MLT GlobeBacteriology: Mycobacteria – page 2
75 Bacteriology MCQs on Mycobacteria with answers and explanations.
Malachite green is included in Löwenstein-Jensen medium to:
Malachite green is a selective dye that suppresses many contaminating bacteria. The egg base is solidified by inspissation, not by the dye.
Identification and drug-susceptibility testing on a growing culture of Mycobacterium tuberculosis should be performed under:
Manipulating cultures produces aerosols with very high numbers of bacilli, so BSL-3 containment is required. A clean-air bench protects the product, not the worker.
A surgeon sends only a dry swab from a deep abscess for mycobacterial culture. Why are swabs discouraged for this test?
Swabs hold only a small volume, and waxy mycobacteria stick to the fibres, lowering recovery. Tissue or aspirated fluid is preferred.
On a Gram stain of sputum, Mycobacterium tuberculosis typically appears as:
The lipid-rich wall resists crystal violet penetration, so mycobacteria stain weakly gram-positive, beaded, or not at all. An acid-fast stain is needed to show them.
The BCG vaccine contains:
BCG (bacille Calmette-Guérin) is a live attenuated strain of M. bovis.
In the Ziehl-Neelsen stain, which counterstain is usually applied after decolourisation?
Methylene blue stains the background and non-acid-fast cells blue, contrasting with red acid-fast bacilli.
In the Ziehl-Neelsen method, carbol fuchsin is heated on the slide in order to:
Heat softens the lipid-rich wall so carbol fuchsin can enter; once inside, it resists acid-alcohol.
Leprosy mainly damages the skin and which other tissue?
M. leprae prefers cooler tissues and invades Schwann cells, causing sensory loss and deformity.
Patients taking rifampicin for tuberculosis should be told that the drug commonly:
Rifampicin is a red-orange pigment that colours urine, sweat and tears; this is harmless.
Which of these is a first-line anti-tuberculosis drug?
Standard first-line TB treatment uses isoniazid, rifampicin, pyrazinamide and ethambutol.
Multidrug-resistant tuberculosis (MDR-TB) is defined as resistance to at least:
MDR-TB means resistance to at least isoniazid and rifampicin, the two most important first-line drugs.
A Mantoux tuberculin skin test is read how long after injection?
The test measures delayed-type hypersensitivity, so the induration is read after 48–72 hours.
Tuberculosis of the spine is known as:
Pott's disease is spinal TB; scrofula is TB of cervical lymph nodes and lupus vulgaris is skin TB.
Which statement about Gram staining is INCORRECT?
The waxy mycobacterial wall resists Gram reagents, so M. tuberculosis stains poorly or appears as 'ghosts'; acid-fast staining is required.
In Runyon's classification of non-tuberculous mycobacteria, 'photochromogens' such as M. kansasii are defined by:
Photochromogens are non-pigmented in the dark and turn yellow after light exposure. Scotochromogens are pigmented in the dark; non-chromogens are not pigmented.
A mycobacterial isolate forms yellow-orange colonies whether incubated in darkness or light. In the Runyon scheme, it is a:
Scotochromogens (Runyon group II, e.g. M. gordonae) are pigmented even in the dark. Photochromogens (group I, e.g. M. kansasii) produce pigment only after light exposure.
Pleural fluid from a patient with tuberculous pleurisy is typically an exudate in which the predominant cell type is:
Tuberculous pleural effusions are lymphocyte-predominant exudates, often with few mesothelial cells. Neutrophils dominate in bacterial parapneumonic effusions.
The standard Mantoux tuberculin skin test injects intradermally:
The standard dose is 5 tuberculin units (PPD-S) or equivalent 2 TU PPD RT23, in 0.1 mL; induration is read at 48–72 hours.
Which species is NOT a member of the Mycobacterium tuberculosis complex?
M. kansasii is a photochromogenic non-tuberculous mycobacterium. M. bovis, M. africanum and M. microti belong to the MTB complex.
A slow-growing, rough, buff-coloured acid-fast bacillus on Lowenstein-Jensen that is niacin positive and nitrate reduction positive is most likely:
M. tuberculosis forms dry 'rough and buff' colonies after 2-6 weeks and is niacin and nitrate positive. M. kansasii is photochromogenic and M. fortuitum grows rapidly.