Mycobacterium Flashcards
General Characteristics
- Thin, rod shaped
- Obligate aerobe
- Tough cell wall and membrane
- Acid-Fast stain
- Slow growing- 4-8 weeks to form colonies
The Unusual Structure of the Mycobacterial Cell Wall
Very Hearty; Very Hard to Kill
- Lipid-rich wall is highly impermeable to regular antibiotics and dyes
- Hardiness -> high transmissibility
- Acid Fast Stain (3% HCl; heated): stains mycolic acid
- Effective therapeutic targets for anti-TBs
Acid Fast Stain
- Stains myocolic acid
- HCl 3%, heated
- Ziegler’s-Nielsen (ZN) Stain
- Carbol Fuchsin binds to mycolic acid
- Fluorochrome Stain (Sputum) - modified acid-fast; more sensative
Outer Membrane (mycomembrane)
- High lipid content
- Mycolic Acid - up to 90 Carbons!
- Long chain folded and stacked
- stacking leads to increased permeability
Mtb Colonies
- Non-pigmented
- Lowenstein-Jensen agar: allows to observe colony and color for diagnostics
Mtb Gram Staining
- Gram stain gives unique Ghost Stain or GP indeterminate
- if you don’t suspect Tb but see this… suspect Tb
- CANNOT DESTAIN MYCOBACTERIUM
Tuberculosis Epidemiology
1/3 of world infected
- S. Africa, S. Asia, Pacific Islands, N. Asian, Mongolia, N. Korea
Mtb Disease Overview
Pathogenesis
- caught via inhalation; not everyone gets sick; long incubation
- can also be inoculated or ingested
- Tubercle bacilli multiply in alveolar macrophages
- Tissue injury consequence of immune response
- HOST DESTROYS ITSELF to control bacterial growth
Latent Tb
- dormancy via immunity
- no disease present because shielded by macrophages
- most Tb infections remain latent due to delayed type hypersensitivity
- reaction which helps diagnose via skin test
TB Pathology
- necrosis: over growth of connective tissue
- loss of lung function leading to death
TST
Tuberculin Skin Testing
- aka Mantoux Test
- Antigens injected intradermally
- look for ring of induration (not redness)2-3 days later
- result of delayed-type hypersensitivity
Do results depend on risk factors and patient history
- result of delayed-type hypersensitivity
> 15mm considered positive for any person
Problems with TST
- incorrect reading
- inconsistent interpretation
- failure to follow up
- cross reactivity to other mycobacterium and positive in BCG vaccinated persons
LTBI vs Active Disease
Hard to test for latent vs active because both would test positive
Immunodiagnosis: Interferon-y Release Assays (IGRAs)
- Directly measure a person’s immune reactivity to Mtb
- Takes time for lymphocytes to proliferate
3 Separate Measurements Obtained:
- whole blood along: baseline
- whole blood plus Mtb peptides: not cross reactive with BCG
- whole blood plus non-specific activator of WBCs (nitrogen): demonstrates WBCs are present and capable of secreting IFN-y (make sure the test has potential to be correct)
Microbiology Test for Active TB Disease
- Specimens
- Specimen Processing
- Direct. Detection & NAAT
- Cultivation
- Identification
- Antimycobacterial Susceptibility Testing
TB Tests: Specimens
- Sputum
- Urine
- Stool
- Tissue and body fluids
- Blood (immunocompromised persons)
- Wounds (aspirate>swab)
TB Tests: Direct Pathogen Visualization
- Acid-fast Stain - sensitivity 20-80% (depends on patient population)
- Fluorochrome Stain - increased sensitivity to AF
Contamination important consideration
Cord Factor
Characteristic of Mtb
Cord Factor ( trehalose dimycolate)
Cording - bacteria binding to each other and aggregating
- virulence factor: toxic to phagocytes and other cells
- anti-cord antibodies are protective
- cording = probs no antibodies
Mtb NAATs
Nucleic Acid Amplification Test
Use in conjunction to other testing
Turn around time similar to smear, faster than culture (remember its very slow growing)
Superior to Smear because:
- greater specificity
- greater sensitivity
Mtb Cultivation
Always culture even if already proven to be Tb
- Lowenstein-Jensen medium incubated for 8 weeks
- confirms diagnosis and guides treatment
- Liquid faster because of increased oxygen and nutrients
MTb Identification
- Acid Fast Stain
- Nucleic Acid Probes
- Probe negative isolates require additional work
- morphology
