1.1 Mycobacteria infection (Tb) Flashcards
(40 cards)
MYCOBACTERIA
Mycobacteria are ___________________ organisms (may also survive extracellularly but prefers an intracellular environment):
• __________, non-spore forming, non-motile bacilli (spores are implicated in resistance to heat and chemical sterilisation and facilitates survival in various environments) → M. tuberculosis can be managed with ______________
• Possess lipid-rich cell walls (containing _____________________) which protect the bacteria from lysosomal attack within macrophages
Mycobacteria takes a while to grow under incubation, and due to the difficulty in staining, it is important to alert the microbiology lab when suspecting TB:
• Proper incubation and plating must be used to identify the pathogens
• Standard bacterial cultures are only incubated for 2 – 3 days, and will not yield any Mycobacterium tuberculosis pathogens
facultative intracellular;
Aerobic;
proper sterilisation;
mycolic acid and long-chain glycolipids;
M. tuberculosis is an _______________ (must have oxygen for survival) and a facultatively intracellular pathogen which usually infects ________________:
• Slow-growing bacterium with a generation time of 12 – 18 hours (contrasted to 20 – 30 minutes for E. coli) → visible growth in 3 – 8 weeks on solid media incubation
• Tends to grow in characteristic _________________
• Hydrophobic with a high ___________ content in the cell wall → tends to clump together (impermeable to usual stains like the Gram stain)
• Special stains (acid-fast stain/Ziehl-Neelsen stain) must be used
obligate aerobe;
mononuclear phagocytes (e.g. macrophages);
parallel groups (serpentine cording) ;
lipid
Acid-fast bacilli (AFB) like M. tuberculosis are impermeable to various basic dyes unless they are combined with _________.
Ziehl-Neelsen stain: Slide is flooded with ______________ and phenol (causing retention) → resist decolourisation by acidified organic solvents (acid-fast) → identified with ________
Fluorescent auramine stain: Using a phenolic auramine or auramine-rhodamine stain and potassium permanganate (fluoresces) → more sensitive than ZN stain but less specific
phenol;
carbol fuchsin; LM
[Tests for TB: AFB stain]
Advantages & Disadvantages?
Advantages
- same day results
- simple
- cheap
Disadvantages
- less sensitive (60%) then culture
- need >= 10 000 organisms/ ml (small samle inaccurate
- cannot distinguish between M Tb vs NTM
[Test for Tb: Culture on solid media]
Advantages
- Gold standard (must use in conjunction with other methods)
- detects 10-100 organisms/mL
- colony morphology
- speciation & strain ID
- Quantification
- Susceptibility testing
Disadvantages?
Take 3-8 weeks for growth, usually 6 weeks +
[TB Test: Liquid Broth]
Advantages
- sens and spec similar to solid media
- less workload
- speciation & strain ID
- susceptibility testing
- quicker than solid media (7-21d)
disadvantages?
cannot do colony morphology, cannot quantify growth
[TB test: BATEC (liquid) ]
Advantages
- preliminary ID
- drug susceptibility results in 5-14 days
Disadvantages?
- must use in conjunction with solid media –> costly
- cannot quantify % of drug resistance
[TB test: NAAT/PCR ]
- same day results
- sensitivity between AAFB and culture
- Identifies MTB complex
Disadvantages?
- cannot distinguish between dead & viable organisms
- must use in conjunction with culture for speciation, susceptibility etc
[TB test: HPLC]
Advantages
- sensitivity and specificity close to 100%
- can identify _______________
Disadvantages: must use in conjunction with culture
M. bovis bcg
[Tb test: molecular tests for c/s mutations]
- same day results, especially for _____________ (conferred by rpoB gene)
disadvantages
- validation in different clinical settings
- only for RIF resistance (not for any antibiotics)
RIF-resistance mutations
IMMUNE RESPONSE
The immune system has 3 levels of defence (surface barriers, innate and adaptive immunity):
- Surface barriers: Enzymes, mucus, skin
- Innate immunity: Lacks immunological memory; involves monocytes (DCs/macrophages in tissues/skin), neutrophils, eosinophils, complement etc.
- Adaptive immunity: Involves B and T cells (mature T cells leaving thymus are CD4/8+)
During the first encounter with an antigen, naïve lymphocytes proliferate as part of the primary immune response, generating effector and memory B and T cells:
• Effector T cells: CD4+ helper T cells, CD8+ cytotoxic T cells
• Effector B cells: antibody-secreting plasma cells
During the adaptive immune response, lymphocytes are activated by antigen-presenting cells (DCs or macrophages):
• Macrophage: presents antigen to the __________, which secretes ___________________ (activates both the ______________________ to kill the intracellular pathogen)
• Dendritic cell: presents antigen to the ____________, which secretes ____________ which helps B cells secrete antibodies (B cells recognise antigen directly via MHC presentation or indirectly via immune complexes)
Th1 cell;
IFN-γ and IL-2;
macrophage and CD8+ cytotoxic T cells;
Th2 cell;
IL-4, 5, 6
CD4+ T helper cells are responsible for regulating the immune response to tuberculosis:
• Macrophages phagocytose M. tuberculosis, then presents it to the CD4+ cells, and secretes cytokines which stimulate or suppress CD4 response
• If the macrophage produces more ______________, the CD4+ T cell produces IFN-γ, which induces the macrophage to kill the intracellular pathogen
IL-12 (or IL-1/TNF-α/ IL-6)
Mycobacteria have developed several strategies to survive within macrophages: - Mycobacterial urease: Preventing _________________ (reduces efficacy of host bactericidal enzymes)
- Reduction of CD8+ cytotoxic response
- ROS detoxification: ____________________ helps to detoxify the ROS of phagocytes
- Direct protection from oxidants: Protection from host oxidants
- Suppression of early oxygen-mediated immune responses: Immune responses needed for efficient antigen presentation (including macrophage activation and apoptosis)
acidification of phagosome;
Superoxide dismutase, catalase, thioredoxin
PATHOGENESIS
TB infection is a spectrum, ranging from asymptomatic (immediately cleared) to the clinical disease state; latent infections refer to replicating organisms with no clinical signs/symptoms:
• Initial exposure to TB: airborne droplets containing TB bacilli deposit in the terminal alveoli (initial focus usually in the ________________ (lower part of upper lobe and upper parts of middle/lower lobes) due to better airflow)
• ______________ (primary lesion): initially there is only 1 focus of infection (with a small area of granulomatous inflammation) developing about 4 weeks after infection → minimal or no symptoms as infection is contained to a point
• Progression: to overt pulmonary TB (with/without dissemination) or to ___________________ (with potential to reactivate at later stage)
o Overt infection: bacilli are ingested by macrophages, leading to macrophage death → more lymphocytes and monocytes migrate to site of infection in an attempt to control the infection → more inflammation and pneumonitis
• If left unchecked: infected macrophages enter bloodstream (in immunocompromised patients) or to regional lymph nodes (favouring _____________________ → but may spread anywhere (e.g. pericardium causing TB pericarditis)
• Tuberculin reactivity (sensitive skin): marks development of microbial growth inhibition by the body (adaptive immunity) 3 – 9 weeks after infection
o Abundant antibodies are produced (but minimal role in host defence)
subpleural mid-lung zone;
Ghon focus;
calcified nodule/subpleural plug;
lung apices, lymph nodes, kidneys, epiphyses of long bones, vertebrae and meninges
Granulomas (chronic process) consist of ____, ___________, ___________ (fusion of several cells; usually macrophages) with some central necrosis:
• Protects host from a persistent irritant (causative agent is walled off and sequestered by cells of the macrophage lineage → containment/destroyed)
• May lead to tissue necrosis (caseating/cavitating) as the cells in the centre of the lesion die and release their toxic contents:
o Caseating necrosis: _____________ material ringed by the inflammatory component (hallmark of TB → until proven otherwise)
o Cavitating necrosis: associated with ___________________
- Langerhans giant cells (fusions of about 20 macrophages) have nuclei lined up along one edge of the cell → epithelioid macrophages are elongated with long pale nuclei and pink cytoplasm
- Ghon complexes are Ghon foci associated with ________________
macrophages, epithelioid cells and Langerhans giant cells;
amorphous pink cheese-like;
strong delayed-type hypersensitivity;
hilar or mediastinal lymphadenopathy
TB re-infection refers to a latent infection becoming reactivated (endogenous) or patient becoming exposed to another TB infection (exogenous):
• Manifests as ____________ infiltrates (on CXR), cavitation but no _______________
• Risk factors: extremes of age (leading to local progressive disease +/- dissemination), immunosuppression, stress, malnutrition, pregnancy, malignancy, gastrectomy, jejunal-ileal bypass, ESRF, destructive local pulmonary processes (e.g. lung abscess)
apical or sub-apical ;
hilar lymph node enlargement
[TB Spread]
TB is transmitted by airborne transmission of very small (1 - 5μm) respiratory droplets (with a very low infectious dose of _____________ → highly contagious):
• Adapted to intracellular survival within human macrophages → persist for a long time and cause lifelong infections
• Most contacts with an infected person do not get infected (only 30% are infected with _________________)
• Of those infected, 90 – 95% develop latent infection (mostly do not ever cause disease), and the remaining develop active TB (mostly develop pulmonary TB; only some develop non-infectious extra-pulmonary disease)
The clinical signs and symptoms of overt clinical disease include:
Symptoms (at least 2 – 3 weeks)
- General: ___, __________, ________, _________
- Organ-specific: pulmonary (cough – at least 3 weeks), extrapulmonary (CNS, bone, lymph nodes)
Signs
- General: fever, weight loss
- Organ-specific: pulmonary, extrapulmonary (cervical lymphadenopathy – __________)
1 – 10 bacilli;
positive tuberculin test;
fever, weight loss, anorexia, night sweats, malaise
scrofula
Pulmonary TB is usually asymptomatic and is thus often undetected:
• M. tuberculosis is phagocytosed by _______________ but is resistant to alveolar killing, allowing it to slowly proliferate and spread locally (causing pneumonia) and to local lymph nodes
o Most patients recover without treatment; a few develop symptomatic disease
• Occurs mainly in the ___________ (due to reactivation of latent infection) with ___________ in immunocompetent patients (aggressive immune response attempting to control the infection) → healing via _______________________
• Standard course of treatment: lasts 6 months (HREZ2-HR4 regimen); given to patients with drug-susceptible localised pulmonary infection
o Duration of therapy is extended for disseminated infections or drug resistance
alveolar macrophages;
upper lobes;
cavitation;
fibrosis and calcification
Miliary TB occurs due to ________________ to within the lungs or elsewhere:
• Presence of ______________ (resembling millet seeds)
• High risk of fatality
• Standard course of treatment: 9 – 12 months
massive haematogenous spread;
numerous small nodular lesions
Tuberculous meningitis occurs due to haematogenous spread from a distant focus (usually the lungs) which lodges immediately deep to the pia mater (forms rich focus):
• May rupture into the ______________ to form exudates (purulent liquid) near the basal cisterns (including the floor of the 4th ventricle) → site of CSF drainage
• Provides infection with a route to spread around the brain and cause _____________(mass-like regions of caseous necrosis) or ___________ (causing ischaemic infarcts)
• Spread of infection may cause _________________ by blocking CSF outflow
The symptoms of tuberculous meningitis are variable depending on which neuroanatomical structure is affected:
- Non-specific: Weight loss, fever, night sweats, subacute presentation (for several weeks)
- Neurological: Personality change, focal neurological deficit, drowsiness/loss of consciousness (low GCS), cerebral oedema, brain herniation
• Oedema related to the tuberculoma could _____________________
• If oedema worsens, the brain could ____________________
A ___________________ is used to diagnose tuberculous meningitis, and presents with lymphocytosis in the CSF, low glucose (bacterial consumption), raised inflammatory protein levels:
• Standard treatment: at least 12 months of _______________________
subarachnoid space;
choroid plexitis;
arteritis;
obstructive hydrocephalus;
push against the ventricles and the midline of the brain;
herniate through the foramen magnum (causing death)
lumbar puncture (draining CSF);
anti-TB therapy and steroids
Vertebral TB (Pott’s disease) occurs due to haematogenous spread (causing initial discitis), and further local spread of inflammation to vertebral bodies causes vertebral end-plate destruction and collapse:
• May extend anteriorly to cause a _____________, or posteriorly to ____________________ (medical emergency → immediate surgery indicated to prevent spinal cord damage and paralysis)
• Symptoms: fever, night sweats, weight loss, back pain
• Investigations: MRI/CT (with/without biopsy/aspirate)
• Treatment: 9 – 12 months of anti-TB therapy + surgery (if indicated)
iliopsoas abscess;
compress on the spinal cord
DIAGNOSIS OF TB
The mainstays of traditional diagnosis of tuberculosis include:
1. Clinical history and epidemiological risk (general clinical picture)
2. CXR and other imaging (e.g. CT/MRI for spinal TB)
3. Smear microscopy (look for AFB using the ZN stain)
4. ___________ (gold standard) + sensitivity + identification
The tuberculin skin test and the IFN-γ release assays (IGRA) can be used to test for previous TB exposure or latent TB infection:
- Tuberculin/PPD skin test (Mantoux test): Tuberculin (purified protein derivative of TB) pricked into skin → positive test would be _______________________
- Not very sensitive (false negatives)
- IFN-γ release assays (IGRA): Done on whole blood → IFN-γ production by _____________________ indicates previous exposure to TB
- Involves quantiferon or T-spot
*These tests may have false negatives in immunocompromised patients (cannot mount proper immune response despite previous exposure) and cannot distinguish latent TB infection from overt clinical/active TB.
Solid culture;
localised skin inflammation (Type IV delayed hypersensitivity);
previously primed T cells
BIOMEDICAL PREVENTION
Prevention of overt clinical tuberculosis can be done by giving the Bacillus Calmette-Guerin (BCG) vaccine or treating the latent TB infection (after detection above):
• BCG is a live attenuated vaccine created from ________________, which does not protect against primary TB infection or reactivation against latent TB (main reasons for TB transmission → limited efficacy against transmission)
• Prevents ________, ______ , and also the prevention of ____________
• No added benefits for giving a booster jab (may even create more side effects)
• Limitations: different strains of BCG, exposure to environmental Mycobacteria (affects patient immune response to vaccination → blocking: no response; masking: reduced response), cannot administer to immunocompromised or pregnant patients (risk of disseminated disease)
Mycobacterium bovis;
disseminated TB and TB meningitis;
leprosy
Since tuberculosis is a slowly replicating pathogen (and thus develops resistance easily), 4 drugs are given as a first-line combination therapy, with other drugs as second-line therapy:
First-line medications : __________________
Second-line medications
- Quinolones
- Injectables (kana, ami)
- Ethionamide/Prothionamide
- Cycloserine
- Para-amino salicylic acid (PAS)
- Linezolid
- Clofazamine
Rifampicin (R), Isoniazid (H), Ethambutol (E), Pyrazinamide (Z)
