22. Infectious process - microorganism-host interaction, toxic effects of microorganisms.

Infectious process and infectious disease – microorganism-host interaction, toxic effects of microorganisms

Microorganism–Host Interaction

The dynamic relationship between a microorganism and the host, determining whether infection is asymptomatic, causes disease, or results in severe pathology.

Types of Interactions

1. 1 Symbiosis

Definition: General term for any close association between two different species.

1. 2 Mutualism

Definition: Both microorganism and host benefit.

Examples:

• Normal gut flora synthesize vitamins (e.g., vitamin K) while getting nutrients and a niche.
• Skin bacteria help prevent colonization by pathogens.

1. 3 Commensalism

Definition: Microorganism benefits; host is neither harmed nor helped.

Examples:

• Non-pathogenic Staphylococcus epidermidis on skin.
• Oral cavity flora living on mucus and food debris without affecting the host.

1. 4 Parasitism

Definition: Microorganism benefits; host is harmed.

Examples:

• Plasmodium falciparum (causes malaria).
• Mycobacterium tuberculosis (causes TB).

1. 5 Opportunistic infections

Occur when normal commensals cause disease in immunocompromised hosts or when they enter sterile sites.

Examples:

• Candida albicans causing oral thrush in HIV patients.

Possible interaction outcomes

Elimination: Host immune system clears pathogen.

Latent infection: Virus persists without active replication (e.g., HSV, VZV).

Chronic infection: Continuous low- level replication (e.g., hepatitis B).

Disease: If pathogen overcomes defenses, leading to tissue damage.

Toxic Effects of Microorganisms

1. 1 Exotoxins

Definition

Proteins secreted by bacteria; highly potent and specific.

Types & Examples

• Neurotoxins:
• Clostridium tetani → tetanospasmin → spastic paralysis.- incr muscle tone- hyperreflexia, UMN
• Clostridium botulinum → botulinum toxin → flaccid paralysis.
• Enterotoxins:
• Vibrio cholerae → cholera toxin → watery diarrhea.
• E. coli (ETEC) → heat-labile toxin → diarrhea.
• Cytotoxins:
• Corynebacterium diphtheriae → diphtheria toxin → inhibits protein synthesis, myocarditis.

Mechanism

• Bind to specific host receptors.
• Interfere with cellular processes (e.g., ADP-ribosylation of host proteins).
• Can act locally or systemically.

1. 2 Endotoxins

Definition

Components of the outer membrane of Gram-negative bacteria (lipopolysaccharide —LPS).

Effects

• Fever (via IL-1, TNF-α release).
• Hypotension, shock.
• Disseminated intravascular coagulation (DIC).
• Example: Sepsis due to E. coli.

Mechanism

LPS binds to CD14 and TLR4 on immune cells → massive cytokine release → systemic inflammatory response.

1. 3 Enzymatic tissue damage

Examples:

• Hyaluronidase (from streptococci) → degrades connective tissue → spreads infection.
• Collagenase (from Clostridium perfringens ) → promotes gas gangrene.

1. 4 Immunopathological damage

Overactivation of immune responses leading to tissue damage.

Examples:

• Post-streptococcal glomerulonephritis (immune complex-mediated).
• Hepatitis B —cytotoxic T- cell–mediated hepatocyte injury.