Mycobacteria Tuberculosis Flashcards
1
Q
Tuberculosis and mycobacteria: Introduction:
Tuberculosis is caused by what bacteria?
A
- TB is caused by Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacteria. africanum, Mycobacteria microti, Mycobacteria. canetii.
- Mycobacteria tuberculosis accounts for 98% of infections in the UK.
- Mycobacteria other than tuberculosis (atypical/MOTT) or non-tuberculosis mycobacteria (NTM) can also cause infections (respiratory/non-respiratory).
2
Q
Mycobacteria species and clinical diseases:
- 5 species of interest.
- Reservoir?
- Virulence in humans?
- Clinical disease?
A
- Mycobacteria tuberculosis:
- Reservoir in humans
- High virulence in humans
- Clinical disease: TB
- M. bovis:
- Reservoir: animals
- High virulence in humans
- Bovine TB
- M. leprae:
- Reservoir: Humans
- High virulence in humans
- Clinical disease: leprosy.
- M. abcessus:
- Reservoir: Environment water systems.
- Low virulence in humans
- Only infects people with underlying conditions (CF)
or bronchiectasis. - M. avium-intracellularae
- Reservoir: Environment birds
- Low virulence in humans
- Causes TB-like lung infections or disseminated
infections in AIDS patients.
3
Q
Characteristics of Mycobacteria species:
- Phylum?
- Acid fast?
- Most abundant virulence factor?
- Trehalose dimycolate (TDM) cord factor causes what?
- Generation time?
A
- Mycobacteria are actinobacteria and are
filamentous. - Acid fast organism: cell envelop contains 60% long chain branched hydrocarbons (waxes) (means difficult to stain).
- Mycolic acid is the most abundant- virulence factor.
- Trehalose dimycolate (TDM) cord factor:
- Reduces permability to many
molecules: confers resistance to
chemicals, stains and antibiotics. - Confers to resistance to drying:
increased survival in environment.
- Reduces permability to many
- Slow growing: generation time 15 -22
hours.
4
Q
Pathogenesis of M. tuberculosis:
5 stages?
A
- Inhalation of infectious particles. Droplet nuclei (5mm approx. 3 bacilli)
- (7-21 day incubation) M. tuberculosis multiples within macrophages. (TDM cord factor prevents the phagosome fusing with the lysosome) (intracellular). Macrophages secrete IL-12 and present MTB antigen on surface, eventually bursting releasing MTB.
- IL-12 stimulates CD4/CD8 T cells to infiltrate: recognizes MTB antigen: become activated (sensitized): CD4 T cells release inflammatory factors INF-y resulting in a tubercle formation (primary lesion).
- MTB continues to multiple within inactivated or poorly activated macrophages and tubercle expands.
- Primary lesion heals (ghon focus) type of granuloma (dormant lesion, contains MTB: may re-activate). In disseminated infection the a cluster of granulomas are formed ghon complex which can be detected on x-ray.
N.B: it is the cell mediated response in healthy individuals that heals the primary lesion.
5
Q
Primary tuberculosis: (primary exposure): Outcomes:
- Ghon focus?
Immunocompetent individuals?
Immunocompromised individuals?
A
- Ghon focus is a calcified nodule within the subpleural of the lung. (inflamed, infected, lymph node).
Immunocompetent individuals:
- Cell mediated immunity (CMI) prevents the spread of MTB, minimal/no symptoms in 90%.
- MTB remains latent (latent TB)
- Reactivation may occur.
Immunocompromised individuals:
- Primary lesion worsens (pneumonia develops)
- systemic dissemination (lymph nodes, meninges, upper parts of the lungs).
- Symptoms result from host cell mediated immunity response: chronic inflammation. (MTB has no endotoxins or exotoxins.
6
Q
Secondary TB (reactivation of TB): - Endogenous reactivation is? - What can cause endogenous reactivation? Caseous lesions?
A
- Endogenous reactivation of initial infection: commonly within 2 years but may occur any time after.
- Associated with any impairment in the cell mediated immune response. (immunosuppression) & local disturbances to the dormant tubercle.
- The granuloma undergoes caseous necrosis which liquefies and discharge into bronchi.
- well-aerated environment.~
- Distribution to other parts of the lung.
- Contents of caseous lesions are
coughed up becoming infective
droplet nuclei. - TB pneumonia may result.
7
Q
Mycobacteria & AIDS:
- Why is it important and what does it cause?
- Developing nations aids problem?
A
- Cell mediated immune response is important in TB infections. In HIV/AIDs the CD4 T helper cells are depleted impairing this response.
- 2/3 of AIDs patients in sub-Saharan Africa have TB.
- Pt prone to rapid primary infection and reactivation.
- Advanced AIDs pt (<50% T cell/uL) are very susceptible to M. avium-intracellularae (environmental)
- Disseminated M.avium-intracellularae complex (MAC) infections: chronic, fever, wasting, multiple organ involvement and death.
8
Q
Tuberculosis: Symptoms:
-General stats (%)
- symptoms?
Other possible complications?
A
- 90% are asymptomatic.
- 10% develop primary tuberculosis and experience symptoms; re-activation may occur.
Symptoms:
- LRTI
- Cough (bloody sputum)
- Significant weight loss
- Night sweats
- Fatigue
- Fever
Complications: May spread to other parts of the body (miliary TB):
- Meningitis
- Septicaemia
- Kidney infection
- Joint infection; Potts disease.
9
Q
Diagnosis of TB: Clinical diagnosis:
- Clinical indication and rapid test?
- Laboratory diagnosis?
- Non-culture?
- Culture?
A
- Presenting symptoms.
- Radiological changes on x-ray, tuberculan skin test (TST): Mantoux test (tradition)- injection of purified protein derivate it is an extract from Mycobacteria sp.(PPD).
Laboratory:
Non-Culture:
- Interferon gamma release assay (IGRA)
- Molecular detection in clinical samples (Nuclei acid amplification tests (NAAT) PCR.
- Microscopy
Culture:
- Gold standard: MGIT (Mycobacteria growth indicator culture.
10
Q
Mycobacteria sample collection:
- Primary Sample?
- Renal TB?
- Meningitis?
- Other possible samples?
Safety considerations?
A
- Early morning sputum x 3
- Renal TB - Complete early morning urine (EMU x 3)
- CSF (meningitis)
- Lymph node biopsy
- Blood/bone marrow aspirate
- Whole blood (for IGRA)
Safety considerations: Tuberculosis is a category 3 pathogen!!
11
Q
Non Culture techniques: Interferon gamma release assay (IGRA):
Theory?
The process?
The vaccination false positives?
Sample used?
What is the difference in genetic antigen from TB and NTM? + the antigens they encode?
Commercial kits?
A
- Theory: A person’s T-cell that previously were sensitized to TB antigen (i.e Tb infection). Produce a high level of INF-y when re-exposed to the same mycobacteria antigen.
Process:
- Antigen specific T cell + antigen and APC = Antigen specific T -cell to release high amounts of INF-y.
- Unlike the Mantoux (TST) this assay does not produce false positives against BCG vaccinated people.
- Samples is blood.
- Antigen utilized are encoded in region of difference (RD-1) a genomic sequence that is absent from NTM (e.g. BCG vaccine, environmental mycobacteria).
RD1 encodes antigens:
- early secretory antigenic target 6 (ESAT-6)
- Culture Filtrate protein (CFP-10)
Commercial kits:
- QuatiFERON -TB Gold: INF-y secretion measured.
- T-spot. TB; INF-y secreting T-cells enumerated.
12
Q
Non-culture techniques: Microscopy:
- primary stain? difficulties with diagnosis?
- New stain? Sensitivity?
- TB read?
A
- Ziehl-neelson is an acid fast stain primarily used for TB. how ever need a large quantity of bacteria need to be present for detection!!
New stain in use:
- Fluorescence auramine stain.
- Rapid diagnosis (within 1 hour)
- Negative does not rule out TB (low numbers in sample)
- more effective than the ZN stain.
- overall microscopy sensitivity = 50%
- TB read (extra reading)
13
Q
TB Culture: (GOLD STANDARD): - Sample preparation: 3 steps. Culture: - Solid culture? - Broth culture
A
Sample preparation: e.g Sputum (non-sterile)
- Liquefaction of sputum (sputasol; N-acetyl cystine)
- Concentration: (Centrifugation)
- Decontamination: 4% NaOH
N.B: sterile samples do not require decontamination.
Solid culture:
- Lowenstein-Jenson slopes (LJ)
- contains: whole eggs, salts, glycerol, potatoes flour, malachite green, penicillin, nalidixic acid.
- Incubation at 37 degrees for up to 8 weeks.
- Broth culture: e.g Bactec mycobacteria broth.
- Semi-automated system Bactec.
14
Q
Full identification of TB colonies:
- what is required?
- biochemical test?
- molecular testing?
A
- Colonial appearance macroscopy and microscopy.
- Ziehl-neelson stain positive.
- Biochemical tests:
- Niacin: MTB POS . Atypical mycobacteria NEG.
- Nitrate reductase: MTB POS. Atypical mycobacteria: NEG.
Molecular testing:
- Nucleic acid amplification tests: PCR.
- utilizing species specific DNA probes.
- or ribosomal rRNA probes which are more sensitive.
15
Q
Treatment of MTB: Current guidelines:
- British thoracic society and NICE guidelines?
- Improving patient compliance? DOT?
- Combination therapy?
A
- Current guidelines for treatment of active TB infections is a 6 month course of antibiotics split into 2 phases.
- initial phase 2 months: Isoniazid, rifampicin, Pyranzinamide and ethambutol .
- Continuation phase: 4 months: Isoniazid and rifampicin.
- Patient compliance is key to prevent resistance forming. controlled by directly observed therapy from a trained physician. DOT is used for pts at high risk i.e alcoholic/vagrants.
- Combination therapy: aids in pt compliance as reduces the pill burden.
- Rifater: initial phase: isoniazid, rifampicin, Pyranzinamide.
- Rifinah 300 or rimactazid 300: continuous phase: isoniazid and rifampicin.
