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Microbiology

Skin, Soft Tissue, & Bone

This comprehensive overview synthesizes the key aspects of bacteriological analysis for skin, soft tissue, and bone infections, covering specimen handling, identifying normal flora, recognizing major pathogens, and understanding their disease-causing mechanisms

Specimen Sources & Collection

  • Specimen Sources
    • Skin: Wounds, abscesses, biopsies, skin scrapings (rarely for bacteria)
    • Soft Tissue: Wounds, abscesses, aspirates, biopsies
    • Bone: Biopsies, aspirates, bone fragments (from surgery)
    • Bite Wounds: Aspirates, tissue samples, swabs
  • Collection Techniques
    • Aseptic Technique: Essential to minimize contamination
    • Wounds: Swabs (superficial), aspiration (abscesses), tissue biopsy (deeper infections)
    • Abscesses: Aspiration (preferred), incision & drainage
    • Bone: Biopsy (preferred), aspiration (less common)
    • Bite Wounds: Aspiration, tissue biopsy
  • Specimen Transport & Storage
    • Prompt Delivery: To the lab
    • Transport Media: If delays are anticipated (e.g., Amies transport medium)
    • Refrigeration: If there is a delay (4°C)

Indigenous Flora: Colony Morphology & Gram Stain

  • Purpose: Differentiate between normal, harmless bacteria and potential pathogens
  • Skin Flora
    • Staphylococcus epidermidis
      • Gram Stain: Gram-positive cocci, clusters
      • Colony: White/cream, smooth, non-hemolytic
    • Corynebacterium spp. (Diphtheroids)
      • Gram Stain: Gram-positive rods, pleomorphic (variable shapes)
      • Colony: Small, variable appearance
    • Propionibacterium acnes
      • Gram Stain: Gram-positive rods
      • Colony: Small, often requires anaerobic conditions
    • Micrococcus spp.
      • Gram Stain: Gram-positive cocci, tetrads or clusters
      • Colony: Yellow, smooth
    • Malassezia spp. (yeast)
      • Gram Stain: Yeast cells, may see hyphae
      • Colony: Cream-colored, “yeasty” odor
    • Acinetobacter spp.
      • Gram Stain: Gram-negative coccobacilli
      • Colony: Can be mucoid
  • Soft Tissue Flora: Similar to skin, but less diverse, often includes anaerobes
  • Bone Flora: Normally sterile; any growth is significant

Colony Morphology and Identification of Major Pathogens

  • Key Pathogens (and characteristic features)
    • Staphylococcus aureus
      • Gram Stain: Gram-positive cocci, clusters
      • Colony: Golden-yellow, beta-hemolytic, mannitol-positive
      • Identification: Coagulase positive, Latex agglutination positive
    • Beta-Hemolytic Streptococci (e.g., Streptococcus pyogenes)
      • Gram Stain: Gram-positive cocci, chains
      • Colony: Beta-hemolytic
      • Identification: Group A (Bacitracin susceptible, PYR positive)
    • Pseudomonas aeruginosa
      • Gram Stain: Gram-negative rods
      • Colony: Greenish pigment, grape-like odor, oxidase positive
    • Pasteurella multocida
      • Gram Stain: Gram-negative coccobacilli
      • Colony: Small, musty odor, oxidase positive
    • Eikenella corrodens
      • Gram Stain: Gram-negative rods
      • Colony: Pitting or corroding appearance, oxidase positive
    • Bacillus anthracis
      • Gram Stain: Gram-positive rods, spores
      • Colony: “Medusa head” appearance, non-motile
      • Identification: Gamma phage lysis
    • Brucella spp.
      • Gram Stain: Gram-negative coccobacilli
      • Colony: Small, slow-growing, CO2 required
    • Francisella tularensis
      • Gram Stain: Gram-negative coccobacilli, very small
      • Colony: Requires enriched media (chocolate agar)
    • Yersinia pestis
      • Gram Stain: Gram-negative coccobacilli, bipolar staining
      • Colony: “Fried egg” appearance
  • Identification Methods
    • Gram Stain: Initial Gram reaction and morphology
    • Catalase Test: Differentiates Staphylococcus (positive) from Streptococcus (negative)
    • Oxidase Test: Differentiates Pseudomonas (positive) from Enterobacteriaceae (usually negative)
    • Other Rapid Tests: Coagulase, Bacitracin susceptibility, PYR
    • Biochemical Tests: Sugar fermentation, enzyme production, etc
    • Commercial Identification Systems: API, Vitek 2, MALDI-TOF MS
    • Molecular Methods: PCR

Organism Pathogenicity

  • Staphylococcus aureus
    • Etiology: Gram-positive cocci, clusters
    • Transmission: Direct contact, autoinfection, healthcare-associated
    • Virulence: Adhesins, enzymes (coagulase, hyaluronidase), toxins (ETs, TSST-1, PVL), biofilm formation
  • Beta-Hemolytic Streptococci
    • Etiology: Gram-positive cocci, chains (Lancefield groups)
    • Transmission: Direct contact, respiratory droplets
    • Virulence: Adhesins, M protein, enzymes (streptolysins, hyaluronidase), toxins (SPEs)
  • Pseudomonas aeruginosa
    • Etiology: Gram-negative rod, opportunistic
    • Transmission: Environmental, healthcare-associated
    • Virulence: Adhesins, capsule, enzymes, toxins (exotoxin A, pyocyanin), biofilm formation
  • Pasteurella multocida
    • Etiology: Gram-negative coccobacillus
    • Transmission: Animal bites
    • Virulence: Capsule, enzymes
  • Eikenella corrodens
    • Etiology: Gram-negative rod
    • Transmission: Human bites
    • Virulence: Adhesins, enzymes, LPS
  • Bacillus anthracis
    • Etiology: Gram-positive rod, spore-forming
    • Transmission: Contact, inhalation, ingestion
    • Virulence: Capsule, anthrax toxin
  • Brucella spp.
    • Etiology: Gram-negative coccobacilli
    • Transmission: Ingestion, contact, inhalation
    • Virulence: Intracellular survival
  • Francisella tularensis
    • Etiology: Gram-negative coccobacillus
    • Transmission: Tick bites, contact, inhalation
    • Virulence: Intracellular survival
  • Yersinia spp. (Y. pestis)
    • Etiology: Gram-negative coccobacilli
    • Transmission: Flea bites, respiratory droplets
    • Virulence: Capsule, T3SS, Pla