Study Notes on Infection and Pathogens

Infection Overview

Objectives

  • Identify how microbes can become pathogens to human host cells.
  • Relate the development of infections to breaks in the lines of defense.
  • Differentiate the types of pathogens.
  • Discuss the phases of acute infection.
  • Examine complications of infection.
  • List common clinical manifestations of infection.
  • Understand laboratory tests related to diagnosing infection.
  • Apply concepts of infection to clinical models.

Bacteria and Pathogens

Relationships Between Invaders and Hosts

  • Symbiosis: Benefits only the human; no harm to the organism.
  • Mutualism: Benefits both the human and organism.
  • Commensalism: Benefits only the organism; no harm to the human.
  • Pathogenicity: Benefits the organism; harms the human.
  • Opportunism: A benign organism becomes pathogenic due to decreased host resistance.

Pathogens

  • Definition: Disease-producing microbes that must be capable of binding to specific receptors on human host cells.
  • Mechanisms for Causing Disease:
      - Direct destruction of host cell by pathogen.
      - Interference with host cell’s metabolic function.
      - Exposure of host cell to toxins produced by the pathogen.

Factors Affecting Pathogenicity

  • Virulence: Potency of the pathogen.
  • Infectivity: The exposure level necessary to cause an infection.
  • Toxigenicity: The ability to produce toxins.
  • Antigenicity: The ability to evade the immune system.
  • Antigenic variability: Genetic variations that can prevent effective host immune response.
  • Pathogenic defense mechanisms: Strategies pathogens use to avoid destruction.
  • Coinfection: Presence of two or more pathogens simultaneously.
  • Superinfection: Secondary infection occurring on top of an existing infection.

Types of Pathogens

  • Categories of Pathogens:
      - Obligate Parasites: Require a host to survive.
      - Facultative Parasites: Can survive without a host.
  • Examples of Pathogen Types:
      - Bacteria
      - Virus
      - Fungi
      - Rickettsiae
      - Mycoplasmas
      - Protozoa
      - Chlamydiae

Specifics of Pathogen Types

Bacteria
  • Characteristics of Bacteria:
      - Type: Prokaryotic organisms; can be aerobic or anaerobic.
      - Shapes:
        - Spherical = cocci
        - Rodlike = bacilli
        - Spiral = spirochetes
      - Gram Staining: Bacteria can be classified as Gram-positive or Gram-negative.
Virus
  • Definition: Intracellular parasites that take over host cell metabolic machinery for survival.
  • Function in Treatment: In cancer treatments, medications can be attached to viruses for targeted delivery into cells.
Fungi
  • Description: Includes large, single-celled yeasts or multicellular molds.
  • Types:
      - Fungi affecting skin, hair, or nails (tineas).
      - Normal Flora: Organisms such as Candida albicans that can become pathogenic when host bacteria are reduced.

Normal Flora

  • Definition: Organisms that normally exist on/in the body.
  • Examples:
      - Gram-positive cocci and rods.
      - Gram-negative bacteria and spirochetes.

How Normal Flora Become Pathogens

  • In cases where the immune and inflammatory system are compromised, microorganisms may leave their normal site and cause infection.
  • Normal flora can become opportunistically pathogenic.

Communicable Diseases

  • Reservoir: The holding tank for the pathogen.
  • Portal of Exit: How the organism leaves the reservoir.
      - Examples: Direct contact, droplet, airborne, vector.
  • Mode of Transmission: Access point for the microorganism to enter the host.
      - Examples of Entry Points: Mouth, nose, eyes, cuts in skin.

Chain of Infection Model

  • Components:
      - Susceptible Host: Individuals such as the elderly, infants, or immunocompromised individuals.
      - Infection occurs when there is a break in the chain, allowing pathogens to infect susceptible hosts.

Phases of Infection

  • Exposure: The initial stage when the pathogen enters the host.
  • Incubation: Period during which the pathogen multiplies without causing noticeable symptoms.
  • Prodrome: Early symptoms that may indicate a contagious period.
  • Clinical Illness: Typical symptoms of disease appear as the immune response develops.
  • Convalescence: Recovery phase following the acute phase.
Usual Incubation Period for Foodborne Diseases
  • Documented ranges for various pathogens such as:
      - Hepatitis A: 14-20 days.
      - E. coli O157:H7: 2-5 days.

Complications of Infection

  • Chronic Infection: Long-lasting infections that can cause ongoing health issues.
  • Septicemia: Presence of bacteria in the blood leading to systemic inflammation.
  • Septic Shock: A severe drop in blood pressure due to infection leading to organ dysfunction.

Manifestations of Infection

  • Local Symptoms:
      - Heat
      - Edema
      - Pain
      - Redness
      - Lymphadenitis
      - Purulent exudate
  • Systemic Symptoms:
      - Fever
      - Weakness
      - Headache
      - Malaise
      - Anorexia
      - Nausea

Laboratory and Diagnostic Tests

  • White Blood Cell Count: Indicators of infection include leukocytosis (increased WBC) or leukopenia (decreased WBC).
  • Cultures and Sensitivities: Critical for identifying the pathogen before initiating antibiotic treatment.
  • Elevated Erythrocyte Sedimentation Rate (ESR): Indicates inflammation.

Treatment of Infection

  • Antimicrobial Drugs:
      - Antibacterials
      - Antifungals
      - Antivirals
  • Symptom Reduction:
      - Fluids, rest, and analgesics.

Specific Pathogenic Bacteria and Related Infections

Neisseria meningitidis
  • Type of Infection: Meningitis
Streptococcus pyogenes
  • Types of Infections: Septicemia, Myositis, Necrotizing fasciitis.
Streptococcus pneumoniae
  • Types of Infections: Pneumonia, meningitis, acute otitis media.
Staphylococcus aureus
  • Related Infections: Endocarditis, cellulitis, pneumonia, osteomyelitis, septicemia.
Escherichia coli
  • Type of Infections: Urogenital tract infections, diarrhea.
Salmonella typhi
  • Manifestations: Enterocolitis, bacteremia, typhoid fever.
Pseudomonas aeruginosa
  • Type of Infections: Urinary infections, wound infections.

Urinary Tract Infection (UTI)

  • Pathophysiology: Ascending infection of the urinary tract.
  • Common Pathogen: E. coli is the most common pathogen associated with UTIs.
  • Clinical Manifestations: Dysuria, urgency, frequency, hematuria, cloudy (purulent) urine.
  • Diagnostic Criteria: Include urinalysis and urine culture, especially for leukocyte esterase.

Viral Hepatitis

  • Pathophysiology: Acute or chronic inflammation of the liver caused by infection with one or more hepatitis viruses, transmitted typically via fecal-oral route or contact with blood/body fluids.

  • Types and Transmission:
      | Type | Transmission | Incubation | Carrier State | Chronic | Vaccine |
      |-------------|----------------------------------|--------------|---------------|---------|---------|
      | Hepatitis A | Fecal-oral; contaminated food | 1-2 months | No | No | Yes |
      | Hepatitis B | Blood/fluids; mother to fetus | 2-3 months | Yes | Yes | Yes |
      | Hepatitis C | Blood contact | 2-3 months | Yes | Yes | No |
      | Hepatitis D | Coinfection with Hep B | 2-3 months | Yes | Yes | No |
      | Hepatitis E | Fecal-oral | 1-2 months | No | No | No |

  • Clinical Manifestations:
      - Prodrome: Fatigue, anorexia, low-grade fever.
      - Icterus: Jaundice, hepatomegaly, clay stools, dark urine.
      - Recovery Phase: Improvement with residual hepatomegaly.

Tinea (Fungal Infections)

  • Pathophysiology: Group of superficial fungal infections transmitted via direct contact, affecting keratinized cells.

  • Clinical Manifestations: Vary by type/location:   - Corporis (body): “ringworm”
      - Versicolor: hypopigmentation
      - Capitis (scalp): hair loss/breakage
      - Pedis (feet): maceration between/toes
      - Cruris (groin): erythema, itching
      - Unguium (nails): thickening/discoloration

  • Diagnostic Criteria:
      - History and Physical Examination
      - Microscopic Examination
      - Fungal Cultures
      - Wood Light Examination

  • Treatment:
      - Antifungal drugs (topical and oral)
      - Prevention through proper hygiene and avoiding contact with infected individuals.

Conclusion and Questions

  • Review of key points related to infection and treatment.
  • Invitation for questions regarding the material covered.