A 40-year-old woman with cavitary pulmonary tuberculosis has a sputum isolate identified as M. tuberculosis complex; it is niacin positive and nitrate positive. Growth-based MGIT susceptibility results: isoniazid S, rifampin S, ethambutol S, pyrazinamide R. Sequencing of pncA shows a wild-type gene, and a subculture from the PZA tube grows only M. tuberculosis complex. On review, the PZA tube had been inoculated from a suspension much denser than the procedure specifies. The most likely explanation for the PZA result is:

False resistance from a heavy inoculum

A mixed culture with M. avium complex

Early multidrug-resistant tuberculosis

Natural PZA resistance of M. bovis

A 50-year-old kidney transplant recipient has tender, ulcerating nodules on both forearms. A skin biopsy shows granulomas, and the auramine-rhodamine stain shows many acid-fast bacilli. After 3 weeks, the MGIT tube at 37 °C and the Löwenstein-Jensen slant at 30 °C show no growth, and an M. tuberculosis complex NAAT on the tissue is negative. A chocolate agar plate from the routine bacterial culture, left at 30 °C by mistake, grows tiny colonies of acid-fast bacilli at day 14. Which culture condition should have been added at setup?

Hemin-supplemented media at 30 °C

Holding the LJ slant for 12 weeks

Mycobactin J-supplemented media

A line probe assay on an M. tuberculosis isolate shows wild-type rpoB and katG, and a mutation in the inhA promoter. This result most likely means:

Low-level isoniazid resistance; ethionamide likely inactive

High-level isoniazid resistance; ethionamide still active

Fully susceptible; inhA promoter changes are silent

Rifampin resistance; the strain should be treated as MDR-TB

Mycobacteria and Nocardia

29 ASCP MLS practice questions on Mycobacteria and Nocardia, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 71-year-old man with COPD has a new upper-lobe cavity, and three sputum cultures over 2 months grow the same slowly growing acid-fast bacillus. Results: M. tuberculosis complex NAAT negative; growth at 42 °C better than at 37 °C; no growth at 25 °C; small yellow colonies in the dark that do not change after light exposure; niacin negative; nitrate negative; 14-day arylsulfatase positive. The hospital's hot-water system had recently tested positive for nontuberculous mycobacteria. Which organism is most likely?
1. AMycobacterium gordonae
2. BMycobacterium xenopi
3. CMycobacterium scrofulaceum
4. DMycobacterium kansasii
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Answer: B · Mycobacterium xenopi
The isolate is a slow-growing scotochromogen, yellow in the dark with no change after light exposure. Its growth at 42 °C but not at 25 °C is the key: M. xenopi is thermophilic and lives in hot-water systems. M. gordonae and M. scrofulaceum are also scotochromogens but grow well at 25 °C, and M. gordonae is usually a contaminant. M. kansasii is a photochromogen that reduces nitrate. Three positive sputum cultures with a new cavity meet the clinical criteria for disease, so this isolate should not be dismissed as water contamination.
- A: M. gordonae grows at 25 °C, not at 42 °C, and is usually a contaminant.
- C: M. scrofulaceum grows at 25 °C and does not grow well at 42 °C.
- D: M. kansasii is a photochromogen and reduces nitrate.
Question 2 · Hard
A 40-year-old woman with cavitary pulmonary tuberculosis has a sputum isolate identified as M. tuberculosis complex; it is niacin positive and nitrate positive. Growth-based MGIT susceptibility results: isoniazid S, rifampin S, ethambutol S, pyrazinamide R. Sequencing of pncA shows a wild-type gene, and a subculture from the PZA tube grows only M. tuberculosis complex. On review, the PZA tube had been inoculated from a suspension much denser than the procedure specifies. The most likely explanation for the PZA result is:
1. AA mixed culture with M. avium complex
2. BEarly multidrug-resistant tuberculosis
3. CFalse resistance from a heavy inoculum
4. DNatural PZA resistance of M. bovis
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Answer: C · False resistance from a heavy inoculum
Pyrazinamide is active only at acid pH, so the MGIT PZA medium is set at about pH 5.9. A heavy inoculum raises the pH and lets M. tuberculosis grow in the drug tube, giving false resistance, a well-known weakness of growth-based PZA testing. Most true PZA resistance comes from pncA mutations, so a wild-type pncA supports a false result. The niacin-positive, nitrate-positive isolate is M. tuberculosis rather than M. bovis, which is niacin and nitrate negative and naturally PZA resistant. With isoniazid and rifampin susceptible this is not MDR-TB, and the PZA test should be repeated with a correct inoculum.
- A: The subculture from the drug tube grew only M. tuberculosis complex.
- B: MDR-TB requires resistance to both isoniazid and rifampin.
- D: M. bovis is niacin and nitrate negative; this isolate is positive for both.
Question 3 · Hard
A 50-year-old kidney transplant recipient has tender, ulcerating nodules on both forearms. A skin biopsy shows granulomas, and the auramine-rhodamine stain shows many acid-fast bacilli. After 3 weeks, the MGIT tube at 37 °C and the Löwenstein-Jensen slant at 30 °C show no growth, and an M. tuberculosis complex NAAT on the tissue is negative. A chocolate agar plate from the routine bacterial culture, left at 30 °C by mistake, grows tiny colonies of acid-fast bacilli at day 14. Which culture condition should have been added at setup?
1. AHemin-supplemented media at 30 °C
2. BHolding the LJ slant for 12 weeks
3. CMycobactin J-supplemented media
4. DIncubation at 42 °C
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Answer: A · Hemin-supplemented media at 30 °C
Skin nodules full of AFB in a transplant recipient, with growth only on chocolate agar at 30 °C, point to Mycobacterium haemophilum. This species needs iron in the form of hemin or ferric ammonium citrate and grows best at 28-32 °C, so unsupplemented LJ and MGIT stay negative. Skin, joint and bone specimens from immunocompromised patients should therefore also go onto chocolate agar or hemin-supplemented mycobacterial agar at 30 °C. Mycobactin J is needed by M. avium subsp. paratuberculosis, and 42 °C suits M. xenopi. Holding the unsupplemented LJ longer would not supply the hemin the organism needs.
- B: Longer incubation does not supply the hemin that the organism needs.
- C: Mycobactin J is the growth factor for M. avium subsp. paratuberculosis.
- D: Growth at 42 °C helps M. xenopi, not this hemin-requiring species.
Question 4 · Hard
A line probe assay on an M. tuberculosis isolate shows wild-type rpoB and katG, and a mutation in the inhA promoter. This result most likely means:
1. ALow-level isoniazid resistance; ethionamide likely inactive
2. BHigh-level isoniazid resistance; ethionamide still active
3. CFully susceptible; inhA promoter changes are silent
4. DRifampin resistance; the strain should be treated as MDR-TB
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Answer: A · Low-level isoniazid resistance; ethionamide likely inactive
InhA is the enzyme that activated isoniazid blocks, and ethionamide acts on the same target. Promoter mutations increase InhA production, causing low-level isoniazid resistance and cross-resistance to ethionamide. katG mutations, in contrast, stop the catalase-peroxidase from activating isoniazid, causing high-level resistance without affecting ethionamide. A wild-type rpoB region makes rifampin resistance unlikely, so this is not MDR-TB.
- B: High-level resistance with ethionamide spared is the katG pattern.
- C: inhA promoter mutations raise InhA levels and cause resistance.
- D: rpoB is wild type, so rifampin resistance is not shown.
Question 5 · Easy
In the Runyon scheme, which group includes Mycobacterium avium complex?
1. ANonchromogens
2. BPhotochromogens
3. CScotochromogens
4. DRapid growers
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Answer: A · Nonchromogens
MAC is a slow grower whose colonies stay buff or cream whether grown in the dark or after light exposure, so it is a Runyon group III nonchromogen. Photochromogens such as M. kansasii turn yellow only after light exposure, and scotochromogens such as M. gordonae are yellow even in the dark. The Runyon groups sort nontuberculous mycobacteria by pigment and growth rate. Molecular methods and MALDI-TOF now identify most species directly.
- B: Photochromogens, such as M. kansasii, turn yellow only after light exposure.
- C: Scotochromogens, such as M. gordonae, are pigmented even in the dark.
- D: MAC is a slow grower, needing more than 7 days to form colonies.
Question 6 · Medium
Multidrug-resistant tuberculosis (MDR-TB) is defined as resistance to at least:
1. AIsoniazid and ethambutol
2. BRifampin and pyrazinamide
3. CIsoniazid and rifampin
4. DAny two first-line drugs
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Question 7 · Hard
A man with advanced HIV (CD4 count 30/µL [0.03 x 10^9/L]) has fever and weight loss. His blood culture grows a slowly growing AFB: nonpigmented in light and dark, niacin negative, nitrate negative, M. tuberculosis complex NAAT negative. The most likely organism is:
1. AM. gordonae
2. BM. kansasii
3. CM. avium complex
4. DM. tuberculosis
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Question 8 · Medium
An acid-fast smear from a positive liquid culture shows bacilli lying side by side in long, rope-like (serpentine) cords. This pattern most suggests:
1. AM. gordonae
2. BM. avium complex
3. CM. tuberculosis complex
4. DNocardia species
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Question 9 · Medium
A sputum specimen is positive for M. tuberculosis complex by NAAT. Why is a mycobacterial culture still needed?
1. ATo get an isolate for susceptibility testing and genotyping
2. BTo tell latent tuberculosis infection from active disease
3. CBecause a positive NAAT must be confirmed before reporting
4. DBecause NAAT misses M. tuberculosis in smear-negative sputum
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Question 10 · Medium
How does the BD MGIT liquid culture system detect mycobacterial growth?
1. APressure changes in the tube's headspace
2. BTurbidity read by a photometer
3. CA color change in a CO2 sensor at the bottom
4. DFluorescence that appears as oxygen is consumed
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Question 11 · Hard
An aquarium owner has a slowly enlarging nodule on his finger, with new nodules up the forearm. A biopsy culture grows AFB at 30 °C within 2 weeks but poorly at 37 °C. Colonies are buff in the dark and turn yellow after light exposure. Which organism is most likely?
1. AMycobacterium marinum
2. BMycobacterium fortuitum
3. CMycobacterium haemophilum
4. DMycobacterium kansasii
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Question 12 · Hard
An isolate of M. abscessus subsp. abscessus has a clarithromycin MIC in the susceptible range on day 3, but resistant when reread on day 14. The best explanation is:
1. AAn acquired 23S rRNA (rrl) point mutation
2. BIts chromosomal beta-lactamase
3. CAn inducible erm(41) methylase gene
4. DContamination with M. chelonae
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Question 13 · Hard
A kidney transplant recipient has a lung nodule and a brain abscess. The abscess Gram stain shows beaded, branching gram-positive filaments that stain with the modified Kinyoun method (1% sulfuric acid). Aerobic culture grows dry, chalky colonies at 4 days. The most likely organism is:
1. ANocardia species
2. BMycobacterium tuberculosis
3. CActinomyces israelii
4. DStreptomyces species
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Question 14 · Medium
For Mycobacterium avium complex isolates, the drug most important to test, because the result predicts treatment response, is:
1. ARifampin
2. BClarithromycin
3. CIsoniazid
4. DPyrazinamide
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Question 15 · Hard
A man treated with intravesical BCG for bladder cancer develops a psoas abscess. Slowly growing AFB are identified as M. tuberculosis complex. The isolate is niacin negative, nitrate negative and resistant to pyrazinamide. Which organism is most likely?
1. AM. africanum
2. BM. tuberculosis
3. CM. bovis BCG
4. DM. avium complex
Answer and explanation in LearnFree account
Question 16 · Easy
For drug-susceptible pulmonary tuberculosis, the standard initial phase uses isoniazid, rifampin, pyrazinamide and:
1. AEthambutol
2. BClarithromycin
3. CAmikacin
4. DBedaquiline
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Question 17 · Easy
Which drug is usually the first choice for treating nocardiosis?
1. AAmphotericin B
2. BIsoniazid and rifampin
3. CHigh-dose penicillin G
4. DTrimethoprim-sulfamethoxazole
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Question 18 · Hard
A man with advanced HIV has a cavitary lung lesion. Sputum grows mucoid colonies that turn salmon-pink over several days. The organisms are gram-positive coccobacilli, partially acid-fast, with no branching or aerial hyphae. The most likely organism is:
1. ANocardia farcinica
2. BMycobacterium fortuitum
3. CRhodococcus equi
4. DStreptomyces species
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Question 19 · Medium
Sputum from patients with cystic fibrosis often overgrows mycobacterial cultures with Pseudomonas aeruginosa. Which processing change helps?
1. AA shorter NaOH exposure time
2. BA higher concentration of NALC
3. CInoculating liquid medium only
4. DAn added 5% oxalic acid treatment step
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Question 20 · Medium
A physician suspects pulmonary nocardiosis in a lung transplant recipient and sends sputum for routine bacterial culture. What should the laboratory do?
1. AHold the plates longer than the usual 48 hours
2. BReject it, because Nocardia cannot be grown
3. CSet up an anaerobic culture in place of aerobic
4. DProcess it only by the AFB digestion method
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Question 21 · Easy
Rapidly growing mycobacteria are defined as species that form mature colonies on solid subculture in:
1. AFewer than 24 hours
2. BFewer than 7 days
3. CAbout 6 to 8 weeks
4. DAbout 2 to 3 weeks
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Question 22 · Hard
In agar proportion testing of M. tuberculosis, the drug-free quadrant inoculated with a 1:10,000 dilution grows 120 colonies. The drug quadrant inoculated with a 1:100 dilution grows 30 colonies. Resistance is defined as growth of more than 1% of the population. The result is:
1. A0.0025%, susceptible
2. B0.25%, susceptible
3. C25%, resistant
4. D2.5%, resistant
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Question 23 · Medium
One of three sputum cultures from a man with chronic cough grows a few colonies of Mycobacterium gordonae. The smears and the other two cultures are negative. How should this isolate be regarded?
1. AProbably a contaminant, often from tap water
2. BA marker of hidden M. tuberculosis infection
3. CProbable cause of his chronic lung disease
4. DA reason to begin M. avium complex therapy
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Question 24 · Medium
Mycobacterial cultures of skin and soft tissue biopsies are also incubated at 30 °C because:
1. ASkin flora that contaminate cultures are suppressed at 30 °C
2. BM. tuberculosis grows faster at 30 °C than at 37 °C
3. CSome skin pathogens, such as M. marinum, grow poorly at 37 °C
4. DRapid growers need 30 °C to produce their yellow pigment
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Question 25 · Hard
A rapidly growing mycobacterium from a post-surgical wound gives these results: nitrate positive, iron uptake positive, growth in 5% NaCl, 3-day arylsulfatase positive. Which species fits best?
1. AM. chelonae
2. BM. fortuitum
3. CM. marinum
4. DM. abscessus
Answer and explanation in LearnFree account

4 more Mycobacteria and Nocardia questions

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Study Microbiology

Mycobacteria and Nocardia

A line probe assay on an M. tuberculosis isolate shows wild-type rpoB and katG, and a mutation in the inhA promoter. This result most likely means:

In the Runyon scheme, which group includes Mycobacterium avium complex?

Multidrug-resistant tuberculosis (MDR-TB) is defined as resistance to at least:

A man with advanced HIV (CD4 count 30/µL [0.03 x 10^9/L]) has fever and weight loss. His blood culture grows a slowly growing AFB: nonpigmented in light and dark, niacin negative, nitrate negative, M. tuberculosis complex NAAT negative. The most likely organism is:

An acid-fast smear from a positive liquid culture shows bacilli lying side by side in long, rope-like (serpentine) cords. This pattern most suggests:

A sputum specimen is positive for M. tuberculosis complex by NAAT. Why is a mycobacterial culture still needed?

How does the BD MGIT liquid culture system detect mycobacterial growth?

An aquarium owner has a slowly enlarging nodule on his finger, with new nodules up the forearm. A biopsy culture grows AFB at 30 °C within 2 weeks but poorly at 37 °C. Colonies are buff in the dark and turn yellow after light exposure. Which organism is most likely?

An isolate of M. abscessus subsp. abscessus has a clarithromycin MIC in the susceptible range on day 3, but resistant when reread on day 14. The best explanation is:

A kidney transplant recipient has a lung nodule and a brain abscess. The abscess Gram stain shows beaded, branching gram-positive filaments that stain with the modified Kinyoun method (1% sulfuric acid). Aerobic culture grows dry, chalky colonies at 4 days. The most likely organism is:

For Mycobacterium avium complex isolates, the drug most important to test, because the result predicts treatment response, is:

A man treated with intravesical BCG for bladder cancer develops a psoas abscess. Slowly growing AFB are identified as M. tuberculosis complex. The isolate is niacin negative, nitrate negative and resistant to pyrazinamide. Which organism is most likely?

For drug-susceptible pulmonary tuberculosis, the standard initial phase uses isoniazid, rifampin, pyrazinamide and:

Which drug is usually the first choice for treating nocardiosis?

A man with advanced HIV has a cavitary lung lesion. Sputum grows mucoid colonies that turn salmon-pink over several days. The organisms are gram-positive coccobacilli, partially acid-fast, with no branching or aerial hyphae. The most likely organism is:

Sputum from patients with cystic fibrosis often overgrows mycobacterial cultures with Pseudomonas aeruginosa. Which processing change helps?

A physician suspects pulmonary nocardiosis in a lung transplant recipient and sends sputum for routine bacterial culture. What should the laboratory do?

Rapidly growing mycobacteria are defined as species that form mature colonies on solid subculture in:

In agar proportion testing of M. tuberculosis, the drug-free quadrant inoculated with a 1:10,000 dilution grows 120 colonies. The drug quadrant inoculated with a 1:100 dilution grows 30 colonies. Resistance is defined as growth of more than 1% of the population. The result is:

One of three sputum cultures from a man with chronic cough grows a few colonies of Mycobacterium gordonae. The smears and the other two cultures are negative. How should this isolate be regarded?

Mycobacterial cultures of skin and soft tissue biopsies are also incubated at 30 °C because:

A rapidly growing mycobacterium from a post-surgical wound gives these results: nitrate positive, iron uptake positive, growth in 5% NaCl, 3-day arylsulfatase positive. Which species fits best?