MLT GlobeBacteriology: Mycobacteria – page 4
75 Bacteriology MCQs on Mycobacteria with answers and explanations.
Which microbiological finding meets the ATS/IDSA criterion for nontuberculous mycobacterial lung disease in a patient with compatible symptoms and imaging?
NTM are common in the environment, so one sputum isolate may be contamination. Two separate positive sputa (or one bronchial wash/lavage) with the same species are required.
A man treated with intravesical BCG for bladder cancer develops fever and prostatitis. A slow-growing Mycobacterium tuberculosis complex isolate grows from his urine. The most likely organism is:
BCG is a live attenuated M. bovis strain, and local or disseminated infection can follow bladder instillation. M. kansasii is not a member of the M. tuberculosis complex.
About how many acid-fast bacilli per mL of sputum are needed for a Ziehl-Neelsen smear to be reliably positive?
Smear microscopy needs roughly 5,000–10,000 bacilli/mL, which is why culture and NAAT are more sensitive.
Reactivation pulmonary tuberculosis typically affects the upper lobes (apices) of the lungs because M. tuberculosis:
The lung apices have the highest oxygen tension, which suits the strictly aerobic tubercle bacillus.
A commonly used pleural fluid adenosine deaminase cut-off supporting tuberculous pleurisy is about:
Pleural ADA above roughly 40 U/L, in a lymphocytic exudate, strongly supports tuberculosis in high-prevalence settings. Serum ADA reference ranges vary by method.
When Ziehl–Neelsen staining for Mycobacterium leprae, the usual decolouriser is:
M. leprae is less acid-fast than M. tuberculosis, so a weaker decolouriser (5% H2SO4) is used. 20% sulphuric acid is used for M. tuberculosis in some protocols.
An M. tuberculosis complex isolate is niacin negative, nitrate negative, susceptible to thiophene-2-carboxylic acid hydrazide (TCH) and naturally resistant to pyrazinamide. It is most likely:
M. bovis (including BCG) is intrinsically pyrazinamide resistant, niacin and nitrate negative, and inhibited by TCH. M. tuberculosis is niacin and nitrate positive and TCH resistant.
A line probe assay detects a mutation in the inhA promoter of an M. tuberculosis isolate, with no katG mutation. This usually indicates low-level isoniazid resistance and cross-resistance to:
inhA encodes the target shared by isoniazid and ethionamide, so inhA promoter mutations give low-level isoniazid resistance and ethionamide cross-resistance. katG mutations give high-level isoniazid resistance without ethionamide cross-resistance.
According to the 2021 WHO definitions, an MDR-TB isolate that is also resistant to levofloxacin and to bedaquiline is classified as:
XDR-TB is MDR/RR-TB with resistance to any fluoroquinolone plus at least one of bedaquiline or linezolid. Pre-XDR-TB is MDR/RR-TB with fluoroquinolone resistance only.
Over one month, 9% of a laboratory's NALC-NaOH processed sputum cultures on solid media are overgrown by bacteria and fungi. The most likely cause is:
An acceptable contamination rate is about 2–5%. A higher rate suggests weak NaOH or too short exposure; a rate below 2% suggests over-harsh processing that also kills mycobacteria.
A kidney transplant recipient has painful skin nodules. Biopsy shows acid-fast bacilli. Culture grows only on chocolate agar incubated at 30 °C, not on Löwenstein-Jensen at 37 °C. The most likely organism is:
M. haemophilum needs hemin or ferric ammonium citrate and grows best at 28–32 °C, so it is missed on routine LJ at 37 °C. M. marinum does not need hemin.
A non-pigmented mycobacterium from a surgical wound grows in 4 days. It is positive for 3-day arylsulfatase, nitrate reduction and iron uptake. The most likely species is:
M. fortuitum is a rapid grower that reduces nitrate and takes up iron. M. chelonae and M. abscessus are also arylsulfatase positive but nitrate negative.
In the agar proportion method, a drug-free quadrant inoculated with a 1:100 dilution grows 60 colonies. The isoniazid quadrant, inoculated undiluted, grows 90 colonies. The isolate is:
The undiluted control equals 60 × 100 = 6000 CFU. Growth on drug is 90/6000 = 1.5%, which exceeds the 1% critical proportion, so the isolate is resistant.
An Xpert MTB/RIF Ultra result on sputum reads 'MTB detected trace; rifampicin resistance indeterminate'. This means:
'Trace' is the lowest positive category, detected by multi-copy targets; the rpoB region is not amplified enough to call resistance. The result is valid, not an internal-control failure.
A Mycobacterium abscessus isolate has a clarithromycin MIC in the susceptible range on day 3. CLSI recommends reading clarithromycin again at day 14 because:
Many M. abscessus strains carry a functional erm(41) gene that induces macrolide resistance during exposure, visible only with extended incubation. The organism itself grows in 3–5 days.