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Microbiology

Organism Pathogenicity

Understanding the pathogenicity of microorganisms that cause infections in normally sterile body fluids is critical for accurate diagnosis, effective treatment, and prevention of serious complications. Pathogenicity is defined as the ability of a microbe to cause disease, and it is determined by the pathogen’s capacity to:

  1. Colonize: a host
  2. Invade: tissues
  3. Evade: the host’s immune defenses
  4. Produce: toxins or other harmful substances

Organisms from Normally Sterile Sites

Streptococcus pneumoniae

  • Etiology: Gram-positive, lancet-shaped diplococci
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis
    • Adhesins: Aid in attachment to host cells
    • Pneumolysin: Damages host cells
    • IgA Protease: Degrades host antibodies
  • Pathogenicity
    • Pneumonia, meningitis, and septicemia
    • In body fluids: Pleural empyema, septic arthritis
    • Inflammation and tissue damage
  • Pathogenesis: Entry into the body, spread to the normally sterile site

Haemophilus influenzae

  • Etiology: Gram-negative coccobacilli or pleomorphic rods
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Antiphagocytic, primarily type b (Hib)
    • Adhesins: Promote adherence to host cells
    • Endotoxin (LPS): Triggers inflammation
  • Pathogenicity
    • Meningitis, septic arthritis, and other invasive infections
    • Inflammation and tissue damage
    • Can cause empyema, septic arthritis
  • Pathogenesis: Colonization, invasion of the bloodstream, and spread to a sterile site

Neisseria meningitidis

  • Etiology: Gram-negative diplococci (kidney bean-shaped)
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis
    • Adhesins: Promote adherence
    • Endotoxin (LPS): Triggers inflammation and sepsis
    • Outer membrane proteins: Adherence and immune evasion
  • Pathogenicity
    • Meningitis, sepsis (meningococcemia)
    • Can cause septic arthritis
    • Inflammation, damage to tissues, septic shock
  • Pathogenesis: Spread to the meninges or joints

Escherichia coli

  • Etiology: Gram-negative rod
  • Transmission: Fecal-oral route or contaminated medical devices
  • Virulence Mechanisms
    • Adhesins: Fimbriae
    • Capsule: Prevents phagocytosis
    • Endotoxin (LPS): Triggers inflammation and sepsis
    • Exotoxins: Produce various toxins, depending on the strain
  • Pathogenicity
    • Bacteremia, peritonitis, septic arthritis, and other infections
    • Neonatal meningitis
    • Inflammation, tissue damage, sepsis
  • Pathogenesis: Entry into the body (from the gut), spread to a sterile site. Can be a primary infection or arise from another infection

Listeria monocytogenes

  • Etiology: Gram-positive, short rods or coccobacilli
  • Transmission: Ingestion of contaminated food
  • Virulence Mechanisms
    • Internalins: Promote invasion
    • Listeriolysin O (LLO): Pore-forming toxin
    • ActA: Promotes actin-based motility
  • Pathogenicity
    • Meningitis (especially in immunocompromised, pregnant women, neonates)
    • Can cause septic arthritis
    • Can cause amniotic fluid infection
    • Inflammation and tissue damage
  • Pathogenesis: Ingestion, invasion of the bloodstream, spread to the CNS

Enterobacteriaceae

  • Etiology: Gram-negative rods (e.g., Klebsiella, Proteus, etc.)
  • Transmission: Various (e.g., healthcare-associated, fecal-oral)
  • Virulence Mechanisms
    • Adhesins: Facilitate attachment
    • Capsule: Prevents phagocytosis
    • Endotoxin (LPS): Triggers inflammation
    • Siderophores: Acquire iron
  • Pathogenicity
    • Peritonitis, septic arthritis, and other infections
    • Sepsis, inflammation, and tissue damage
  • Pathogenesis: Entry into a sterile site, can be a primary infection or spread from another infection

Staphylococcus aureus

  • Etiology: Gram-positive cocci in clusters
  • Transmission: Direct contact or contaminated medical devices
  • Virulence Mechanisms
    • Adherence: Surface proteins
    • Enzymes: Coagulase, hyaluronidase
    • Immune Evasion: Protein A
    • Toxins: Cytotoxins, superantigens
  • Pathogenicity
    • Bacteremia, septic arthritis, empyema, and peritonitis
    • Wide variety of other infections
    • Sepsis, tissue damage, and severe inflammation
    • Forms biofilms on medical devices
  • Pathogenesis: Direct entry, through medical devices, or spread from another infection

Beta-Hemolytic Streptococci

  • Etiology: Gram-positive cocci in chains (e.g. Streptococcus agalactiae, Group B; Streptococcus pyogenes, Group A)
  • Transmission: Various
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis (GBS)
    • Adhesins: Promote attachment
    • Exotoxins: Streptolysin S, etc
  • Pathogenicity
    • Septic arthritis, empyema, peritonitis, and sepsis
    • GBS is a major cause of neonatal sepsis
    • Amniotic fluid infection
    • Inflammation and tissue damage
  • Pathogenesis: Entry via breaches in skin or mucosal surfaces, or through medical procedures

Enterococcus spp.

  • Etiology: Gram-positive cocci in pairs or short chains
  • Transmission: Fecal-oral route
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Biofilm Formation: Forms biofilms on medical devices
    • Cytolysin: A pore-forming toxin
    • Antibiotic Resistance: High resistance to multiple antibiotics
  • Pathogenicity
    • Peritonitis, septic arthritis, and empyema
    • Sepsis, inflammation, and endocarditis
  • Pathogenesis: Entry from the gut, medical devices, or via a contaminated environment

Pseudomonas aeruginosa

  • Etiology: Gram-negative rod
  • Transmission: Environmental (water, soil, contaminated surfaces), healthcare settings
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Capsule: Prevents phagocytosis
    • Exotoxins: Exotoxin A
    • Enzymes: Proteases, elastases
    • Biofilm Formation: Forms biofilms on medical devices
    • Antibiotic Resistance: Significant resistance
  • Pathogenicity
    • Peritonitis, septic arthritis, empyema, and sepsis
    • Difficult to treat
    • Inflammation, tissue damage, and antibiotic resistance
  • Pathogenesis: Entry from the environment, or via medical devices

Acinetobacter spp.

  • Etiology: Gram-negative coccobacilli or short rods
  • Transmission: Healthcare settings, contaminated surfaces
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Biofilm Formation: Forms biofilms
    • Antibiotic Resistance: Significant resistance
  • Pathogenicity
    • Peritonitis, septic arthritis, empyema, and sepsis
    • Difficult to treat
    • Inflammation and tissue damage
  • Pathogenesis: Entry from the environment or healthcare settings

Clostridium perfringens

  • Etiology: Gram-positive rod
  • Transmission: Wound contamination, or from the gut
  • Virulence Mechanisms
    • Exotoxins: Many, including alpha-toxin (phospholipase C)
  • Pathogenicity
    • Peritonitis, wound infections, myonecrosis
    • Gas gangrene, severe tissue destruction
  • Pathogenesis: Entry via a wound, or through a breach of the gut

Bacteroides fragilis group

  • Etiology: Gram-negative rod
  • Transmission: From the gut
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis
    • Adhesins: Promotes attachment
    • Enzymes: Proteases
  • Pathogenicity
    • Peritonitis, and intra-abdominal abscesses
    • Can cause empyema
    • Tissue destruction
  • Pathogenesis: Breach of the gut wall. Often polymicrobial infection

Implications of Pathogenicity

  • Clinical Presentation: Determines the signs and symptoms (e.g., fever, pain, swelling, altered mental status)
  • Diagnosis: Guides selection of appropriate diagnostic tests (e.g., Gram stain, cultures, molecular methods)
  • Treatment: Determines appropriate antibiotic selection, or antifungal, or antiviral
  • Prevention: Enables targeted infection control measures

Key Terms

  • Pathogenicity: The ability of a microorganism to cause disease
  • Virulence: The degree of pathogenicity
  • Etiology: The cause of a disease
  • Transmission: How an organism spreads
  • Adhesins: Surface structures or proteins that facilitate the attachment
  • Capsule: A polysaccharide or protein layer that protects against phagocytosis
  • Endotoxin (LPS): A component of the outer membrane of Gram-negative bacteria that triggers a strong inflammatory response
  • Exotoxins: Toxins secreted by bacteria
  • Biofilm: A community of microorganisms that adhere to a surface and are encased in a matrix of extracellular substances
  • Sepsis: A life-threatening organ dysfunction caused by a dysregulated host response to infection
  • Antimicrobial Resistance: The ability of bacteria to survive and multiply in the presence of antibiotics
  • Amniotic Fluid: Fluid that surrounds the fetus
  • Empyema: Pus in the pleural space
  • Peritonitis: Inflammation of the peritoneum
  • Arthritis: Inflammation of a joint
  • Septic Arthritis: Joint infection
  • Amniotic fluid infection: Infection of the amniotic fluid
  • Intracellular pathogen: A pathogen that can survive in the cells
  • Breach: A break or rupture