Mycobacteria (Fan) - 4/28/16 Flashcards
General Characteristics of mycobacteria (8)
- Thin, rod shaped
- Obligate aerobe
- Non-motile
- Cell wall contains N-glucolylmuramic acid (instead of N-acetylmuramic acid) and has very high lipid content
- Acid fast (3% HCl, heated)
- Slow growing (doubling time: 30 h –> 4-8 weeks to form colonies)
- Do not form endospores
- Over 100 species (Mtb complex vs NTMs)
Gram- bacteria cell wall vs. Mycobacterial-specific cell wall features
Gram- bacteria:
- Lipid layers
- Peptidoglycan
- Porins
- LPSs
Mycobacterial-specific features:
- Lipid layer
- Peptidoglycan
- Porins
- Arabinogalactan
- Mycolate (C54-C78) - very long lipids (protective)
- Acyl lipids
Difficult to stain
Describe the Mtb complex.
Components: M. tuberculosis, M. bovis, M. bovis BCG, M. africanum, M. microti, M. canetti
All may cause TB in humans and animals
Non-pigmented colonies
Mtb disease overview.
1/3 of the world’s population is infected
Pathogenesis:
- Inhalation of organisms produces disease in 15-20%
- M. bovis from milk
- M. bovis BCG from vaccination (in immune-compromised people)
- Disease may occur many years after exposure
- Small % of patients develop disseminated disease
Spectrum of disease:
- Low grade fever, night sweats, anorexia, weight loss
- Productive cough w/ pulmonary infection + fever, chills, myalgia, and sweating
How is TB transmitted?
Spread person to person through the air via droplet nuclei –> bacilli reach alveoli of lungs –> bacilli multiply in alveolar macrophages –> within 2-8 wks, cell mediated immunity develops and activated macrophages surround tubercle bacilli (these cells form a barrier shell called a granuloma, that keeps bacilli contained and under control)
Mtb may be expelled when an infectious person (coughs, sneezes, speaks, sings)
Two types of TB testing:
- Tuberculin Skin Testing (TST)
- Interferon-gamma release assays (IGRAs)
TST:
Measure ring of INDURATION (not ring of redness) 2-3 days after (delayed-type hyper-sensitivity)
IGRAs:
- Like TSTs, IGRAs measure a person’s immune reactivity to Mtb
- Principle is the same but you cut off the last step so you don’t wait for interferon gamma to induce local inflammation
- 3 separate measurements of IFN-gamma are obtained:
1) Whole blood alone (baseline level of IFN-gamma)
2) Whole blood + Mtb peptides (TB specific antigen): IFN-gamma release in response to RECOMBINANT, SPECIFIC Mtb antigens (not cross reactive w/ BCG)
3) Whole blood + non-specific activator of WBCs (mitogen): demonstrates that WBCs are present and capable of secreting IFN-gamma
IGRA or TST?
IGRA may be used in place of (but not in addition to) TST
IGRA preferred when:
- Testing patients that are unlikely to return for reading
- Testing BCG-vaccinated individual
TST preffered when:
- Testing children
Risk factors for Mtb progression
- Persons with HIV infection
- Infants/children
Latent TB infection (LTBI) vs. Active Disease
Latient:
- No symptoms
- Does not feel sick
- Cannot spread TB bacteria to others
- Usually has a skin test or blood test result indicating TB infection
- Normal chest x-ray and negative sputum smear
- Needs treatment for LTBI to prevent active TB disease
Active:
- Has symptoms (bad cough, coughing up blood/sputum, weakness, weight loss, fever)
- Usually feels sick
- May spread TB bacteria to others
- Usually has a skin test or blood test result indicating TB infection
- May have abnormal chest x-ray or positive sputum smear/culture
- Needs treatment to treat (multiple drug therapy)
Laboratory diagnosis of mycobacterial infections:
- Specimens
- Specimen processing
- Direct detection
- Cultivation
- Identification
- Antimycobacterial susceptibility testing
Specimen
- Respiratory (sputum)
- Urine (clean catch)
- Stool
- Tissue and body fluids
- Blood - immunocompromised patients
- Wounds - aspirate > swab
Specimen processing
Specimens collected from normally sterile sites can be directly stained and inoculated to media
Specimens from non-sterile sites must first be decontaminated before further analysis
Direct pathogen visualization
- Acid-fast stain (cording)
- Fluorochrome stain (more sensitive than acid fast stain)
Contamination is an important consideration
Cultivation
Solid media and liquid media inoculated or all specimens, incubated for 8 wks
Liquid media reduces average turn-around time (TAT) from 17 days to 10 days
Identification
Acid fast stain
Nucleic acid probes for Mtb complex, MAI, M. kansasii and M. gordonae
Probe negative isolates require additional work: morphology, pigment production, biochem mostly replaced by DNA sequencing and other molecular methods
