Mycobacteria Flashcards
M. tuberculosis causes…
Tuberculosis
M. avium complex (MAC) causes…
Disseminated infection in AIDS, chronic lung infection
M. kansasii causes…
Chronic lung infection
M. marinum causes…
Fish tank granuloma
M. ulcerans causes…
Buruli ulcer
Rapidly growing mycobacteria (M. foruitum complex) causes…
Skin and soft tissue infections.
M. leprae causes…
Leprosy
Tuberculosis numbers
1.5M deaths per year
2.5BN infected people
10.4M new cases per year
> 500K ABx resistance
Mycobacteria
Slightly curved, beaded bacilli.
High lipid content with mycolic acids in cell wall makes Mycobacteria resistant to Gram stain
Ziehl-Neelsen stain
- Carbol fuchsin
- Acid alcohol (AFB are resistant to de-staining)
- Methylene blue
Need 10,000 bacilli per ml sputum to diagnose
Microbiology of Mycobacteria
- Aerobic, non-spore forming, non-motile bacillus.
Cell wall has high molecular weight lipids. (Weakly gram-positive or colourless).
Cell wall characteristics
A significant part of the 4.4 Mb genome of Mtb encode genes involved in lipogenesis and lipolysis.
Mycolic acids
Lipoarabinomannan
What is the generation time for M. tuberculosis?
15-20 hours so it is a slow growing bacteria.
How is M. tuberculosis transmitted?
Air droplets.
Primary tuberculosis
Initial contact made by alveolar macrophages.
Bacilli taken in lymphatics to hilar lymph nodes.
Latent tuberculosis
Cell mediated immune response from T-cells.
Primary infection contained but CMI persists -> latent TB.
Latent TB:
- no clinical disease
- detectable CMI to TB on tuberculin skin test.
Pulmonary tuberculosis
Granulomas form around bacilli that are found in the apex.
TB may spread in lung causing causing other lesions.
Could occur immediately following primary infection (post-primary) or months later after reactivation.
Why are granulomas more common in the apex of the lung?
In apex of lung there
Is more air and less
blood supply
(fewer defending
white cells to fight).
What makes up the primary complex?
Granuloma + Lymphatics + Lymph nodes
TB can spread beyond the lungs
Bacilli in:
- lung apex
- in lymph nodes
- in bone and joint
- in pleura
- in genito urinary system
- meningitis
- Miliary TB
Tuberculosis of the spine
Gibbus formation.
Killing of Mycobacteria
Phagocytosed by macrophages and trafficked to a phagolysosomes.
The bacterium has adapted to the intracellular environment = withstand phagolysosomal killing and escapes to the cytosol.
What is required for the intracellular killing by macrophages?
Effective immunity requires CD4 T-cells which generate interferon gamma and this helps activate them.
TB-Macrophage-Trojan Horses
TB can survive inside macrophages and therefore evade the immune system.
Hallmarks of granuloma formation
If the granuloma works: Mycobacteria shut down metabolically in order to survive – dormancy.
But if fails, e.g. in the lung, this can result in the formation of a cavity full of live mycobacteria and eventual disseminated disease (consumption).
What is a granuloma?
Macrophages and type 1 helper T lymphocytes (Th1) capable of synthesising IFN-γ and other cytokines such as TNFα.
Unstable granuloma
DC4 depletion leads to unstable granulomas.
Explanation why HIV positive individuals are 20x more likely to fall ill with TB.
What is another mechanism that leads to unstable granulomas?
TNF-alpha depletion.
Why is diagnosing Mtb challenging?
Slow bacterial growth in culture.
First decontamination: all rapid growing bacteria (mycobacteria is protected by the lipid coat).
Solid culture L.J. SLOPE
Egg-based Lowenstein Jensen.
Agar based Middlebrook 7H11.
It takes 2-8 weeks.
Liquid culture
Automated systems BACTEC mycobacterial growth indicator tube.
Fluorometric detection in liquid media.
It takes 1-3 weeks.
Nucleic acid detection - more rapid diagnosis.
GeneXpert MTB/RIF cartridge based test.
Purifies and concentrates MTB, sonicates to release genomic material and then performs PCR.
Rapid result for MTB and detects Rifampicin resistance using fluorescence.
Can detect 131 bacilli/ml. Sensitivity 88%, Specificity 98%.
Recommended for rapid diagnosis in TB endemic countries.
Tuberculin skin test (Mantoux)
The highly immunogenic nature of mycobacterial lipids stimulates T-cell responses in 3-9 weeks after exposure to M. tuberculosis
This reactivity is measured in the tuberculin skin test (TST) an intradermal injection of purified protein derivatives.
What is indicative of a positive tuberculin skin test?
Induration of 5 or more mm in HIV PT, recent contact with someone who has TB.
Induration of 10 or more mm - recent immigrants, children <4.
Induration of 15 or more mm - anyone with no known risk for TB.
What is the first line treatment for TB?
isoniazid (INH), rifampicin (RIF), pyrazinamide (PZA) and ethambutol (ETH) x 2 months followed by isoniazid and rifampicin for further 4 months
What is the second line treatment for TB?
injectable agents (streptomycin, cycloserine, capreomycin)
Side effects of treatment:
Wide-ranging and severe, include liver damage.
How long is TB-treatment?
4-9 months of combination therapy.
Resistance mechanisms
Drug inactivation: bacteria produces beta-lactamase.
Drug titration: target overproduction.
Alteration of drug target:
Missense mutation.
Altered cell envelope:
Increased permeability and drug efflux.
Multi-drug resistant TB
XDR-TB: resistant to four commonly used TB drugs. 6% of all TB cases.
Resultant from inadequate TB therapy and failure to clear patients of bacteria.
Treatment is lengthy and expensive.
What is the treatment of XDR-TB?
BPaL regimen:
Bedaquiline
Pretomanid
Linezolid
All oral treatments for 6 months
These can fail too with totally drug resistant (TTR) TB. No known solution as yet.
What is the model organism to study TB?
Zebra fish.
Embryos are visually transparent.
Genetically manipulable.
Pharmaceuticals can be added to the embryo water.
Host-derived therapies
What activates white blood cells?
INJURY > loss of blood
> HYPOXIA
(low oxygen)
Utilising Hypoxia Inducible Factor (HIF)-
genetic master switch to trick cells into hypoxia.
