Introduction to Medical Microbiology: Microbial Relations

Microbial Relations

• Mutualism: beneficial association between microorganisms and hosts, e.g., colon bacteria receive a warm, nutrient-rich environment, and humans absorb vitamin precursors released.
• Commensalism: one member benefits without affecting the other, e.g., non-pathogenic Staphylococcus on skin.
• Parasitism: a parasite derives benefits from the host and harms it, e.g., various microorganisms involved.

Normal Microbiota in Hosts

• Normal flora and indigenous microbiota: organisms that colonize the body's surfaces without normally causing disease.
• Resident microbiota: permanent residents of the body.
• Transient microbiota: temporary residents of the body.
• Microbes on tongue: example of normal microbiota.

Reservoirs of Infectious Diseases

• Most pathogens cannot survive long outside of the host.
• Three types of reservoirs:
  • Animal reservoir: e.g., zoonoses, diseases that spread from animal hosts to humans.
  • Human carriers: infected individuals who are asymptomatic but infective to others.
  • Nonliving reservoir: e.g., soil, water, and food can be reservoirs of infection.

The Nature of Infectious Disease

• Manifestations of disease:
  • Symptoms: subjective characteristics of disease felt only by the patient.
  • Signs: objective manifestations of disease that can be observed or measured by others.
  • Syndrome: a group of symptoms and signs that characterize a disease or abnormal condition.
  • Asymptomatic, or subclinical, infections: lack symptoms but may still have signs of infection.

Koch's Postulates

• Proof that a specific microorganism causes a specific disease.
• Four postulates:
  1. The microorganism must be present in the diseased and not in a healthy animal.
  2. The microorganism must be cultivated in pure culture.
  3. The pure culture in a second animal must cause disease.
  4. The pure culture from the second animal should be the same as the first.
• Exceptions to using Koch's postulate:
  • Some pathogens cannot be cultured.
  • Some diseases are caused by combinations of pathogens, physical, environmental, and genetic factors.
  • Ethics: inoculation of healthy susceptible hosts not always possible.

Nosocomial Infections

• Infections acquired in healthcare settings.
• Figure 14.9: Microbiology an introduction.
• Table 14.5: Causes of nosocomial infections in the United States.

Emerging Infectious Diseases

• Contributing factors:
  • New microorganisms: genetic recombination, e.g., E. coli O157, avian influenza (H5N1), V. cholerae O139.
  • Misuse of antibiotics and pesticides: antibiotic-resistant strains.
  • Changes in weather patterns: e.g., Hantavirus.
• Crossing the Species Barrier: Clinical Focus, p. 371.

Virulence Factors of Infectious Agents

• Pathogenicity: ability of a microorganism to cause disease.
• Virulence: degree of pathogenicity.
• Virulence factors contributing to virulence:
  • Adhesion factors.
  • Biofilms.
  • Extracellular enzymes.
  • Toxins.
  • Antiphagocytic factors.
• Extracellular enzymes: secreted by pathogens, help maintain infection, invade further, and avoid body defenses.
• Toxins: harm tissues or trigger host immune responses causing damage.
• Toxemia: toxins in the bloodstream.
• Two types of toxins:
  • Exotoxins: e.g., cytotoxins, neurotoxins, enterotoxins.
  • Endotoxins: Gram-negative bacteria.

This quiz covers the concepts of microbial relations in medical microbiology, including mutualism, commensalism, and parasitism. It also explores the role of normal microbiota in hosts, discussing normal flora and indigenous microbiota.