Mycobacteria

Question 1

Features of mycobacterium

Answer 1

Aerobic non-motile rod-shaped bacteria
Slow growing
Cell wall of long-chain (mycolic) fatty acids = waxy coating
Acid-fast - cannot be Gram-stained due to waxy layer

Question 2

Two types of stain used to identify acid-fast bacteria

Answer 2

Auramine (cheaper, more sensitive, easier - no heading required)
Ziehl-Neelson (more expensive, more specific - used to confirm)

Question 3

Pulmonary diseases resembling TB
Not caused by Mycobacterium TB
RF = Pre-existing lung disease

Answer 3

Non-tuberculous mycobacterium (NTM)

- Resistant to normal TB treatment

Question 4

Ulceration in cold parts of the body

Answer 4

M ulcerans

Question 5

Swimming pool granuloma

Painful red nodules on limbs

Answer 5

M marinum

Question 6

Bronchiectasis in immunocompetent

Disseminated disease in immunocompromised

Answer 6

M avium intracellulare

Question 7

Skin + soft tissue infections

Answer 7

Rapid-growing NTMs

Question 8

Risk of close contact acquiring TB from infected person

Answer 8

10%

Question 9

TB appearance Lowenstein-Jensen

Answer 9

Brown coffee-coloured granular deposits that stick to bottom of plate + are hard to remove

Question 10

TB treatment

Answer 10

RIPE - RIfampicin, Isoniazid for 6 months

Pyrazinamide, Ethambutol for first 2 months

Question 11

TB Ziehl-Neelson appearance

Answer 11

Carbofuchsin pink dye binds to myolic acid in cell wall

Question 12

TB auramine stain

Answer 12

Binds myolic acid to give yellow fluo

Question 13

TB screening for latent / active TB infection

Answer 13

Important to screen if someone has been in close contact with infected person or for healthcare workers starting work (don’t want TB to be passed on)
Neither of these screening tests differentiate active vs. latent disease
Mantoux will be positive if have had BCG, active or latent infection
IGRA will be positive if have active or latent infection (not BCG)

1. Mantoux (Tuberculin skin test)
   - Intradermal injection 0.1ml tuberculin protein, read 48-72h later
   - Positive result >15mm in all, >10mm if child / from TB country / work in myco lab, >5mm if immunocompromised
   Type IV hypersensitivity reaction
2. IGRA (e.g. ELISPOT) is preferred if patients have had BCG - blood sample mixed with TB antigens + look for IFN-gamma production - will not give positive result if have had BCG but still doesn’t differentiate active / latent disease

Question 14

Contraindication for ethambutol use

Answer 14

Renal impairment - cannot be cleared properly - has ocular toxicity

Question 15

TB meningitis

Answer 15

Viral picture (high lymphocytes) + turbid + very high protein

Question 16

CXR TB features

Answer 16

Upper lobe cavitation
Bihilar lymphadenopathy
Enlarged mediastinal LNs

Question 17

6y old child

Father just diagnosed with TB

Answer 17

Prophylactic Isoniazid

Question 18

TB lab Ix

Answer 18

Quick initial screen - smear microscopy (sputum in adults, gastric aspirate in children as tend to swallow sputum)

Gold standard - culture (usually takes around 6w - solid culture takes longer but more detail, broth culture more rapid but less detail)

NAAT by PCR

Question 19

Skin depigmentation, plaques + nodules

Thickened nerves

Answer 19

Leprosy ‘Hansen’s disease’

M Leprae + Lepromatosis

Question 20

BCG

Answer 20

Live attenuated vaccine
Attenuated strain of M bovis
Bad for pulmonary TB, good for other types
Contraindicated in HIV patients
Targetted programme - babies in high TB areas, immigrants, lab staff, abbatoir workers, travellers to TB-endemic areas