TB I Flashcards
Bacteria responsible for TB
mycobacterium u=tuberculosis
What causes leprosy
Mycobacterium leprae
Non-Tuberculous Mycobacteria causes what infections?
Opportunistic infections in immunocompromised patients, but not TB
Explain the Mycobacterium Tuberculosis complex
TB is caused by a family of closely related species, including Mycobacterium bovis (infecting cattle).
However Mycobacteria tuberculosis is the most prominent in this complex
All members of this family can cause tuberculosis in humans and or animals
structural features of mycobacteria
Cell wall with high lipid content - hydrophobic
Cell wall contains 60% mycolic acids -Long chain fatty acids
How does mycobacteria stain with Gram stain?
Doesn’t stain with conventional Gram stain: absent/only a ghost outline of mycobacteria. It requires special stains
How to encourage entrance of stain into the cell?
Use special stains and heating during the staining process, followed by the decolourisation step with acid and alcohol
Why are mycobacteria called acid fast?
The decolourisation step with acid and alcohol usually removes stain from ordinary bacteria.
But mycobacteria hold onto the stain they are ACID AND ALCOHOL – FAST
Microscopic lab characteristics of mycobacteria (6)
Slender (thin) straight or slightly curved bacilli (also sometimes called rods)
* Aerobic
* Slow growing – multiplies and divides only about every 24 hrs compared to E. coli which divides
every 30 minutes
* Resistant to drying and to many chemical disinfectants
* Sensitive to UV light
* Sensitive to sustained heat
what cell types are commonly found in chronic inflammation?
Lymphocytes, plasma cells macrophages
why does mycobacteria cause chronic inflammation?
Oneof the causes of chronic inflammation is infection with mycobacteria and other organisms that are
resistant to phagocytosis and intracellular killing
What special type of chronic inflammation is caused
granulomatous inflammation
6 key concepts of TB pathogenesis
. Intracellular pathogen
2. Primary host cell in humans is the macrophage
3. Require cell mediated immunity to control infection
4. Characteristic pathological feature of tuberculosis is granulomatous inflammation or the
formation of necrotising granulomas
5. Potentially lifelong infection
6. Outcome of infection with TB is a continuous balance between the host’s immune control and
the virulence of the mycobacteria
pathogenesis of TB
Occurs on first exposure to TB bacilli
* Following inhalation TB bacilli are phagocytosed by alveolar macrophages
* Normally phagocytosis leads to killing of micro-organism that has been engulfed
* But TB bacilli manage to upset this normal killing: they are not killed and in fact TB bacilli can multiply inside macrophages
How is cell mediated immunity activated in during TB pathogenesis
- Inside macrophages TB bacilli are transported via lymphatics to regional (hilar) lymph nodes
- In the lymph nodes TB bacilli stimulate an immune response, mainly by the cell mediated immune response :
- Infected macrophages present TB antigen to lymphocytes
- TB-specific lymphocytes proliferate and produce various cytokines including
- Interferon-gamma
- Tumour necrosis factor (TNF)
What is the functions of the cytokines released by the TB specific lymphocytes?
Cytokines activate macrophages and increase their ability to kill the mycobacteria by the formation of granulomas
What does a granuloma consist of?
Central area of necrosis
* Surrounded by activated macrophages (some of which may fuse together to form giant
cells) Note activated macrophages also called epitheliod cells
* Surrounded on outside by T lymphocytes and fibrous tissue
* The granuloma contains the organism by walling it off
After some time, he material within the granulomas becomes necrotic and__________
caseous
How does primary TB infection heal?
In most cases the tubercles /small granulomas heal spontaneously
* Sometimes form a scar or even calcify
* May show up on CXR as nodule especially if calcified
How long does primary infection take to heal?
3-6 weeks
Explain latent TB
Dormant TB. The mycobacteria from primary infection did not all die, some remain dormant causing latent TB.
potential to reactivate when immunocompromised
what is progressive primary disease?
in a minority of people, mycobacteria are not contained within the primary complex, at the time
of primary infection or within the next 1-2 years but spread causing ‘progressive primary
disease’
* Spread can be by local extension or via bloodstream or lymphatics
Progressive primary infection results in what?
Result in:
* more extensive lung disease
* foci of infection in almost any site of the body which can develop into disease immediately or
at some stage in the future
* disseminated disease in many organs – called ‘miliary’ tuberculosis
What is secondary TB?
Due to reactivation of dormant mycobacteria, usually due to declining immune system
