MLT GlobeBacteriology: Mycobacteria – page 3
75 Bacteriology MCQs on Mycobacteria with answers and explanations.
In NALC-NaOH processing of sputum for mycobacterial culture, the role of N-acetyl-L-cysteine (NALC) is to:
NALC is a mucolytic that breaks disulfide bonds in mucus. Sodium hydroxide is the decontaminating agent, and phosphate buffer is used to dilute and neutralize afterwards.
In liquid culture, Mycobacterium tuberculosis grows as parallel, rope-like 'serpentine cords'. This appearance is linked to:
Cord factor (trehalose 6,6'-dimycolate) makes the bacilli stick side by side in serpentine cords, a presumptive sign of the M. tuberculosis complex. Niacin accumulation is a biochemical test, not a morphology.
A slow-growing acid-fast bacillus forms cream colonies in the dark, which turn yellow after exposure to light. Which species fits this Runyon group?
Pigment formed only after light exposure defines photochromogens (Runyon group I), such as M. kansasii. M. gordonae is a scotochromogen (pigmented in the dark), M. avium is a non-chromogen and M. fortuitum is a rapid grower.
An aquarium keeper has a nodular skin lesion on the hand. Acid-fast bacilli are seen in a biopsy. For best recovery of the most likely organism, the laboratory should also incubate cultures at:
M. marinum causes 'fish tank granuloma' and grows best at 28–32 °C, often poorly at 37 °C. Skin and soft-tissue specimens should therefore also be cultured at about 30 °C.
A yellow scotochromogenic, slow-growing mycobacterium is isolated once from a bronchial wash. It is known to be common in tap water and rarely causes disease. It is most likely:
M. gordonae, the 'tap water bacillus', is a scotochromogen and a frequent contaminant from water used in procedures or processing. A single isolate is usually not significant.
A patient with advanced HIV (CD4 count below 50 cells/µL) has fever, weight loss and anaemia. Which specimen is most useful to diagnose the most likely disseminated non-tuberculous mycobacterial infection?
Disseminated Mycobacterium avium complex is common at very low CD4 counts and causes continuous bacteraemia, so blood cultured in mycobacterial media is the best specimen. Routine urine culture media do not support mycobacteria.
A positive liquid mycobacterial culture is tested with an immunochromatographic test for MPT64 antigen and gives a positive result. This indicates:
MPT64 is a protein secreted by M. tuberculosis complex organisms but not by most non-tuberculous mycobacteria, so the test rapidly identifies MTBC from positive cultures. Some BCG strains can be negative.
The Xpert MTB/RIF assay detects rifampicin resistance by finding mutations in:
About 95% of rifampicin resistance is caused by mutations in the 81-bp rifampicin resistance-determining region of rpoB, which the assay probes. katG relates to isoniazid, gyrA to fluoroquinolones and pncA to pyrazinamide.
A new TB patient's sputum gives 'MTB detected; rifampicin resistance detected' on Xpert MTB/RIF. The most appropriate interpretation is:
The result shows rifampicin-resistant TB, which is managed as MDR/RR-TB. Further tests, such as culture-based or molecular testing for isoniazid and fluoroquinolones, are needed to guide treatment. Xpert does not test isoniazid.
Growth in a Mycobacteria Growth Indicator Tube (MGIT) is detected because:
MGIT contains a fluorescent compound in silicone that is quenched by oxygen; as mycobacteria consume oxygen, fluorescence increases and the instrument flags the tube. CO2 colorimetric detection is used by some blood culture systems.
A Ziehl-Neelsen sputum smear shows 35 acid-fast bacilli in 100 oil-immersion fields. Using the WHO/IUATLD grading scale, the result is reported as:
On the WHO/IUATLD scale, 10–99 AFB per 100 fields is 1+. Scanty is 1–9 AFB per 100 fields; 2+ needs 1–10 AFB per field.
Middlebrook 7H10 and 7H11 agar plates for mycobacterial culture should be incubated at 35–37 °C in:
Middlebrook agar media need 5–10% CO2 for best growth and are kept away from light. Anaerobic or microaerophilic atmospheres do not support M. tuberculosis.
A slow-growing mycobacterium from sputum grows well at 42 °C and is linked to hospital hot-water systems. The most likely species is:
M. xenopi is thermophilic, grows at 42–45 °C and lives in hot-water pipes. M. ulcerans, M. marinum and M. haemophilum prefer lower temperatures (about 30 °C).
A child from West Africa has a large painless skin ulcer with undermined edges. Acid-fast bacilli are seen. The likely agent is best cultured at:
Buruli ulcer is caused by M. ulcerans, which grows only at about 25–33 °C and very slowly (6–12 weeks). Culture at 37 °C often fails.
Most Mycobacterium tuberculosis strains give which result in the heat-stable (68 °C) catalase test?
M. tuberculosis catalase is heat-labile, so the 68 °C test is negative, whereas most NTM keep activity. Loss of catalase also occurs in some isoniazid-resistant (katG) strains.
A slow-growing acid-fast isolate is inoculated onto a medium containing p-nitrobenzoic acid (PNB). It fails to grow. This supports identification as:
PNB inhibits members of the M. tuberculosis complex but most NTM grow in its presence. This makes PNB a simple way to separate MTBC from NTM.
An interferon-gamma release assay (IGRA) is preferred to the tuberculin skin test in BCG-vaccinated people because IGRA antigens (ESAT-6 and CFP-10):
ESAT-6 and CFP-10 are coded in the RD1 region, which is deleted in all BCG strains, so vaccination does not give a positive IGRA. The tuberculin (PPD) mixture contains antigens shared with BCG.
A healthy nurse has a positive interferon-gamma release assay at pre-employment screening. She has no symptoms and a normal chest X-ray. The best interpretation is:
IGRA shows immune sensitisation to M. tuberculosis but does not separate latent infection from active disease; clinical and radiological findings suggest latent infection. BCG does not cause a positive IGRA.
A gastric aspirate from a child with suspected tuberculosis cannot be processed within 4 hours. The correct action is to:
Gastric acid rapidly kills mycobacteria, so a delayed specimen is neutralised with sodium carbonate. Formalin would kill the organisms and prevent culture.
Which urine specimen is recommended for mycobacterial culture?
Three consecutive first-morning specimens give the best yield. Pooled 24-hour urine is rejected because of contamination overgrowth and dilution.