Mycobacteria Flashcards
What are five properties shared by all mycobacteria?
- Aerobic
- Non-spore forming
- Bacilli
- Grow slowly in culture (1-8 weeks)
- Cell walls contain long chain fatty acids
What are the 6 layers of the mycobacterial “cell wall”
- outer lipid
- Lipoarabinomannan
- Mycolic acid
- Polysaccharides
- Peptidoglycan
6 Lipid bilayer
What are the 2 subdivisions used to group mycobateria?
Tuberculosis complex or non-tuberculosis (atypical) mycobacteria
What is the morphology of Mycobacterium tuberculosis?
- thin/straight/curved rods
- obligate aerobes
- non spore-forming
- single, pairs, or in masses
- high lipid content in cell wall (60% mycolic acid)
What kinds of staining can/can’t be used on M. tuberculosis?
- Not stained by gram staining
- Stains with Ziehl-Neelsen or Kinyoun Staining
What does acid fast bacilli mean? how does the staining work?
Once stained with red dye, resist de -colorizationwith 3% HCl in alcohol
appear red against a blue background
What is the generation time for M. tuberculosis?
18-20 hours so very long
What kind of lesions are formed by M. tuberculosis when there are large numbers of organisms?
Cavitary lesions
How do drug resistant strains of M. tuberculosis arise? Why?
spontaneous chromosomal mutations at a predictable low frequency
M. tuberculosis does not interact/exchange genetic material with other bacteria
What two factors contribute to the selection pressure on drug resistant strains of M. tuberculosis?
misuse of antituberculosis drugs, such as monotherapy or the addition of single drugs to failing regimens
In Canada, which groups of the population have the highest instances of TB?
Aboriginal and foreign born populations
Which province/territory has the highest rates of TB?
Nunavut
What is the source of infection typically for M. tuberculosis?
other infected humans
What is the ghon focus?
the granuloma formed at the primary infection site
What is milary TB? when does it occur?
Occurs when infective foci in the lungs seed or rupture into one of the branches of the pulmonary venous return to the heart
- widespread hematogenous dissemination of bacteria to most organs of the body.
What organs tend to be affected by millary TB?
liver, bone marrow, spleen, adrenals, meninges, kidneys, fallopian tubes and epidydymis
What kind of endo/exotoxins does M. tuberculosis produce?
none!
What is a physical adaptation that virulent strains of M. tuberculosis demonstrate?
Cording - parallel bundles of organisms intertwined
What 3 things are responsible for the virulence of M. tuberculosis?
- survival and multiplication in macrophages
- Delayed hypersensitivity reaction to tuberculo proteins
- Cell wall lipids that produce granulomatous lesions
What is Pott’s Disease?
Extra-pulmonary TB infection of the bones, joints, and neurons
For the tuberculin skin test, where do you need to inject the tuberculin protein? When do you read it?
into the dermis of the skin
read after 48-72 hours
What is the tuberculin skin test for?
used to determine previous exposure to Tuberculo-protein
What are the criteria for a positive vs. negative tuberculin skin test?
positive: induration of =/> 10 mm OR 5 mm in certain circumstances
- ex: HIV patients, people who have been exposed to infectious TB in the past 2 years, patients displaying fibronodular disease on x-ray
What does a positive tuberculin test tell you?
that they have been exposed to M. tuberculosis at some point in their life
What two factors can generate false positive tuberculin tests?
- Previous vaccination with BCG
2. Infection with a strongly cross-reacting non-tuberculosis mycobacteria
What 4 factors can generate false negative tuberculin tests?
- very recent TB infection (will only turn positive after about 4-6 weeks)
- Overwhelming (ie. Millary TB) infection
- Immunosuppression or anergy
- Incorrect administration of test
What is another method (besides the tuberculin test) to diagnose latent TB?
Interferon Gamma Release Assays (IGRAS)
How do IGRAS work?
measure T cell release of interferon gamma following stimulation by antigens specific to Mycobacterium tuberculosis
- measuring the cell mediated immune response
What are some benefits of IGRAS > tuberculin test? A downside?
- IGRAS will crossreact with fewer non TB mycobacteria
- Only need 1 visit
- Not affected by the BCG vaccine
- similar sensitivity between the two
problem: more expensive
What kind of testing (4) is done to diagnose active TB? What are the requirements?
- Microscopic examination of sputum that has been acid fast stained
- need at least 3 specimens collected 1 hr apart - Do PCR on smear positive specimens
- Chest radiograph
- Culture
What are some symptoms of active TB?
Weight loss, night sweats, fever, chills, active productive cough
What is the % sensitivity for chest radiographs as a method of diagnosing active TB? What are they useful for?
about 70-80 % but only if done with other testing methods
useful for evaluating symptoms and treatment
What is the most sensitive and specific method of testing for active TB?
Culture
Clinical and radiographic findings are ____ in diagnosis
non-confirmatory
How sensitive is AFB staining of sputum samples/microscopy?
about 60%
What two treatments need to be performed on Mycobacteria detected in direct smears (if they have been contaminated by normal flora)
- Decontaminating agent (NaOH)
2. Mucolytic agents also used for sputum specimens to dissolve mucus (N-acetyl L- cysteine)
What 5 forms of samples can you obtain to test for pulmonary TB?
- At least 3 sputum specimens
- Bronchial washings after bronchoscopy
- Bronchoalveolar lavage (BAL)
- Gastric aspirates (especially in children)
- Pleural fluid in Pleural Effusion
What kind of samples do you need to obtain for extrapulmonary TB? What are 3 examples?
Often need to be biopsies
- Liver and bone marrow biopsy in disseminated TB
- CSF in meningitis
- Urinary tract disease: First-void morning urine for 3-6 days
What is different between the culture protocol for sputum vs CSF ?
Sterile body fluids (e.g. CSF, pleural fluid) and tissue cultured directly while sputum & urine treated prior to culture
- treatment: digestion and decontamination
What is the difference between solid and liquid media for the culture of TB?
Liquid media: quicker growth and more sensitive but more prone to contaminants
- tend to use one of each media types
What 3 methods are used to determine growth of M. tuberculosis in culture?
- Colonial morphology, pigment, growth rate
- Biochemical tests: e.g. M. tuberculosis produce niacin and others do not
- Molecular methods
What are the 4 first line drugs used to treat TB? Which two are the most active and important? WhY? which one is bacteriostatic
- Isoniazid
- Rifampicin
- these are the most active/important
- bactericidal
- effective intra and extracellularly - Pyrazinamide
- bactericidal
- active at acid pH within cells - Ethambutol
- bacteriostatic
How long do you treat someone with the first line treatment of drugs for TB?
Treat with all 4 drugs for 2 months and then drop it down to just Isoniazid and Rifampicin for the remaining 4 months
When are patients non-infectious?
after about 2 -3 of chemo
When are second line drugs added?
added to combination if resistance or toxicity contraindicate first line drugs
What are some examples of second line drugs?
Para-Aminosalicylic acid (PAS) Streptomycin Ethionamide Cycloserine F Fluoroquinolones Kanamycin, ect .....
What 4 factors are responsible for the long treatment time for TB?
- Long doubling time of tubercle bacilli (12-24 h)
- Intra-phagocytic survival and multiplication.
- Metabolically inactive bacilli are not killed by drugs
- Caseous material interferes with drug action
What are mono and poly resistant TB defined as?
Mono-resistant: Resistance to a single drug
Poly-resistant: Resistance to more than one drug, but not the combination of isoniazid and rifampicin
What is MDR TB? XDR TB?
Multidrug-resistant (MDR): Resistance to at least isoniazid and rifampicin
Extensively drug-resistant (XDR): MDR plus resistance to fluoroquinolones and at least 1 of the 3 injectable drugs (amikacin, kanamycin, capreomycin
Where are atypical mycobacteria (NTM) usually acquired from?
the environment
What is the distribution of atypical mycobacteria?
worldwide
What is the instance of case to case transmission for NTM?
no evidence of it occurring
What kind of media do NTM grow on? how do they grow compared to TB?
grow on the same media just more quickly
What are 4 traits of NTM that differentiate them from TB?
- Colonial morphology
- Niacin test – negative
- Relatively more resistant to anti-TB drugs
- Diseases typically less invasive
what is the pathogen responsible for leprosy/Hansen’s disease?
Mycobacterium leprae
Most new cases of leprosy originate where?
India, Brazil and Indonesia
What are three characteristics of M. leprae?
long incubation period (usually 5, up to 20 years)
won’t grow on artificial culture media
low virulence/pathogenicity
Shedding of M. leprae from which part of the body is important in transmission?
shedding from the nose
What is the range of presentation for M. leprae?
discolored patches of skin, growths (nodules) on the skin, painless ulcers, painless swellings, skin numbness
The severity of leprosy lesions depends on..?
the cell mediated immune response mounted by the person
A good CMI response to M. leprae will usually result in?
Lesions that have numerous lymphocytes with few organisms in them - “Tuberculoid leprosy”.
A poor CMI response to M. leprae will usually result in?
Lesions that have numerous bacilli in them - “Lepromatous leprosy”.
An intermediate CMI response to M. leprae will usually result in?
between tuberculoid and lepromatous - called borderline leprosy
What is another name for tuberculoid leprosy? what do the lesions typically look like?
aka. Paucibacilary
Few skin lesions
peripheral nerve involvement/thickening (sensory>motor)
What is another name for lepromatous leprosy? what do the lesions typically look like?
aka. Multibacilary
Multiple lesions, symmetrical
Less nerve involvement
How do you test for M. leprae? what kind of specimens do you obtain?
Skin biopsy snips from ear lobe
AFB (Fite), PCR…does not grow in artificial culture
- special acid fast stain with a weaker decolourizer and PCR
How do you treat M. leprae? what determines the length of treatment? (3 drugs)
Duration depends on severity (Paucibacillary X 12m, Multibacillary X 24 m)
Dapson, Rifampin, Clofazimin
