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.