Chapter 24

• Actinomycetes


  Also called "actinobacteria"
• Actinomycetes


  • Aerobic, Gram-positive bacteria
  • Form branching filaments or "mycelium", some genera grow as rods/cocci
  • Usually non-motile
  • Widely distributed in soil = major taxa in soil
  • Produce most of medically important natural antibiotics
  • Most are saprophytes, a few genera contain strains which are pathogenic to humans, other animals, and plants
• Pathogenic Actinobacteria

  • Mycobacterium
  • Nocardia
  • Corynebacterium

  
• Actinomycetoma

  • Mycetoma (Madura foot) - localized, slowly progressive, chronic infection that begins in the subcutaneous tissue and spreads to adjacent tissues
  • Destructive and often painless
  • Most often caused by soil fungi
  • Actinomycetoma is a mycetoma caused by filamentous, branching bacteria

  
• Actinomycosis


  • Rare, sub-acute to chronic, suppurative and granulomatous infection that produces pyogenic lesions
  • Caused by several closely related members of the normal flora of the mouth and gastro-intestinal tract

  
• Causes of Actinomycosis


  • Actinomyces israelii
  • Actinomyces gerencseriae
• Suborder Corynebacterineae


  • Acid-fast, nocardioform
  • Produce mycolic acids (waxy layer, virulence factor and makes it difficult to kill the microbe)
• Genera in Suborder Corynebacterineae

  • Corynebacterium
  • Mycobacterium
  • Nocardia

  
• Genus Nocardia

  • Develops substrate mycelia that readily break into rods or coccoid forms
  • Aerial mycelium can develop conidia (sacs of spores)
  • Most are free- living saprophytes
  • Some are opportunistic pathogens causing nocardiosis
• Nocardiosis


  • Sub-acute to chronic lung infection that may disseminate to other organs (usually brain and skin)
  • Infection is initiated by inhalation of bacteria

  
• Causes of Nocardiosis


  • Nocardia asteroides complex
  • N. brasiliensis
• Genus Mycobacteria


  • Obligate aerobes, rods that do not form spores
  • Grow very slowly when cultured - 2- 40 days (special media with Ab)
  • Cell walls have high lipid content and waxes with 60- 90 mycolic acids
  • They are acid- fast - basic fuchsin dye cannot be removed by acid or alcohol treatment. Doesn't stain with Gram stain must therefore do a Ziehl Neelsen stain
• Mycobacterium species


  • M. bovis - tuberculosis (TB) in cattle and other ruminants
  • M. tuberculosis - tuberculosis (TB) in humans
  • M. leprae - leprosy
  • M. avium-intracellulare or M. avium complex (MAC) and other atypical mycobacteria frequently infect patients with AIDS

  
• Mycobacterium tuberculosis (MTB)


  • MTB bacilli are thin, straight rods measuring 0.4 x 3µm
  • Mycobacteria are more resistant to chemical agents than other bacteria
  • Acid fast - do not Gram stain "Gram-invisible"
  • Often cultured on "Middlebrook 7H10 or 7H11 agar, or 7H9 broth
  • MTB has some unique virulence factors
  • Mycobacterial cell walls are rich in lipids that include mycolic acids, waxes and phosphatides (waxy layer makes them resistant to water-soluble antibiotics), proteins that elicit the tuberculin reaction, and polysaccharides that cause hypersensitivity immune response

  
• Spread of Tuberculosis


  1. Bacteria in droplets are inhaled

  2. Phagocytosed by macrophages

  3. 4-12 weeks

  4. Primary TB

  5. Tubercles (mature into granulomas)

  6. MDR-TB

  7. XDR-TB

  
• Global incidence of TB (2021)
• Mycobacterium avium complex (MAC)


  • Often called the MAC or MAI
  • Ubiquitous in environment
  • Rarely cause disease in immunocompetent hosts
  • In the USA disseminated MAC is one of the most common opportunistic infections of bacterial origin in AIDS patients
• Other Mycobacteria


  • M. kansasii - can produce lung and systemic disease indistinguishable from TB especially in patients which are immunocompromised
  • M. scrofulaceum - chronic lymphadenitis in children
  • M. marinum, M. ulcerans - superficial skin lesions (ulcers, swimming pool granulomas)
  • M. fortuitum - superficial and systemic disease in humans

  
• Mycobacterium leprae


  • Causative agent of leprosy (leper is a person suffering from the disease)
  • Still not been cultured on non-living bacteriological media
  • Global decline from 5.2 million in 1980s to 200,000 in 2020 - mainly in Asia
  • Despite stigmatism it is not highly contagious, spread via respiratory route
  • Onset is insidious
  • Involve the coolest parts of the body - skin, superficial nerves, nose, pharynx, larynx, eyes and testicles
  • Disfigurement due to skin infiltration and nerve involvement in untreated cases can be extreme