MLT GlobeBacteriology: Mycobacteria
75 Bacteriology MCQs on Mycobacteria with answers and explanations.
Sputum from a suspected pulmonary TB patient is decontaminated for culture. Which solid medium is standard for growing Mycobacterium tuberculosis?
Lowenstein-Jensen contains egg, glycerol and malachite green to support slow mycobacterial growth and suppress contaminants. The other media are for routine bacteria.
Slender red rods resisting acid-alcohol decolourisation are seen on a Ziehl-Neelsen-stained sputum. Their staining is due to lipid-rich walls. The organism is most likely:
Mycolic acids make the mycobacterial wall waxy, so carbol fuchsin is retained after acid-alcohol (acid-fast). The other organisms are not acid-fast.
Which staining method is used to detect acid-fast bacilli of M. tuberculosis in a sputum smear by light microscopy?
The Ziehl-Neelsen (hot carbol fuchsin, acid-alcohol, methylene blue) stain shows red acid-fast bacilli. Auramine fluorescence is an alternative; Gram and Wright stains do not reliably show mycobacteria.
What structural feature makes Mycobacterium and Nocardia acid-fast?
Long-chain mycolic acids make the wall waxy, retaining carbol fuchsin despite acid-alcohol. Capsules and LPS do not confer acid-fastness.
In the Ziehl-Neelsen stain, tubercle bacilli retain carbol fuchsin after treatment with which decolouriser?
'Acid-fast' means the waxy mycolic acid wall holds carbol fuchsin against acid decolourisation, while non-acid-fast organisms lose it and take up the methylene blue counterstain.
Caseating granulomas in pulmonary tuberculosis result from which type of hypersensitivity?
Th1 cells activate macrophages to wall off mycobacteria, forming granulomas; this delayed hypersensitivity also underlies the tuberculin skin test.
An airborne pathogen causes latent infection in most people, detected by IGRA, with about 10% progressing to active lung disease. It is:
M. tuberculosis spreads by respiratory droplets, often remains latent, and reactivates in about 10% of infected people. M. fortuitum is an environmental rapid grower.
Which of these mycobacteria is an obligate human pathogen rather than an environmental saprophyte?
M. tuberculosis is a strict pathogen with humans as the reservoir. M. smegmatis, M. phlei and M. gordonae are environmental species usually regarded as contaminants.
A slit-skin smear from a patient with numb, hypopigmented skin patches shows acid-fast bacilli; the organism cannot be grown on artificial media. Most likely agent?
M. leprae causes leprosy with anaesthetic skin lesions and has never been cultured in vitro. M. ulcerans and M. marinum cause skin ulcers but grow in culture.
A sputum smear stained with auramine-rhodamine shows bright yellow bacilli under fluorescence microscopy. This stain is used to screen for:
Auramine-rhodamine binds mycolic acid and resists acid-alcohol, so acid-fast mycobacteria fluoresce. It is more sensitive than ZN for screening sputum.
The solid egg-based LJ medium for mycobacteria is named after:
LJ stands for Löwenstein–Jensen. Levey–Jennings is a quality-control chart, and Loeffler serum is used for C. diphtheriae.
In a lymphocytic pleural effusion, which enzyme is measured to support a diagnosis of tuberculosis?
Adenosine deaminase, released by activated T lymphocytes, is raised in tuberculous effusions. Amylase suggests pancreatitis or oesophageal rupture.
Which form of leprosy has the highest bacillary load and is most infectious?
Poor cell-mediated immunity in lepromatous leprosy allows huge numbers of bacilli (multibacillary) shed from nasal mucosa. Tuberculoid disease is paucibacillary.
Which of these organisms is an obligate aerobe?
M. tuberculosis requires oxygen, favouring the apex of the lung. E. coli and S. aureus are facultative; C. perfringens is anaerobic.
A lung biopsy in pulmonary tuberculosis would characteristically show which necrosis?
Caseous necrosis gives a cheese-like centre within granulomas. Liquefactive necrosis is seen in bacterial abscesses and brain infarcts.
In the Ziehl-Neelsen stain, the primary stain and decolouriser are:
Heated carbol fuchsin enters the waxy mycolic acid wall; acid-alcohol cannot remove it from mycobacteria, so they stay red against a methylene blue counterstain.
The main advantage of auramine-rhodamine fluorochrome staining over Ziehl-Neelsen staining for acid-fast bacilli is:
Fluorescent bacilli are easy to see at lower power, so more fields are read quickly, giving higher sensitivity. Positive smears can be confirmed with a carbol fuchsin stain. It does not identify species.
For diagnosis of pulmonary tuberculosis by smear and culture, the usual recommendation is to collect:
Three good-quality sputa, 8–24 hours apart and including an early-morning sample, increase detection. Pooled 24-hour collections are rejected because of overgrowth and dilution, and saliva is not sputum.
Runyon group IV mycobacteria, such as M. fortuitum and M. abscessus, are defined by:
Rapid growers form mature colonies in less than 7 days on subculture, unlike slow growers that need several weeks. Pigment after light exposure defines group I.
The Kinyoun acid-fast stain differs from the Ziehl-Neelsen stain because it:
Kinyoun is a 'cold' method: a higher concentration of basic fuchsin and phenol drives the dye into the waxy wall without heating. Auramine is used in fluorochrome staining, not Kinyoun.