Tuberculosis Flashcards
What is the organism responsible for TB?
Mycobacterium tuberculosis
What is the name of the stains that can stain the mycobacterium tuberculosis bacteria?
Ziehl Neelsen Stain - most common
Auramine staining - not as specific as the Ziehl Neelsen stain
What are the other subtypes of Mycobacteria?
M.Tuberculosis M.Bovis M.Kansasii M.Chelonae M.Abscessus M.Fortuitum M.Leprae Some are environmental and some are fast growing
How does TB become an established infection?
M.Tuberculosis is inhaled into the lungs. Cavities open into the bronchi allowing the spread of the bacteria through coughing Haematogenous spread (occur in most people). This can progress to the next stage if reactivated (which some people get immediately) - cavitary TB
What is a Ghon Focus?
A primary lesion (usually sub-pleural) caused by mycobacterium tuberculosis.
- found in the mid to lower zones of the lungs
- developed in a nonimmune host
- occurs in primary tuberculosis
Once someone has inhaled the bacteria and become infected, what two things can happen to them?
Immediately develops a primary infection
No initial infection occurs and the disease remains latent within the body
Describe the pathogenesis of a primary TB infection.
The bacteria end up in the alveoli where they are recognised as foreign and are attacked by macrophages
The macrophages engulf the bacteria and try to dissemble them, but given the microbiology of TB, this doesn’t work
The surviving bacteria hijack the macrophages and infect them, while also increasing the bacteria population.
The bacteria within the macrophages then multiply until they burst, causing extracellular infection
The infected area becomes a granuloma (a wall of macrophages designed to contain the infection)
- the cells within the centre of this start to be killed by the bacteria, causing a necrotising centre = blood and sputum in the lungs
Once a granuloma has formed in the lungs, what happens to the infected person?
Most cases
- necrotising lesions heal with some scarring and calcification
- disease hasn’t gone, just become latent (for years/decades)
- less than 10% of these progress to secondary TB (80% of active TB cases)
Or the host can’t contain the infection -> secondary TB
Describe the pathogenesis of secondary TB.
The host tries to contain the growing infected areas by killing off the tissue around them and deploying T-cells - leads to inflammation
The bacteria still attack the immune system cells leading to steadily growing areas infested with bacteria known as tubercles
- these can break off and travel through the blood and cause extrapulmonary TB infections
- in some cases the tubercles liquefy, causing bacteria-rich environments and thick sputum. The lungs then fill with free floating bacteria and clusters in the linings of the lungs
What is miliary TB?
Widespread dissemination of the bacteria via haematogenous spread
Looks like seeding of TB bacilli in the lung on an X-Ray
Makes up 1-3% of all TB cases
What are the three clinical categories of CNS TB?
TB meningitis
Intracranial tuberculoma - appears as abscesses in the brain on MRI
Spinal tuberculous arachnoiditis
Briefly describe renal TB.
Multiple granuloma form at the site of metastatic foci (typically bilateral and adjacent to the glomeruli and may remain inactive for decades)
Although both kidneys are seeded, clinically significant disease (capillary rupture and delivery of proliferating bacilli to the proximal tubules) usually only develops in one kidney
Describe what you would expect to see in a mycobacterium tuberculosis granuloma biopsy.
Giant cells as the body tries to remove the infection - multinuclear
Lymphocytes
Epitheliod cells
Caseous necrosis
How can a person be immune to mycobacterium tuberculosis?
Cell mediated immunity is the most important aspect of this
- macrophages are the key controlling cell
- T cells produce interferon gamma
- cytokines also help destroy the bacteria
Why doesn’t our immune response normally clear the mycobacterium tuberculosis bacteria?
Once the macrophage tries to engulf the bacteria, a protein called Ptpa is secreted
- this disables the mechanisms of the macrophage to create an acidic environment in which to destroy the bacteria
