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Microbiology

Common Contaminant

This section covers the characteristics that help identify a common contaminant, Mycobacterium gordonae, and differentiate it from true pathogens

General Principles

  • Ubiquitous in the Environment: M. gordonae is widely distributed in the environment, particularly in water sources (tap water, distilled water, etc.). This makes it a common contaminant in clinical specimens
  • Low Pathogenicity: M. gordonae is generally considered to be of low pathogenicity, meaning it rarely causes disease in humans. Most isolates represent colonization or contamination rather than true infection
  • Clinical Significance: Distinguishing M. gordonae from pathogenic mycobacteria is crucial to avoid unnecessary treatment and anxiety for patients

Mycobacterium gordonae

  • Common Name: “Tap water bacillus”
  • Clinical Significance: Typically a contaminant; rarely causes disease
  • Diseases: Very rarely, M. gordonae has been implicated in pulmonary infections, skin infections, and disseminated infections, primarily in immunocompromised individuals

Acid-Fast Reaction

  • Acid-Fast: Strongly acid-fast. Stains readily with carbolfuchsin and resists decolorization with acid-alcohol

Colony Morphology

  • Egg-Based Media (e.g., Löwenstein-Jensen)
    • Smooth, round, dome-shaped colonies
    • Typically produce a deep yellow to orange pigment, even in the dark (scotochromogenic)
    • The intensity of the pigment can vary between strains
  • Agar-Based Media (e.g., Middlebrook 7H10/7H11)
    • Similar to egg-based media, but the colonies may appear slightly more transparent

Growth Characteristics

  • Growth Rate
    • Relatively rapid-growing compared to M. tuberculosis. Colonies typically appear within 3-7 days on suitable media
  • Temperature
    • Grows well at 25-37°C
  • Other Characteristics
    • Catalase: Positive
    • Tween 80 Hydrolysis: Positive
    • Urease: Variable

Differentiation from Pathogenic Mycobacteria

  • Clinical Context: The most important factor is the clinical context. M. gordonae is unlikely to be a true pathogen unless there is strong evidence of infection (e.g., consistent isolation from multiple specimens, histopathological evidence of tissue invasion)
  • Scotochromogenicity: The deep yellow to orange pigment produced in the dark is a key characteristic. While some other mycobacteria can produce pigment, M. gordonae’s pigment is typically more intense and consistent
  • Biochemical Tests
    • Tween 80 Hydrolysis: This test differentiates M. gordonae from other scotochromogenic mycobacteria
  • Molecular Methods
    • PCR and Sequencing: These methods provide definitive identification and can differentiate M. gordonae from closely related species
  • Lipid analysis (HPLC): useful in identifying different mycobacteria

When to Suspect Contamination

  • Single Positive Culture: A single positive culture of M. gordonae, especially from a non-sterile site (e.g., sputum), is highly suggestive of contamination
  • Lack of Clinical Correlation: If the patient has no signs or symptoms of mycobacterial infection, contamination is more likely
  • Isolation from Water Sources: If the same strain of M. gordonae is isolated from laboratory water sources, contamination is highly probable

Laboratory Practices to Minimize Contamination

  • Sterile Water: Use only sterile, distilled water for media preparation and other laboratory procedures
  • Regular Water Testing: Routinely test laboratory water sources for mycobacteria
  • Proper Specimen Collection: Emphasize proper specimen collection techniques to minimize environmental contamination
  • Quality Control: Implement strict quality control procedures to monitor for contamination in media and reagents

Key Terms

  • Contaminant: A microorganism that is unintentionally introduced into a specimen or culture
  • Scotochromogen: A microorganism that produces pigment in the dark
  • Pathogenicity: The ability of a microorganism to cause disease
  • Colonization: The presence of a microorganism on or in a host without causing disease
  • Tween 80 Hydrolysis: A biochemical test used to determine the ability of an organism to hydrolyze Tween 80, a nonionic surfactant
  • Catalase Test: A biochemical test used to detect the presence of the enzyme catalase, which catalyzes the decomposition of hydrogen peroxide into water and oxygen
  • Urease Test: A biochemical test used to determine the ability of an organism to produce the enzyme urease, which hydrolyzes urea into ammonia and carbon dioxide
  • PCR (Polymerase Chain Reaction): A molecular biology technique used to amplify specific DNA sequences
  • Sequencing: A molecular biology technique used to determine the nucleotide sequence of DNA
  • HPLC (High-Performance Liquid Chromatography): An analytical chemistry technique used to separate, identify, and quantify the components in a mixture