Mycobacterium Flashcards
what kind of bacteria does this describe?
- thin, rod shaped
- obligate aerobes
- non-motile
- tough cell wall and envelope with high lipid content
- acid fast
- slow growing
mycobacterium: has mycomembrane with mycolic acid (long lipid chain) —> makes wall highly impermeable, hardy —> high transmissibility and resistance to antibiotics, acid fast staining
describe the clinical effect/importance of the mycomembrane of mycobacterium
mycobacterium: has mycomembrane with mycolic acid (long lipid chain) —> makes wall highly impermeable, hardy —> high transmissibility and resistance to antibiotics, acid fast staining
what are mycobacterial-specific features of the cell wall? (4)
- acyl lipids
- mycolate (mycolic acid): long lipid chain, contributes to impermeability and antibiotic resistance
- arabinogalactan: biopolymer of arabinose + galactose monosaccharides
- LAM (lipoarabinomannan)
what kind of bacteria has the following in its cell wall?
- acyl lipids
- mycolate
- arabinogalactan
- LAM (lipoarabinomannan)
mycobacterium
what is different about the peptidoglycan found on the cell walls of mycobacterium?
peptidoglycan contains N-glucolylmuramic acid instead of N-acetylmuramic acid
explain why mycobacterium is considered “acid fast”
the cell wall and outer mycomembrane is so tough that it absorbs dye too well, and even acid alcohol cannot decolorize it
thus, they are “acid fast”
what 2 staining techniques can be used for mycobacterium? describe them
- Ziehl-Neelsen (ZN) acid-fast staining: Carbol Fuchsin + acid-alcohol decolorize + methylene blue counterstain = red mycobacterium on blue background
- fluorochrome stain: Auramine O-Rhodamine + acid alcohol + potassium permanganate (removes autofluorescence) = yellow mycobacterium on background (more sensitive but less specific)
what are 2 ways to divide mycobacterium, of which there are 100+ species?
- Mtb (Mycobacterium tuberculosis) complex: all cause TB, so species level ID is not clinically necessary (but useful for epidemiology), all form non-pigmented colonies
[M. tuberculosis, M. bovis, M. bovis BCG, M. africanum, M. microti, M. canettii] - NTMs (non-tuberculous mycobacteria): ubiquitous, aka environmental or atypical mycobacteria, classified by pigmentation with/without light
M. tuberculosis, M. bovis, M. bovis BCG, M. africanum, M. microti, M. canettii all belong to what group?
Mtb (mycobacterium tuberculosis) complex: mycobacteria that all cause TB and form non-pigmented colonies
ID is not necessary for clinical purposes (all cause TB), but good for epidemiology
how is TB transmitted?
via mycobacterium tuberculosis (MTB complex) in respiratory droplets —> bacilli travel to alveoli
some patients are never infectious, others infectious for months
describe the pathogenesis of TB
mycobacterium tuberculosis (MTB complex) inhaled via respiratory droplet, bacilli travel to alveoli
tubercle bacilli multiple in alveolar macrophages —> early logarithmic growth prior to immune response
2-8 weeks: cell mediated immunity develops, activated macrophages form granuloma —> LTBI (latent TB infection), Mtb in dormancy
*recall latent TB is delayed/Type IV hypersensitivity
becoming immunocompromised (HIV, age) may allow TB escape and dissemination —> active disease, miliary TB
how does a lung TB infection look histologically?
granulomas with Langerhan’s giant cells (foamy macrophages)
bacilli can persist in granulomas for a very long time
how can delayed (type IV) hypersensitivity of LTBI (latent TB infection) be demonstrated with a diagnostic value? (2)
describe these tests
- interferon-gamma release assay (IGRA): measure immune reactivity based on IFNy release (measure whole blood, whole blood + Mtb peptides, whole blood + mitogen)
- tuberculin skin test (TST): aka Mantoux test, inject Mtb extract or purified protein (PPD) —> ring of induration is measured (48-72h later)
[remember that Type IV hypersensitivity is Th1 mediated]
describe how tuberculin skin test (TST) is read and interpreted
aka Mantoux test: inject Mtb extract or purified protein (PPD) —> specific T cells recruited, which produce IFNy and activate macrophages —> ring of induration is measured (48-72h later)
smaller diagnostic values are considered positive for high-risk populations (sliding scale interpretation)
describe how the IGRA TB test works
IFNy release assay: measure IFNy released from blood T-lymphocytes that have been infected with Mtb, using 3 measurements of IFNy:
- whole blood alone: baseline
- whole blood + Mtb peptides: measures stimulated IFNy in response to recombinant, specific Mtb antigen (NOT cross-reactive with BCG as skin test can be)
- whole blood + non-specific activator of WBC (mitogen): demonstrates WBCs are present and capable of secreting IFNy
what is the major advantage of IFNy release assay over skin test for TB diagnosing? how are they typically used for high vs low risk patients?
TST can cause cross-reactivity to other mycobacteria, giving false (+)
while IGRA measures stimulated IFNy release in response to recombinant, specific Mtb antigens
*for high risk patients, a second diagnostic test should be performed even if initial test is neg. - do not want to miss early diagnosis (increase sensitivity) - diagnosis made with single positive test
*for low risk patients, a confirmatory test is used following an initial positive result (ensure specificity) - LTBI ruled out with single neg. test
active TB may represent reinfection, reactivation, or progressing primary infection
in this case, tubercle bacilli escape granuloma and disseminate
what is this termed?
miliary TB: TB infected lymph node erodes a vessel wall and tubercle bacilli spread via blood
on x-ray, see diffuse miliary pattern (“millet seeds”) throughout lung
which of these is true of active TB disease but NOT LTBI?
a. may spread TB mycobacterium to others
b. usually (+) TST or IGRA
c. needs treatment
person with latent TB (LTBI) CANNOT spread TB mycobacterium to others, also have normal chest x-ray
*also note that persons with active infection are symptomatic while persons with LTBI are asymptotic
tissue injury following TB infection is a consequence of…
immune response (rather than specific toxins and virulence factors)
host destroys itself to control intracellular bacterial growth
caseous necrosis and cavity formation
what kind of necrosis does TB cause
TB is most common cause of caseous necrosis (“cheese like”)
necrotic cellular debris surrounded by zone of suppurative inflammation
describe how non-tuberculous mycobacteria (NTMs) are classified
(aka environmental or atypical mycobacteria)
Runyon classification (4 groups):
- photochromogens: pigmentation only in light - M. kansasii, M. marinum
- Scotochromogens: pigmentation with/without light - M. gordonae, M. xenopi, M. scrofulaceum
- nonphotochromogens: M. avium complex (MAC), M. ulcerans
- rapid growers: M. abscessus, M. chelonae, M. fortuitum
what kind of bacteria (including classification) is M. kansasii, and what kind of illness does it cause?
NTM: non-tuberculous mycobacteria, Runyon classification is photochromogen (Group 1 - pigmentation only in light)
causes chronic pulmonary infections involving upper lobes of lungs (resembles MTB clinically)
*dissemination is rare except in AIDS
where does M. kansasii (NTM, photochromogen) come from, and how can it be identified?
tap water is major reservoir
slow growing, responds quickly to antimicrobial therapy
ID with DNA probe
which bacteria species does this describe?
- photochromogen
- tap water is major reservoir
- causes chronic pulmonary infection
- slow growing, and dissemination is rare except in AIDS
M. kansasii: non-tuberculous mycobacteria (NTM), responds quickly to antimicrobial therapy
remember photochromogen (Group I Runyon classification) means it only pigments in light
what kind of bacteria (including classification) is M. marinum, and what kind of illness does it cause?
non-tuberculous mycobacteria (NTM), photochromogen (Group 1 Runyon, pigmentation in light), grows slowly
causes cutaneous infection associated with exposure to salt/freshwater - “swimming pool granuloma” or “fish tank granuloma”
what bacteria species does this describe?
- most common in southern coastal states
- photochromogenic
- causes “swimming pool” or “fish tank” granuloma
M. marinum: NTM, Group 1 Runyon (photochromogen - pigmentation in light)
causes cutaneous infection associated with exposure to salt/freshwater
grows slowly
what kind of bacteria (including classification) is M. gordonae?
non-tuberculous mycobacteria (NTM), scotochromogen (Group 2 Runyon, pigmentation with/without light)
most common non-pathogenic NTM - no treatment required
found in soil/water, colonizes respiratory tract
DNA probe for ID
what kind of bacteria (including classification) is M. xenopi? what kind of disease does it cause?
non-tuberculous mycobacteria (NTM), scotochromogen (Group 2 Runyon, pigmentation with/without light)
causes chronic pulmonary disease in adults with underlying lung disease (COPD)
what kind of bacteria are those in the M. avium complex (MAC)?
MAC: M. avium, M. intracellular, M. paratuberculosis
non-tuberculous mycobacterium (NTMs), nonphotochromogens (Group 3 Runyon)
most commonly isolated mycobacterium spp. (species)
slow growing, ID with DNA probe
what kind of bacteria are M. abscessus, M. fortuitum, and M. chelonae?
rapid-growing mycobacteria (RGM), Group 5 of non-tuberculous mycobacteria (NTM) - growth in <7 days (fast for mycobacteria)
often healthcare facility-acquired (considered preventable) - post-traumatic/surgical/injection wound infections —> causes pulmonary infections with abscesses that are difficult to treat
describe the nature of infection by Mycobacterium leprae
- cannot be cultivated in vitro (armadillo animal model)
- spread person to person, likely via nasal secretions
- long incubation (could be years)
- either lepromatous (widespread lesion with high bacterial loading) or tuberculoid (limited lesions with low bacterial loading)
what 3 kinds of testing need to be performed for mycobacterium, and what is the timeline for each?
- smearing/staining: acid fast or fluorochrome staining (hours)
- NAAT/PCR (hours)
- cultivation: identification and susceptibility testing, to guide clinical treatment (weeks)
what does “cording” with an acid-fast stain indicate, and what causes it?
cording is characteristic of Mycobacterium tuberculosis (MTB)
due to cording factor trehalose dimycolate - considered virulence factor because it can kill phagocytes (anti-cord antibodies are protective)
what is the CDC guideline regarding culturing mycobacterium?
both solid and liquid media inoculated (liquid media reduces turn-around time)
incubation for 8 weeks (mycobacterium are slow-growing)
