Tuberculosis: microbiology of diagnosis and management

Question 1

What is tuberculosis?

Answer 1

infection caused by mycobacterium tuberculosis

• affects any part of the body
• curable following 6 months therapy

contagious

• if affecting the lungs - pulmonary
• transmission via airborne particles

Question 2

What happens if you inhale TB organisms?

Answer 2

granuloma formation

• macrophages engulf TB (epithelioid histiocytes)
• fuse to form giant cells with central necrosis
• “ghon focus” = lung
• “ghon complex” = + lymph nodes

Question 3

What is the difference between primary disease and latent infection?

Answer 3

primary disease = organisms continue to divide, poor immune systems

Latent infection = organism not dividing, sleeping/dormant TB

Question 4

What does it mean by secondary TB?

Answer 4

latent TB reactivates

• organism wakes up and starts dividing
• decline in health and immunity

Question 5

What are the latent TB screening methods?

Answer 5

Tuberculin skin test
Interferon gamma release assay (IGRA) - quantiferon and T-spot

If they react = do they have TB? if negative do they need BCG?

Question 6

What is TB prevalence associated with?

Answer 6

poor sanitation
overcrowding
unpasteurized milk

Question 7

What are the symptoms of TB?

Answer 7

Long history (slow growing organisms)
- fever => infection
- weight loss and fatigue => prolonged inflammatory state
- night sweats => TNF alpha
consumption
cough, haemoptysis, abdominal pain, headache, back pain

Question 8

What are the differential diagnoses for the symptoms of TB?

Answer 8

fever, weight loss, night sweats

• cancer (lymphoma, leukaemia, lung, bowel, metastasis)
• infection (bacterial, fungal)

granulomas

• sarcoidosis
• crohn’s disease
• granulomatosis with polyangitis
• infection (fungal, parasitic)

Question 9

What is the treatment for TB?

Answer 9

RIPE

• rifampicin - 6 months
• izoniazid - 6 months
• pyrazinamide - 2 months
• ethambutol - 2 months

Question 10

What is the main side effect of rifampicin?

Answer 10

bright orange urine

Question 11

What are the treatments for drug resistant TB?

Answer 11

Longer = 9-24 months depending on resistance pattern

• mono/poly resistance
• multi drug resistant (MDR) = rifampicin and isoniazid
• extensively drug resistant (XDR) = MDR + quinolones and injectables

Question 12

What microbiological samples need to be sent off to diagnose TB?

Answer 12

sputum x3 
broncho-alveolar lavage
gastric lavage
blood 
CSF 
tissue

Question 13

What are the features of mycobacterium tuberculosis?

Answer 13

aerobic bacilli (upper lobes) = acid fast (neither gram +ve or -ve- has a high lipid content (mycolic acid)), slow growing
>85 species
- mycobacterium TB complex = TB, bovis (cows and human hosts), africanum = BCG
- mycobacterium lepraw
- non-tuberculos mycobacteria (NTM) - environmental, cause disease in immunocompromised pts

Question 14

How are acid fast stains used to diagnose TB?

Answer 14

1) auramine stain (auramine phenol) => fluorescent
- “smear positive” = highly infectious
- initial screening of sputum

2) ziehl neelson stain (carbol fuchsin) => red on blue
- confirmation of mycobacteria
- can comment on morphology

Question 15

How can you grow TB?

Answer 15

1) solid media (conventional) => lowenstein jensen (only needs one organism)
2) liquid media (rapid) => MGIT (mycobacteria growth indicator tube) = needs 1-10 organisms)

Question 16

What are the new developments to increase the speed of diagnosing TB?

Answer 16

1) TB polymerase chain reaction (PCR)

2) whole genome sequencing (WGS)

Question 17

How does TB PCR work?

Answer 17

(geneXpert, Xpert, Cepheid)
- straight from sputum 
- detects MTBc
- can predict resistance to rifampicin 
diagnose same day and 2-4 weeks to determine drug resistance

Question 18

What does WGS do?

Answer 18

detects single nucleotide variations (polymorphisms) between 2 isolates
TB mutates at 1 SNP every 2 years
- 0-5 SNPs difference between strains = most probably linked
- 5-12 SNPs may be linked
- >12 SNPs less likely to be linked

Question 19

What are the benefits of WGS?

Answer 19

Faster drug susceptibility prediction
- more confident treatment regimens
- less likely to induce resistance
more informed contact tracing

Question 20

Where does XDR most likely originate?

Answer 20

likely origin is E. Europe - within transmission in the UK

Question 21

Where does MDR most likely originate?

Answer 21

likely origin is E. Africa

Question 22

What is still needed in TB diagnostics?

Answer 22

WGS on sputum directly
smear negative
childhood Tb - sputum hard to get
biomarkers to detect LTBI with high risk of progression to active TB