Host-Microbe Interactions and Pathogenesis: Microbial Symbiosis, Normal Microbiota, and Virulence Factors (lecture 8)

Host-Microbe Interactions

Overview of Host-Microbe Relationships

The human body provides a variety of environments that support the growth of diverse microorganisms. These interactions can be beneficial, neutral, or harmful, depending on the nature of the microbe and the host's condition.

• Human host: Offers favorable environments for microbial growth due to nutrients, moisture, and temperature.

• Body regions: Each anatomical region has unique environmental conditions (e.g., pH, moisture, temperature) that influence the resident microbial populations.

• Microbial populations: Higher densities are found in regions with more exposure to the external environment or abundant nutrients (e.g., skin, mucous membranes).

Key Definitions

• Normal microbiota (flora): Microorganisms that establish more or less permanent residence (colonize) but do not cause disease under normal conditions.

• Transient microbiota: Microorganisms that may be present for days, weeks, or months and then disappear; often acquired from the environment (e.g., hospital-acquired or nosocomial).

• Parasites: Microorganisms that live on or in another host organism, imparting no health benefit and potentially causing harm.

• Pathogens: Microorganisms capable of causing disease by inflicting damage to the host and overcoming host defenses.

• Infection: Invasion or colonization of the body by pathogenic microorganisms.

• Disease: Any change from a state of health, causing damage or injury to the host and impairing host function.

• Pathogenicity: The ability of a pathogen to cause disease.

• Virulence: The degree or measure of pathogenicity (e.g., Salmonella requires thousands of cells for infection, while Shigella requires only hundreds).

Types of Symbiotic Relationships

• Commensalism: One organism benefits, the other is unaffected.

• Mutualism: Both organisms benefit.

• Parasitism: One organism benefits at the expense of the other.

Microbial Antagonism

• Competition among microbes, where normal microbiota protect the host by outcompeting potential pathogens for space and nutrients (e.g., biofilm formation).

Opportunistic Microorganisms

• Normally harmless in their usual habitat but can cause disease if they enter a different environment or if host defenses are compromised (e.g., due to stress, illness, or immunosuppression).

Outcomes of Host-Microbe Interactions

• Host dies

• Pathogen dies

• Coexistence (damage is temporary or permanent)

• Host outcompetes pathogen

Factors influencing outcome: Pathogenicity of the microorganism and host condition (resistance or susceptibility).

Koch’s Postulates

Establishing Causation in Infectious Disease

• The same pathogen must be present in every case of the disease.

• The pathogen must be isolated from the diseased host and grown in pure culture.

• The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible animal.

• The pathogen must be re-isolated from the inoculated animal and shown to be the original organism.

Normal Microbiota of the Human Body

Skin Environment

• Composed of epidermis, dermis, and subcutaneous tissue.

• Associated glands: eccrine (sweat), apocrine, and sebaceous glands.

• Most skin microorganisms are associated with these glands; other areas are dry and inhospitable.

• Normal skin microbiota are primarily Gram-positive and can be resident or transient.

Examples of skin microorganisms: Staphylococcus spp., Streptococcus spp., Propionibacterium acnes, Acinetobacter spp.

Factors affecting skin microbiota: Weather, age, hygiene.

Oral Cavity

• Includes teeth and tongue.

• Saliva provides nutrients and contains antibacterial substances (e.g., lysozyme, lactoperoxidase).

• Bacterial populations develop as teeth appear; biofilm (plaque) forms on teeth.

• If plaque accumulates, dental caries (tooth decay) can result, especially from lactic acid-producing bacteria (e.g., Streptococcus mutans, S. sobrinus).

Gastrointestinal Tract

• Consists of stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum, colon).

• Stomach pH (2-4) acts as a chemical barrier; some bacteria (e.g., Helicobacter pylori) can colonize the stomach wall.

• Highest microbial numbers are found in the colon, which acts as a fermentation vessel (e.g., Bacteroides, Enterococcus faecalis, E. coli).

• Diet significantly influences the composition of the gut microbiota.

• Fecal matter is composed largely of microbial biomass; intestinal gas results from microbial metabolism.

Respiratory Tract

• Divided into upper (nasal cavity, pharynx, larynx) and lower (trachea, bronchi, lungs) regions.

• Upper tract harbors many bacteria (e.g., staphylococci, streptococci, diphtheroid bacilli, gram-negative cocci), including potential pathogens (Staphylococcus aureus, Streptococcus pneumoniae, Corynebacterium diphtheriae).

• Lower tract is usually sterile; ciliary action and mucus help prevent microbial penetration.

Urogenital Tract

• Male and female tracts differ, but the upper tract is generally sterile in both.

• Urethra may be colonized by facultative aerobic gram-negative rods and cocci (e.g., E. coli, Proteus spp.), which can be opportunistic pathogens.

• Vaginal tract is acidic and contains lactobacilli; environment changes with age and hormonal status. E. coli, streptococci, and yeasts may also be present.

Pathogenesis

Establishment of Infection

• Pathogenic microorganisms must establish colonization or invasion in the host to cause disease.

• Expression of specific virulence factors is required to alter host function and cause disease.

Entry and Adhesion

• Many pathogens possess specific adhesion factors (e.g., pili, fimbriae, surface proteins) that allow attachment to host tissues.

• Tissue specificity: Some pathogens target specific tissues (e.g., Neisseria gonorrhoeae for urogenital tract).

• Host specificity: Some pathogens infect only certain hosts (e.g., Salmonella typhi infects humans).

• Other cellular components (e.g., capsules, cell wall proteins) may aid in adhesion.

Invasiveness

• Most pathogens must invade host tissues to initiate pathogenicity.

• May possess enzymes (e.g., hyaluronidase, collagenase) that aid in tissue penetration.

Colonization and Growth

• After successful adhesion and invasion, colonization and growth occur.

• Often, a critical population size is necessary for disease to be initiated.

• Microorganisms must acquire nutrients (e.g., iron) for growth.

• Growth may remain localized or spread, leading to systemic infection.

Virulence Factors

Definition and Types

• Characteristics of pathogens that aid in the establishment and maintenance of disease.

• Most are enzymes or toxins.

Enzymes as Virulence Factors

Enzyme	Function	Example Organism
Hyaluronidase	Breaks down hyaluronic acid in host tissues	Streptococci, Staphylococci, Pneumococci
Proteases	Depolymerize host tissue proteins	Various
Nucleases	Break down nucleic acids	Various
Lipases	Break down lipids	Various
Collagenase	Breaks down collagen	Various
Fibrinolytic enzymes (e.g., Streptokinase)	Break down fibrin blood clots	Streptococcus pyogenes
Coagulase	Causes coagulation of plasma	Staphylococci

Toxins

• Exotoxins: Proteins released extracellularly as the organism grows; highly specific and potent.

• Endotoxins: Lipopolysaccharide (LPS) components of the outer membrane of Gram-negative bacteria; generally cell-bound and released in large amounts only when cells lyse; cause non-specific immune reactions.

Categories of Exotoxins

• Cytolytic toxins: Enzymatically attack cell constituents (e.g., hemolysins, phospholipases).

• A-B toxins: Consist of two subunits; B binds to host cell receptor, A is the active toxic component (e.g., diphtheria toxin).

• Superantigen toxins: Stimulate large numbers of immune cells, causing massive inflammatory reactions.

Examples of Exotoxins

• Diphtheria toxin: Produced by Corynebacterium diphtheriae; an A-B exotoxin that disrupts protein synthesis; iron concentration in the environment regulates toxin production.

• Tetanus and Botulinum toxins: Both are A-B type toxins produced by Clostridium tetani and Clostridium botulinum respectively. Botulinum toxin is produced in preserved food products (strict anaerobe) and causes flaccid paralysis (botulism), leading to respiratory failure. Tetanus results from infection of deep, anoxic wounds and causes rigid paralysis due to neurological impairment.

• Enterotoxins: Exotoxins that act on the gastrointestinal tract, usually the small intestine, causing massive fluid secretion and diarrhea (e.g., cholera toxin produced by Vibrio cholerae, as well as toxins from enteropathogenic Salmonella and E. coli).

Endotoxins

• Cause physiological effects such as fever (elicited by endogenous pyrogens produced by the host in response to endotoxin).

• Large doses may cause death, but toxicity is generally lower than exotoxins.

• Highly sensitive assay for endotoxin: Limulus amoebocyte lysate (LAL) test.

Summary Table: Virulence Factors and Their Functions

Virulence Factor	Function	Example
Cytotoxin	Damages host cells	Salmonella
Enterotoxin	Disrupts intestinal function	Vibrio cholerae
Endotoxin	Induces fever, shock	Gram-negative bacteria
Evasion of phagocytosis	Prevents immune clearance	Capsule formation
Motility	Facilitates colonization	Flagella

Additional info: Virulence factors are often exported out of the bacterial cell and include any structure or substance that aids in infection or contributes to disease symptoms.