Mycobacteria

Question 1

Direct examination of mycobacteria

Answer 1

Gram stain: typically invisible; gram+ rods ultrastructurally

Visible by acid-fast (Ziehl-Neelsen, Kinyoun) or by auramine rhodamine fluorochrome

Question 2

Mycobacteria culture

Answer 2

• Gold standard
• More sensitive than nucleic acid amplification tests
• Needed for susc testing
• Cultured on liquid broth and solid media
• Grows faster on broth (e.g., Middlebrook)
• In broth, organism can be assessed for cording, which is characteristic of M TB
• Solid medium egg based (e.g., Lowenstein-Jensen) or agar based Middlebrook needed for some orgs; helps detect mixed infection

Question 3

Full identification of mycobacteria isolated in culture

Answer 3

• nucleic acid hybridization probes
• multiplex PCR
• growth characteristics and biochemistry:
  
  • first classified as a rapid grower of slow grower
  • further subclassified based on temperature preference and pigmentation (nonchromogen, scotochromogen, or photochromogen)
  • finally classified according to series of biochemical tests that take days to weeks

Question 4

Important Mycobacterial species in the lung

Answer 4

MAC, M TB, M kansasii, M xenopi, M abscessus

Question 5

Important Mycobacterial species in the lymph node

Answer 5

MAC, M TB, M scrofulaceum, M haemophilum

Question 6

Important Mycobacterial species in the skin and soft tissue

Answer 6

M fortuitum, M chelonae, M abscessus, M marinum, M ulcerans, M haemophilum

Question 7

Important Mycobacterial species in GI tract

Answer 7

M TB, MAC

Question 8

Mycobacterium tuberculosis

• key characteristics
• clinical features

Answer 8

• slow growth
• flat, dry, white, wrinkled colonies on solid media
• prefers 37 degrees
• cording in broth
• NAP sensitivity
• nonchromogenic
• Clinical features
  
  • tuberculosis can be causes by
    
    • M tuberculosis
    • M microti
    • M bovis
    • M africanum
    • M canetti
  • Spread by respiratory droplets or aerosols
  • primay infection = pulmonary and then may undergo
    
    • spontaneous eradication
    • undergo resolution (latency), forming tubercles (caseating granulomas) in upper lobes
    • produce active disease
    • reactivation presents as active disease
      
      • may be pulmonary or extrapulmonary: kidneys, bone, GI, meninges

Question 9

Adjunctive diagnostic tests for M tuberculosis

Answer 9

• Pleural effusion
  
  • Smear
  • culture
  • NAAT
  • Adenosine deaminase (ADA) test
• Gastric aspiration for those without adequate respiratory specimen
• tuberculin skin test (TST/PPD): positive may signify
  
  • active TB
  • latent TB
  • nontuberculous mycobacteria infection
  • vaccination with BCG
• False negative 2/2 anergy can occur in HIV
• IFN-gamma release assays: T cells from person with TB release IFN-gamma when stimulated in vitro to ESAT-6 and CFP-10
• Arysulfatase separates fast growing mycobacteria (M fortuitum and chelonae are pathogenic and positive while M phlei and M smegmatis are not positive or pathogenic)

Question 10

Nontuberculous mycobacteria

Answer 10

Isolation does not necessarily indicate infection

Question 11

Mycobacterium avium complex

Answer 11

• M avium and M intracellulare
• infection in immunocompromised and immunocompetent
• Amongst immunocompetent, 3 forms of disease:
  
  • heavy smokers with upper lobe cavitary disease, resembles classical Tb
  • “Lady Windermere syndrome” seen in elderly women with weak cough; akin to colonization
  • Hypersensitivty reaction to MAC after exposure to hot tub water contaminated with organisms
• MAC is most common cause of scrofula
• In culture, slow growing and may be pigmented or nonpigmented

Question 12

M kansasii

Answer 12

• Infection resembles Tb in patients with immunosuppression or underlying pulmonary disease such as pneumoconiosis
• Slow growing photochromogen

Question 13

M marinum

Answer 13

Associated with wound exposure to fresh water fish tanks or salt water, and causes localized cutaneous infection (fishtank granuloma)

Question 14

M ulcerans

Answer 14

indolent, necrotizing, ulcerating cutaneous lesions (Buruli ulcer)

Question 15

M leprae

• disease
• geographic distribution
• diagnosis
• stain

Answer 15

• Leprosy (Hansen disease)
• Hawaii, Texas, Louisiana (harbored by armadillos)
• cannot be cultured on artificial media
• best seen with Fite stain on tissue bx

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Question 20

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Question 26

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