Acid fast bacilli Flashcards
Mycobacterium
> Aerobic, non-spore-forming, nonmotile rods that are usually
straight or slightly curved (Strictly aerobic but increased CO2 will
enhance growth)
> Have distinctive staining characteristics due to high lipid content
of cell walls (mycolic acids)
> Gram stain poorly because lipids block penetration of crystal
violet and safranin (Ghost cells, gram-ghost, gram neutral)
> Acid-Fast stains use phenol to force cells to complex with special
dye (fuchsin or Auramine O). Once stained mycobacterium cells
resist de-staining even against acid alcohol (Hence “Acid Fast”)
mycobacterium group complexes
Closely related species are grouped together in a complex
> M. tuberculosis Complex (MTB)
- M. tuberculosis and
M. leprae
> Non-tuberculous mycobacteria (NTM)
- mycobacteria not M. tuberculosis complex or M.
leprae
> Mycobacteria other than tuberculosis (MOTT)
- atypical mycobacteria
MTB
M. tuberculosis (MTB) > Cells called tubercle bacilli > Member of MTB complex > Most important mycobacterium species > Causes Tuberculosis (TB
MTB infection state
> Primary TB - infection in previously uninfected individual
> Transmitted by droplet nuclei
> Organism multiplies in lungs and can be spread throughout body
> Most immunocompetent individuals contain the infection but it may remain dormant in body for years
> If host becomes debilitated these latent organisms can reemerge and cause secondary or reactivation tuberculosis
> Granulomas may occur (tubercles)- tumor-like inflammatory lesions
> Active Tuberculosis - only occurs in small percentage of cases
> MTB can occur in any body site (lungs, meninges, kidneys, bones, eyes, genital tract (testicles)
> Disseminated TB (miliary TB) - small tubercles spread throughout body
Tuberculin skin test
- Purified Protein Derivative (PPD) (an MTB antigen) injected under the skin
- Detects if patient has been exposed (active/latent)
- Positive - red hard area at injection site within 48-72 hrs
- Negative - no response to the antigen
Progression of TB
- healthy person exposed to low dose- activated macrophages stop infection
some individuals wont have a rapid response
ex:
- bacteria multiply in lung macrophages
- phagocytes attracted to site of infection
- infection may be walled off/ form tubercle
tubercle: nodule that contain caseous necrosis, which form in the lung as a result of infection with mycobacterium tuberculosis
- growth of bacteria causes interior of tubercle to liquify
- rupture of tubercle allows bacteria to escape; leads to infection of other parts of the body
- suppression of immune system may allow bacteria to break out of lesions & multiple ( reactivation)
- old age, cancer, immunosuppressive drugs & HIV infection can lead to reactivation
Mycobacterium ID
- acid fast staining specimens
- positive skin test ( red areas at injection
- tubercles may calcify anf become visible in chest xray
- lung biopsy ( acid-fast stain)
mycobacterium bovis
- MTB complex species by rare in Canada & USA
Mycobacterium avium complex
- includes M. avium and M. intracellulare
- environmental organisms that may harmlessly colonize humans
- may cause pulmonary infection and inflamed lymph nodes in immunocompetent
individuals - causes disseminated disease or gastrointestinal disease in AIDS patients
- most common cause of NTM infection(non tuberculosis mycobacterium)
- resistant to many antimycobacterial drugs (TB drugs)
- abbreviated MAC (M. avium complex) and MAI (Mycobacterium avium-intracellulare)
Mycobacterium leprae
- causes leprosy (disease of the skin, mucus membranes and peripheral nerves)
- known as Hansen’s bacillus
- does not grow in vitro
- can be grown in mice footpads and in armadillos
- diagnosis based on clinical presentation and non-culturable AFB in skin
biopsies
Mycobacterium gordonae
- tap-water contaminant (usually non-pathogenic)
- common laboratory contaminant
specimen collection and transport ( all acid fast )
> Collect in sterile, leak-proof containers and transport promptly to lab
> Refrigerate if not processed immediately except blood which is kept at room temperature
> Do not collect in waxed containers as can cause false positive smear
Laboratory Safety
> TB airborne safety hazard
> Biosafety level (BLS) 2 practices adequate for processing specimens
-gloves, gowns, and all manipulations in a certified biological safety cabinet
> BLS 3 must be used when working with organisms
Specimens of Choice
> Respiratory Tract Specimens (Sputum, Bronchial Washings) > Gastric Aspirates > Urine > Stool > Blood > Bone Marrow > Sterile Body Fluids > Tissue (Biopsies) > Wound Material
Specimen Processing
> Concentration - Most specimens must be concentrated by
centrifugation before inoculation onto culture media
> Decontamination - Specimens that may be contaminated by
normal flora are concentrated and decontaminated before setup with
alkaline or acid agents
> Digestion - mucolytic agent used to liquify mucoid specimens (ex. Sputums)
- Frees the bacteria from clumps of protein allowing it to sediment
during centrifugation
