mycobacterium tuberculosis

Question 1

how do you distinguish mycobacterium tb

Answer 1

• ziehl-neelson stain
• do not do gram stain as it is neither gram positive or gram negative bacteria

Question 2

what are the symptoms for tb

Answer 2

-cough- blood cough
-weight loss
-fever
-rash

Question 3

how is tb diagnosed

Answer 3

• history
• examination
• culture (gold standard)
• ziehl neelson staining - microscopy
• naoh used to distinguish between different mycobacterium
• PCR
• mantoux skin test

Question 4

what is the treatment for tb

Answer 4

• standard anti-tb regim is a combination of rifampicin, isoniazid, pyrazinamide for 2 months
• followed by rifampicin and isoniazid for a further 4 months
• total treatment time for 6 months
• multiple drugs used as bacteria may develop resistance

Question 5

how can tb be prevented

Answer 5

• bcg vaccination- only at risk groups given the vaccination
• tb notifiable disease
  -infected patients should be isolated in a negative pressure room
• healthcare workers should wear masks when in contact with patient
• after correct treatment the patients can come out of isolation

Question 6

what is the global prevalence of tb

Answer 6

• declared a who emergency in 1993
• 5% of those infected than go to develop the disease but patients who are co infected with hiv this rises to 50%
  -over 95% of tb deaths occur in low/middle income countries
• london defined as endemic area

Question 7

how is tb spread

Answer 7

• spread between people by coughing
• enters body via aerosol drops
• aerosols contain droplet nuclei that can resist desiccation due to mycolic acid layer
• tb can be latent

Question 8

structure of m. tb

Answer 8

• neither gram positive or gram negative, stained using neihl zeelson stain as it identifies mycobacterium
• cell wall has peptidogylcan with a layer of arabinogalactan overlying it. the arabinogalactan is covalently bonded to mycolic outer layer
• mycolic acid layer resistant to antibiotics and desiccation
• lipoarabinomannan runs vertically- inactivates macrophages and scavenge oxidative radicals
• mycolic acid impervious to many substances including antibiotics

Question 9

what are the roles of mycolic acid outer layer in tb and what is it bonded to

Answer 9

• impervious to many substances, so antibiotics cannot pass
• mycolic acid also resistant to dessication, antibiotics and disinfectants
• bonded to arabinogalactan which is the middle layer ( arabinogalactan bonded to covalently peptidoglycan cell wall)
• mycolic acid also prevents attack by oxygen radicals released by host cells (granulocytes)

Question 10

what is the role of lipoarabinomannan

Answer 10

• runs vertical
• inactivate macrophages and scavenge free oxidative radicals

Question 11

how does the host respond against the pathogen (tb)

Answer 11

• mycobacterium tb taken up by macrophages
• tb inhibits fusion of phagosome and lysosome- so can survive within the macrophage- multiple intracellularly when host cell dies by apoptosis they are released
  -some m.tb are detected taken up and presented- activated th1 resonse which releases pro inflammatory cytokine ( interferon gamma, tumour necrosis factor)
• cytotoxic tcells also release granulysin
• granulomas are also formed- indicative of chronic inflammation
• tb remians latents in granuloma- limit spread of tb- changes in immunocomoetency can reactivate the bacteria