Mycobacteria

Question 1

Features of mycobacteria

Answer 1

1. Acid Fast Bacilli
   - Ziehl-Neelsen stain, uses heat to drive stain into cells
   - mycobacteria stained with fuchsin withstand discolourisation w acid & alcohol
   - or fluorescent dye auramine, strong affinity for waxy cell wall
2. Slow growing, strict aerobes
3. Waxy cell wall
   - peptidoglycan layer has different chemical basis for crosslinking to lipoprotein layer
   - outer envelope contains a variety of complex lipids (mycolic acids)
   - resistant to drying & other environmental factors + pronounced adjuvant activity (promote immunologic responsiveness)

Question 2

Examples of mycobacteria

Answer 2

Obligate pathogens
1. Mycobacterium tuberculosis complex - M. tuberculosis, M. africanum, M. canetti, M. bovis, M. microti
2. Mycobacteria leprae
Environmental saprophytes 
3. Non-tuberculous mycobacteria

Question 3

Transmission of M tb

Answer 3

Respiratory route (infective droplet nuclei), only need 1-5 bacilli in terminal alveolus

Question 4

Virulence factors of M tb

Answer 4

Cord factor - inhibits acidificatio of phagolysosome - AFB survives & multiplies in macrophages

Question 5

Pathogenesis of M tb

Answer 5

Cell mediated immune response

• Stage 1: contact w macrophage, survival & multiplication of AFB within macrophage
• Stage 2: blood monocytes attracted to site of infection, differentiate into macrophages but unable to kill AFB
• Stage 3: influx of antigen-specific T cells, secrete IFN-

Question 6

Types of TB

Answer 6

1. Primary TB
2. Post primary TB
3. Miliary TB

Question 7

Features of primary TB

Answer 7

• often clinically silent
• result of primary infection in a non-immune host
• Ghon focus = small foci of inflammation consisting of few MTC surrounded by a dense granuloma in the lung
• Primary complex = Ghon focus + enlarged regional lymph nodes

Question 8

Features of post primary TB

Answer 8

• months/years after pri TB, due to reinfection/reactivation of latent TB
• can be in any organ
• large exuberant granulomata w central cheesy caseous necrosis
• in lungs, necrotic tissue coughed away - leaving cavities

Question 9

Features of miliary TB

Answer 9

• when MTC is disseminated via bloodstream, affecting many organs
• multiple granulomata visible in organs macroscopically as small white nodules (like millet seeds)

Question 10

Clinical presentations of TB (4)

Answer 10

1. Systemic features - fever, weight loss
2. Pulmonary - chronic cough, hemoptysis, dyspnea, pleuritic chest pain, pleural effusion
3. CNS - TB meningitis, GIT, bone & joint TB, miliary
4. Lymph node involvement

Question 11

Diagnosis of TB (7)

Answer 11

1. Haematology: FBC (non specific)
2. Biochemical: adenosine deaminase (secreted by T cells) of pleural/peritoneal fluid, CSF cell count (less glucose more protein, lymphocytic instead of neutrophils), urine analysis (sterile pyuria, a lot of pus cells)
3. Imaging (CXR, CT/MRI brain/spine)
4. Culture of AFB - early morning sputum/NG aspirate, first pass urine, CSF, biopsy (lymph nodes, bone, pleura), fluid (peritoneal, pleural, pericardial)
   - in solid media - egg based LJ media or broth based, incubate at 35-37C for up to 8 weeks
5. Microscopy - acid gast/auramine smear, caseating granulomas & AFB
6. Molecular - NAATs, PCR, isothermal amplification, DNA hybridisation
7. Mantoux Testing (latent TB, normal CXR) - inoculation of purified protein derivative intradermally into forearm, read diameter of induration after 48-72h

Question 12

Treatment & management of TB (2)

Answer 12

1. Anti-TB drugs
   - standard therapy: 2m of daily R+I+P, 4m of daily R+I
   - pyridoxine + isoniazid to prevent neuropathy
   - latent TB: 6-9m of I/4m of R
   - 2nd line is toxic & less effective, used in resistance
2. Infection control - isolate pt, contact tracing, TB is notifiable

Question 13

Transmission of M. leprae

Answer 13

• found within macrophages in dense clumps, very slow growing

Close & prolonged contact (resp route most likely)

Question 14

Pathogenesis of leprosy

Answer 14

1. Chronic granulomatous infection targeting Schwann cells of peripheral nerves
2. Nerve damage: paralysis, anaesthesia, trophic ulcers, neuropathic joint disease (Charcot)

Question 15

Clinical presentations of leprosy

Answer 15

1. Initial presentation is mild - hypopigmented & hypoaesthetic skin lesion
2. Look for subtle nerve damage - nerve tenderness/thickening, muscle weakness

Question 16

Types of leprosy

Answer 16

1. Tuberculoid leprosy
2. Lepromatous leprosy
3. Borderline leprosy

Question 17

Features of tuberculoid leprosy

Answer 17

• patients who mount a strong cell mediated immune response (Th1)
• 1 or 2 hypopigmented skin lesions, thickening of peripheral nerves
• biopsy of skin lesions rarely show bacilli

Question 18

Features of lepromatous leprosy

Answer 18

• patients w absent cell mediated immune response (predominantly humoural Th2 response)
• intense edema of affected tissue, facial lip swelling + collapse of bridge of nose (leonine facies), mycobacteremia/dissemination (nasal/pharyngeal mucosa, eye, muscles, testicles, bone marrow)
• tissue biopsy shows undifferentiated macrophages packed w AFBs
• highly infectious due to nasal discharge

Question 19

Features of borderline leprosy

Answer 19

• intermediate form, unstable, small fluctuations in immune response can shift either way along the spectrum
• lymphocytic infiltrate & epitheloid but no grant cells

Question 20

Diagnosis of leprosy (4)

Answer 20

1. Clinical examination
2. Biopsy for histology - skin, nerve, retina, examine slit skin smears for AFB
3. Molecular - PCR
4. cannot culture in vitro

Question 21

Treatment & management of leprosy (3)

Answer 21

1. Paucibacillary leprosy: Rifampicin (monthly, supervised) + Dapsone (daily, unsupervised)
2. Multibacillary leprosy: PL treatment + Clofazimine (Daily, unsupervised) for 2 years
3. Correct deformities, prevent further damage to anaesthetic limbs (eg physio), treat reactions to anti-leprosy drugs, social/psychological welfare

Question 22

Features of non-tuberculous mycobacteria

Answer 22

1. Saprophytes of soil & water
2. Cause opportunistic infections (transplants, cancer, HIV)
3. May colonise non-sterile specimens (contaminant)

Question 23

Clinical presentations of non-tuberculous mycobacteria (4)

Answer 23

1. Pulmonary opportunists (M. kasasii, M. malmoense, M. xenopi) - patients with lower resp tract abnormalities eg bronchiectasis, COPD
2. AIDS related opportunists (MAI complex, M. haemophilum, M. cookei, M. hiberniae)
3. Skin pathogens (M. marinum: fish tank granuloma, M. ulcerans: Buruli ulcer)
4. Rapid growers (M. fortuitum, M. abscessus, M. chelonei complex) - can cause skin infections due to contaminated injection fluids eg diabetics using sq insulin, bacteremia in IV drug users

Question 24

Diagnosis of non-tuberculous mycobacteria (2)

Answer 24

1. Runyon’s classification
   - photochromogens - produce pigment in light only - M. kansasii, M. marinum
   - scotochromogens - produce pigment in dark & light - M. scrofulaceum
   - non-chromogens - unpigmented - MAI complex, M. ulcerans (slow), M. cheloneae, M. fortuitum (rapid)
   - rapid growers - grow on Lowenstein-Jensen agar within 1 week
2. Biochemistry, DNA probes, high performance liquid chromatography (HPLC)

Question 25

Treatment of non-tuberculous mycobacteria

Answer 25

Difficult & prolonged, NTMs resistant to many antibiotics, need multiple drugs with various combinations