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Microbiology

Bite Wound

This section focuses on the colony morphology and identification of major pathogens associated with bite wound infections. The organisms covered here include Pasteurella multocida and Eikenella corrodens

Pasteurella multocida

  • Colony Morphology
    • Agar Plate
      • Blood Agar
        • Size: Small to medium (0.5-1 mm in diameter)
        • Shape: Circular, convex
        • Color: Grayish, translucent, or slightly opaque
        • Texture: Smooth, sometimes mucoid
        • Odor: Often has a characteristic musty or “mousy” odor
        • Hemolysis: Non-hemolytic
    • Gram Stain
      • Gram-negative coccobacilli or short rods
      • Often exhibit a “bipolar staining” appearance (staining more intensely at the ends of the cells)
  • Identification Methods
    • 1. Presumptive Identification
      • Gram Stain: Gram-negative coccobacilli
      • Colony Morphology: Small, gray colonies with a musty odor on blood agar
      • Catalase Test: Positive
      • Oxidase Test: Positive
    • 2. Definitive Identification
      • Biochemical Tests
        • Glucose Fermentation: Positive
        • Lactose Fermentation: Negative
        • Indole Production: Positive
        • Urease Test: Negative
        • Motility: Non-motile
        • Nitrate Reduction: Positive
      • Commercial Identification Systems
        • API 20E: A miniaturized biochemical test system
        • Vitek 2 System: Automated identification system
        • MALDI-TOF MS: Provides a rapid and accurate identification of microorganisms based on their protein profiles

Eikenella corrodens

  • Colony Morphology
    • Agar Plate
      • Blood Agar
        • Incubation: Requires incubation in a microaerophilic or anaerobic environment
        • Size: Small to medium (0.5-1 mm in diameter)
        • Shape: Circular, convex
        • Color: Grayish, translucent
        • Texture: Smooth, often with a “pitting” or “corroding” appearance (especially after prolonged incubation)
        • Odor: May have a bleach-like odor
        • Hemolysis: Non-hemolytic
    • Gram Stain
      • Gram-negative rods
      • Non-motile
  • Identification Methods
    • 1. Presumptive Identification
      • Gram Stain: Gram-negative rods
      • Colony Morphology: Small, gray, and “pitting” colonies on blood agar
      • Catalase Test: Negative
      • Oxidase Test: Positive
    • 2. Definitive Identification
      • Biochemical Tests
        • Glucose Fermentation: Negative
        • Lactose Fermentation: Negative
        • Nitrate Reduction: Positive
        • Lysine Decarboxylase: Positive
        • Ornithine Decarboxylase: Negative
        • Indole Production: Negative
      • Commercial Identification Systems
        • API NH: A miniaturized biochemical test system
        • Vitek 2 System: Automated identification system
        • MALDI-TOF MS: Provides a rapid and accurate identification of microorganisms based on their protein profiles

Additional Considerations

  • Mixed Cultures: Bite wound infections are often polymicrobial, involving multiple organisms from the animal’s oral flora and potentially other pathogens. The most common organisms isolated from bite wounds are Pasteurella multocida and Eikenella corrodens. Other organisms that may be isolated include:
    • Staphylococcus spp.
    • Beta-hemolytic streptococci
    • Anaerobes: (e.g., Bacteroides, Fusobacterium, Prevotella)
  • Antimicrobial Susceptibility Testing: Perform AST on all significant isolates to guide antibiotic therapy. Pasteurella multocida is typically susceptible to penicillin, amoxicillin-clavulanate, and tetracycline. Eikenella corrodens is typically susceptible to penicillin, ampicillin, and amoxicillin-clavulanate
  • Clinical Correlation: Always correlate the laboratory findings with the patient’s clinical presentation, including the type of animal bite, location, and any other relevant signs and symptoms
  • Reporting: Report the identified organisms, their colony counts (if quantitative cultures were performed), Gram stain morphology, and antimicrobial susceptibility results (if applicable) to the clinician

Key Terms

  • Bipolar Staining: A staining pattern where the bacteria stain more intensely at the ends of the cells
  • Microaerophilic: Requiring or tolerating only a small amount of oxygen
  • Pitting/Corroding: A colony morphology describing a colony that appears to have a small depression or “bite” taken out of it
  • Musty Odor: A characteristic smell sometimes associated with Pasteurella multocida
  • Bleach-like Odor: A characteristic smell sometimes associated with Eikenella corrodens
  • AST: Antimicrobial Susceptibility Testing
  • Anaerobe: A bacterium that grows in the absence of oxygen
  • Polymicrobial: Involving multiple microorganisms