parasite doesn't kill the host.

Parasitism (Details)

• Induce harmful effects on the host.
• No benefit to the host.
• Nutrients from host.
• Stable environment from Host
• Protection from Host
• Transport through host

Dysbiosis

• Disruption of the normal microflora can cause disease.
• Elimination of essential organisms or the growth of inappropriate organisms causes dysbiosis.
• Example: C. difficile overgrowth following antibiotic use can lead to inflammation of the colon (antibiotic-associated colitis).

Normal Microbiota (Normal Flora)

• Microbiome: Describes the whole genome of all microorganisms (symbiotic and pathogenic).
• Microbiota: Describes the range of microorganisms found on or in multicellular organisms (commensal, pathogenic).
• Normal microbiota are typically absent from internal tissues and fluids (e.g., blood, brain, muscle).
• Normal microbiota are found on the surface of tissues (e.g., skin, mucous membranes).

Composition of Normal Flora

• Human normal flora consist of over 200 bacterial species.
• Various factors influence normal flora composition (genetics, age, sex, stress, and diet).
• Different parts of the body have different predominant bacterial species.

Normal Microbiota (Colonization)

• Colonization of normal flora begins at birth during passage through the birth canal.
• Handling and feeding of the newborn infant leads to further colonization of the skin, oral cavity, and intestinal tract within 48 hours.
• Normal flora quantity is ~10^ 12 on skin, 10^10 in the mouth, and 10^14 in the gastrointestinal tract of an adult.

Changes in Normal Flora Composition

• Weaning, tooth eruption, and ovarian function changes affect the composition of the normal flora.

Germ-free animals

• Germ-free animals lack microbiota and display specific characteristics due to their lack of normal flora:
• Vitamin deficiencies
• Increased susceptibility to infections
• Poorly developed immune system
• Lack of natural immunity to bacterial invasion.

Beneficial effects of the normal flora

• Synthesis and excretion of vitamins (e.g., Vitamin K, Vitamin B12 and other B- vitamins).
• Prevention of colonization by pathogens (competition and other factors).
• Antagonism of other bacteria (producing specific substances to inhibit growth).
• Stimulation of tissue development (intestinal tissues such as Peyer's patches).
• Stimulation of immune response (producing natural antibodies).

Harmful effects of the normal flora

• Bacterial synergism (cross-feeding).
• Competitions for nutrients
• Toxemia
• Endogenous disease

Tissue Specificity

• Tissue specificity is the tendency of bacteria to preferentially colonize certain tissues.
• Host properties (nutrients, pH, oxygen, temperature) are significant in impacting tissue specificity.
• Bacterial properties like specific adherence and biofilm formation also influence tropism.

Specific Adherence

• Specific adherence is a key factor in tissue tropism.
• Bacteria use specific components (e.g., capsules, fimbriae) to bind to host cell receptors (e.g., glycoproteins).
• The specifics of the ligand (bacterial surface components) and the receptor (host cell components) determine tissue tropism.

Biofilm Formation

• Biofilm formation is a process where bacteria form complex communities on surfaces.
• Steps involved include attachment, microcolony formation, exopolymer secretion, maturation, and dispersal.
• Biofilms are often associated with chronic infections due to their enhanced resistance to antibiotics.