Stages of infection of TB
infection: latent, where the immune system controls the disease
disease: where bacterium escapes the immune response
features of Mycobacterium tuberculosis
- Aerobic (obligate)
- Acid-fast
- Resistant to drying
- Resistant to common antimicrobials
- Very slow growing
○ Requirement for diagnostic methods other than culture
○ Management implications: takes a long time to determine antimicrobial susceptibility
Survives in host macrophages
cell wall of TB
- very thick fatty cell wall
- has extra components such as mycolic acids (for a waxy coat), arabino-galactan, glycolipids, lipoarabinomannan
- very immunogenic components
Ziel-Nielsan stain
Step 1: carbolfuchsin stain
2: heat fixation
3: decolorization with acid alcohol
4: counter stain is methylene blue
acid fast positive is pink
acid fast negative is blue
Parthenogenesis of TB
- Hides in macrophages
- produces protein that interferes with fusion of lysosome to phagosome = reduction of reactive oxygen intermediates and other enzymes
- pH held at 6.2 for replication
Granuloma induction
Antigens are presented on MHC class II
drives strong CD4+ T cells that produce interferon gamma which helps control infection and help produce granuloma structure
activated macrophages fuse -> multinucleate giant cell
+ lymphocytes + epithelioid cells = granuloma
Latent, primary, latent TB
Latent: delayed type hypersensitivity response controls symptoms, individuals are not infectious
Primary TB: bacteria causes infection in initial infection
Secondary TB: reactivation of latent infection
- Most common when bacteria in the tubercles replicate
- Macrophages and bacterial product induce caseous necrosis -> liquification that forms a cavity
Granulomas enlarge, increased immune response, excess cytokines and tissue damage cause symptoms
Tuberculin skin test (Mantoux test)
Purified proteins derivatives that are taken from the organisms (tuberculin) that is injected into the skin
Inflammation at site is evidence of Mtb specific CD4+ T cell response
delayed hyper-sensitive reaction
x vaccinated individuals
IGRA
- collect blood, add purified proteins derivatives that have a different preparation so that it is specific to bacteria but is not present in vaccine
Test for IFNg production from Mtb specific CD4+ cells that has been secreted into media - Doesn’t say whether you have an active infection
Preferred for children over the age of 2 (T cell reaction not easily measured)
Diagnosing active TB
chest X ray: look for tubercles
microscopy: find acid fast bacilli in sputum samples
- Culturing with enriched medium that takes weeks
- Antibody detection unreliable
- Nucleic acid tests -> combine with tests for antibiotic resistance
Treatment of active TB
- 6 months of 3 anti-tuberculosis agents (INH, PZA, RIPE)
- hard to get into granulomas
- Rely on processing by micro bacterial enzymes -> need active bacteria for compounds to work
- Organisms require resistance through mutations in these enzymes
Directly observed treatment essential to avoid resistance -> taking frequent samples during treatment
Treatment for latent TB
- Used for those with a positive result without active infection or vulnerable contacts
- Use isoniazid for 6 months to prevent reactivation
Single drug is used because number of organisms is small
TB Vaccine
BCG vaccine
produced attenuated strain of M bovis
used in areas of high incidence
What are the major immune responses for TB?
Secretion of IFN-g by CD4+ cells that produces granulomas
