In-Depth Notes on Pathogenic Gram-Negative Rods

Other Pathogenic Gram-Negative Rods

Part 3: Pseudomonas, Francisella, Legionella, Coxiella

Pseudomonas aeruginosa

  • General Characteristics:

    • Not a small coccobacilli; not fastidious (grows well on MacConkey agar).
    • Common habitats: water, soil, plants; not part of normal human flora.
    • Aerobic organism requiring oxygen.
    • Oxidase-positive.
    • Produces pigments: green, blue, brown, red (enhance virulence).
    • Commercially used for producing soluble pigments.

  • Pathogenicity:

    • Opportunistic infection if skin is breached; can infect any organ.
    • Problematic in hospitals; can grow in disinfectants, dialysis fluid, whirlpools, respiratory therapy equipment, showers, faucets.
    • Contaminates swimming pools, hot tubs, contact lens solutions.
    • Characteristic odor in culture: tortilla or grape-like.

Infections caused by Pseudomonas aeruginosa

  • Common Clinical Conditions:

    • Swimmer’s ear: most common cause.
    • Eye infections: can lead to blindness.
    • Nosocomial respiratory tract infections: significant concern in cystic fibrosis patients.
    • Infections in burn patients: can cause septicemia, leading to high mortality rates.
    • Septicemia, UTI, pneumonia, chronic lung infections, endocarditis, dermatitis, and osteochondritis (bone inflammation).

  • Skin Infections:

    • Secondary infections in burn patients.
    • Types include pyoderma, hot tub folliculitis, corneal abscess, and invasive infections in immunocompromised individuals.

Pseudomonas aeruginosa & Cystic Fibrosis

  • Cystic Fibrosis:
    • Genetic disease affecting salt channels in cell membranes, leading to thick mucous build-up.
    • Involves multiple organs; primarily affects lungs and GI tract.
    • Colonization: 70-80% of CF patients colonized with mucoid strains of P. aeruginosa, hard to eliminate.
    • Leads to untreatable lung infections; often fatal.

Pathogenesis and Treatment of Pseudomonas aeruginosa

  • Lipid A endotoxin: Causes fever, inflammation, and shock.
  • Pigments:
    • Contribute to coloration and play roles in survival and virulence.
    • Specific pigments include:
    • Pyocyanin (blue)
    • Pyoverdine (yellow-green)
    • Pyorubin (red)
    • Pyomelanin (brown)
  • Defense Mechanisms:
    • Capsule serves as anti-phagocytic shield.
    • Fimbriae assist in biofilm formation.
  • Resistant to disinfectants; can grow in certain soaps.

Francisella tularensis - Tularemia (Rabbit Fever)

  • Overview:

    • A small gram-negative coccobacillus.
    • Transmission methods:
    • Ingesting undercooked infected meat.
    • Bug bites (ticks, deer flies, mosquitoes).
    • Inhalation (pneumonic tularemia is deadly).
    • Mucous membrane exposure (eyes/mouth).
    • Skin exposure leads to lesions and local inflammation.

  • Conditions: Can lead to septicemia, pneumonia, abscesses.

  • Prevalence: Common in certain areas, like Martha’s Vineyard.

  • Fatality Rate:

    • Can be up to 30% if untreated.
    • Treatable with certain antibiotics (excluding beta-lactams).
    • Vaccine available for those at high risk.

Legionella pneumophila

  • Historical Background: Discovered after the 1976 PA American Legion convention outbreak.
  • Characteristics:
    • An aerobic, pleomorphic rod found in various water sources, even chlorinated ones.
    • Infection via aerosolization through water fountains or towers.
  • Clinical Presentation: Begins flu-like, progresses to high fever (up to 105˙F), respiratory failure, and potential death.

Susceptibility to Legionnaires' Disease

  • Risk Factors:
    • Age: Most affected are over 50 years.
    • Gender: Males are more susceptible (2.5:1 ratio).
    • Habits: More frequent in smokers and those with compromised lung health.
    • Immune status: Higher risk in immunocompromised individuals (e.g., on steroids, organ transplants).

Laboratory Isolation of Legionella

  • Characteristics:
    • Thin, faintly staining gram-negative rod; oxidase-positive and motile.
    • Requires L-cysteine for growth; enhanced by iron.
  • Growth Media:
    • Buffered Charcoal Yeast Extract (BCYE) agar is standard.
    • Chocolate agar also works.
  • Culturing Conditions:
    • Incubate for about 5 days at 37°C in a humid atmosphere.
    • Colonies appear ground glass with a greenish speckled look.
  • Alternatives to Culturing: Direct fluorescent antibodies (DFA) stains or serological studies are often used.

Coxiella burnetti

  • Characteristics:
    • Small, aerobic, obligate intracellular parasite.
    • Infects various animal hosts and transmitted through inhalation (not through bites).
  • Diseases:
    • Causes Q Fever, especially in animal handlers.
    • Acute symptoms: Flu-like; chronic may lead to endocarditis after years.
  • Treatment: Doxycycline is the drug of choice.
    • Notable for being highly infectious; can infect a human with a single bacterium.

Anaerobic Gram-Negative Rods

  • Overview:

    • Around 50 species colonize the human body, significantly in the GI, urinary, and reproductive tracts.
    • Predominantly opportunistic pathogens.

  • Examples:

    • Bacteroides: Major contributor to GI diseases; B. fragilis is notable (85% of GI disease cases).
    • Prevotella: Linked to infections in the sinuses, ears, gums, and pelvic sites.