Mycobacteria Flashcards
What is unique about the cell wall of mycobacteria?
Contain mycolic acids, very lipid rich
What staining procedure is used for mycobacteria?
Acid fast
What are the antigens in a Tuberculin Skin Test directed towards?
PPD polypeptides in the cell wall
Which mycobacteria is extremely slow growing?
M. Leprae
From which mycobacteria strain is BCG (the TB vaccine) derived from?
M. bovis
On what medium do you grow M. tuberculosis?
Lowenstein-Jensen (L-J) medium
How does M. tuberculosis appear in liquid medium?
Cording (cord factor)
How can M. tuberculosis and M. bovis be differentiated with biochemical tests?
Both are nitrate reductase positive
M. tuberculosis is niacin positive
M. bovis is niacin negative
What cells primarily mediate the immune response to M. tuberculosis?
CD4+ T cells (specifically Th1) that secrete IFNgamma and macrophages
How is the mycobacteria tuberculosis contained?
Granuloma formation by the T cells and macrophages
What cytokine is necessary for macrophage response and granuloma formation to contain M. tuberculosis?
TNFalpha (remember not to give TNFalpha inhibitors to people at risk of recurrent TB infection)
Which test for tuberculosis can differentiate between BCG vaccine and active M. tuberculosis?
IGRA (QuantiFERON-TB Gold test (blood test))
How does drug resistance develop in Mycobacterium?
Point mutations
NOT plasmid or transposons
How can we grow Mycobacterium leprae?
Armadillos and mice
Cannot culture
Tuberculoid v. Lepromatous M. leprae
Tuberculoid - Th1 predominance, few skin lesions, single nerve involvement
Lepromatous - Th2 predominance, many lesions, several nerves affected
What is the hallmark clinical feature of M. leprae?
Anesthetic skin lesion + thickened peripheral nerve
How is TB most commonly transmitted?
Inhalation of infectious droplets from another person with smear positive pulmonary TB
What is the primary lesion in lung of TB patient called?
Ghon focus
What are the outcomes of primary TB?
- Usually Ghon focus heals with calcification, TB is latent
- Some bacilli can spread in blood stream and cause primary TB disease
What are some defining characteristics of a latent TB infection?
- Positive PPD
- Absence of any clinical symptoms
- Normal CXR (or could have calcification of Ghon focus)
- Negative smear or culture
What are the steps in diagnosis/management of latent TB infection?
- Screen patients at increased risk
- Perform and interpret screening tests (TST or IGRA)
- If test is positive, rule out active TB with phys exam and CXR
- Assess risks and benefits of LTBI treatment and treat if necessary
What is the preferred treatment for latent TB?
Isoniazid daily x 9 months
What is the hallmark of reactivated TB on a CXR?
Cavitation in upper lobes
What are diseases that manifest from extrapulmonary tuberculosis?
- Basilar meningitis
- Pott’s disease (Gibbus formation of vertebral collapse)
- Miliary tuberculosis (Millet seed lesions on CXR)
- Lymphadenitis
