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Microbiology

Less Common Pathogens

This section covers the characteristics of some of the less frequently encountered Mycobacterium species: M. leprae, M. haemophilum, and M. scrofulaceum. While not as common as M. tuberculosis or MAC, it’s important to be aware of these organisms and their unique features

General Principles

  • Clinical Presentation: These species often present with distinct clinical manifestations, which can provide clues to their identification
  • Special Growth Requirements: Some of these species have specific growth requirements that must be met for successful isolation in the laboratory
  • Geographic Distribution: The prevalence of these species can vary depending on geographic location

Mycobacterium leprae

  • Disease: Causes leprosy (Hansen’s disease), a chronic infectious disease affecting the skin, peripheral nerves, mucosa of the upper respiratory tract, and eyes
  • Acid-Fast Reaction: Strongly acid-fast
  • Colony Morphology
    • M. leprae has never been successfully grown on artificial media in the laboratory. It is an obligate intracellular parasite
    • Diagnosis relies primarily on clinical presentation, skin smears, and histopathology
  • Growth Characteristics
    • Cannot be cultured on standard laboratory media
    • Grows very slowly in the footpads of mice or in armadillos
    • Optimal growth temperature: 30-33°C
  • Epidemiology
    • Primarily found in tropical and subtropical regions
    • Transmission is thought to occur through respiratory droplets, but the exact mechanism is not fully understood
  • Diagnosis
    • Clinical Examination: Look for characteristic skin lesions, nerve damage, and sensory loss
    • Skin Smears: Acid-fast bacilli can be detected in skin smears from affected areas
    • Histopathology: Biopsy of skin or nerve tissue can reveal the presence of acid-fast bacilli and characteristic granulomas
    • Molecular Methods: PCR can be used to detect M. leprae DNA in clinical specimens

Mycobacterium haemophilum

  • Disease: Causes skin and soft tissue infections, particularly in immunocompromised individuals. Can also cause disseminated infections
  • Acid-Fast Reaction: Acid-fast
  • Colony Morphology
    • Small, smooth, translucent colonies
    • May have a slightly pinkish or yellowish tint
  • Growth Characteristics
    • Requires the addition of ferric ammonium citrate or hemin (iron) to the culture medium for growth
    • Slow-growing: Colonies may take 2-4 weeks to appear
    • Optimal growth temperature: 30-32°C
  • Epidemiology
    • Relatively rare
    • Found worldwide
    • Risk factors include:
      • Immunocompromise (e.g., HIV/AIDS, organ transplant recipients)
      • Hemodialysis
  • Diagnosis
    • Culture: Requires media supplemented with iron
    • Acid-Fast Stain: Detect acid-fast bacilli in clinical specimens
    • Molecular Methods: PCR can be used to detect M. haemophilum DNA

Mycobacterium scrofulaceum

  • Disease: Causes scrofula (cervical lymphadenitis), primarily in children
  • Acid-Fast Reaction: Acid-fast
  • Colony Morphology
    • Smooth, round, dome-shaped colonies
    • Produces a yellow to orange pigment in the dark (scotochromogenic)
  • Growth Characteristics
    • Slow-growing: Colonies may take 2-3 weeks to appear
    • Optimal growth temperature: 25-37°C
  • Epidemiology
    • Found worldwide
    • More common in children than adults
    • Associated with exposure to contaminated water
  • Diagnosis
    • Culture: Isolate M. scrofulaceum from lymph node aspirates or biopsies
    • Acid-Fast Stain: Detect acid-fast bacilli in clinical specimens
    • Clinical Presentation: Consider scrofula in children with cervical lymphadenitis
    • Molecular Methods: PCR can be used to detect M. scrofulaceum DNA

Key Terms

  • Obligate Intracellular Parasite: An organism that can only survive and reproduce inside a host cell
  • Scrofula: Cervical lymphadenitis caused by mycobacteria, typically Mycobacterium scrofulaceum
  • Hemin: An iron-containing porphyrin complex that is essential for the growth of some bacteria, including Mycobacterium haemophilum
  • Ferric Ammonium Citrate: An iron salt that can be used as a source of iron for bacterial growth
  • Lymphadenitis: Inflammation of the lymph nodes
  • Granuloma: A mass of immune cells that forms when the body attempts to wall off substances it perceives as foreign but cannot eliminate
  • Skin Smear: A sample of skin tissue or fluid that is spread on a glass slide and stained for microscopic examination
  • Histopathology: The microscopic examination of tissue to diagnose disease
  • Molecular Methods: Techniques that use DNA or RNA to identify microorganisms, such as PCR and sequencing
  • PCR (Polymerase Chain Reaction): A molecular biology technique used to amplify specific DNA sequences
  • Skin Lesions: Abnormal changes or growths on the skin, such as sores, bumps, or rashes
  • Nerve Damage: Injury to the nerves that can result in pain, numbness, weakness, or loss of function
  • Sensory Loss: A decrease or absence of the ability to feel sensations, such as touch, pain, temperature, or vibration
  • Disseminated Infection: An infection that has spread from a localized area to other parts of the body
  • Cervical Lymphadenitis: Inflammation of the lymph nodes in the neck
  • Lymph Node Aspirate/Biopsy: A procedure in which a sample of tissue or fluid is removed from a lymph node for examination
  • Tropical and Subtropical Regions: Geographic areas located near the equator that have warm and humid climates
  • Translucent Colonies: Colonies that allow light to pass through, making them appear clear or partially transparent