Mycobacteria Flashcards
The best specimen for recovery of the mycobacteria from a sputum sample is:
A. First morning specimen
B. 10-hour evening specimen
C. 12-hour pooled specimen
D. 24-hour pooled specimen
A. First morning specimen
Note: Contamination by fungi and other bacteria contributes to lower yields of mycobacteria in a 24-hour sample. The first morning specimen
collected by expectoration or nebulization produces the highest concentration of mycobacteria in
sputum.
What concentration of sodium hydroxide (NaOH) is used to prepare a working decontamination solution for the processing of not normally sterile
specimens for mycobacteria?
A. 1% NaOH
B. 4% NaOH
C. 8% NaOH
D. 12% NaOH
B. 4% NaOH
Note: A strong decontamination solution (6% NaOH or greater) may kill or severely damage the mycobacteria. A 4% NaOH solution is mixed with an equal volume of N-acetyl-L-cysteine (NALC), a digestant or mucolytic agent, to yield a final working concentration of 2% NaOH. The time of exposure of
the specimen to the digestion/decontamination solution must be monitored because overtreatment
may result in fewer positive cultures.
Which is the most appropriate nonselective medium for recovery of mycobacteria from a heavily contaminated specimen?
A. Löwenstein–Jensen agar
B. Middlebrook 7H10 agar
C. Petragnani’s agar
D. American Thoracic Society medium
C. Petragnani’s agar
Note: All four media contain malachite green as an inhibitory agent of nonmycobacteria, but Petragnani’s medium contains a higher concentration (0.052 g/dL) than Löwenstein–Jensen (0.025 g/dL), Middlebrook 7H10 (0.0025 g/dL), or American Thoracic Society medium (0.02 g/dL). The last is used for normally sterile specimens, such as from CSF and bone marrow.
Mycobacteria stained by the Ziehl–Neelsen or Kinyoun methods with methylene blue counterstain are seen microscopically as:
A. Bright red rods against a blue background
B. Bright yellow rods against a yellow background
C. Orange-red rods against a black background
D. Bright blue rods against a pink background
A. Bright red rods against a blue background
Note: The carbolfuchsin (fuchsin with phenol) stains the mycobacteria red and does not decolorize after the acid–alcohol is added. The background and any other bacterial elements will decolorize and are counterstained blue by the methylene blue. A fluorescent staining procedure may be used as an alternative to acid-fast staining. Auramine fluorochrome produces bright yellow fluorescent mycobacteria and auramine–rhodamine causes an orange-red (gold) fluorescence against a dark background. A fluorescent microscope must be used, but with this method the smear can be scanned with a 25× objective instead of the 100× objective, permitting more rapid identification of mycobacteria.
Acid-fast staining of a smear prepared from digested sputum showed slender, slightly curved, beaded, red mycobacterial rods. Growth on Middlebrook 7H10 slants produced buff-colored microcolonies with a serpentine pattern after 14 days at 37°C. Niacin and nitrate reduction tests were positive. What is the most probable presumptive identification?
A. Mycobacterium tuberculosis
B. Mycobacterium ulcerans
C. Mycobacterium kansasii
D. Mycobacterium avium–intracellulare complex
A. Mycobacterium tuberculosis
Note: M. tuberculosis is positive for niacin accumulation, while the other three species are niacin negative. M. ulcerans is associated with skin infections (in the tropics), does not grow at 37°C (optimal temperature is 33°C), and is not recovered from
sputum. A serpentine pattern of growth indicates production of cording factor, a virulence factor for M. tuberculosis.
Which organism, associated with tuberculosis in cattle, causes tuberculosis in humans, especially in regions where dairy farming is prevalent?
A. Mycobacterium avium–intracellulare complex
B. Mycobacterium kansasii
C. Mycobacterium marinum
D. Mycobacterium bovis
D. Mycobacterium bovis
M. bovis is also called the bovine tubercle bacillus. A nonvirulent strain, bacillus Calmette–Guérin (BCG), is used as a tuberculosis vaccine throughout the world. Infections with M. bovis resemble infections caused by M. tuberculosis and are seen in circumstances where there is close contact between humans and cattle.
Which of the following organisms are used as controls for rapid growers and slow growers?
A. Mycobacterium fortuitum and Mycobacterium tuberculosis
B. Mycobacterium avium-intracellulare complex
and Mycobacterium tuberculosis
C. Mycobacterium chelonei and Mycobacterium fortuitum
D. Mycobacterium kansasii and Mycobacterium tuberculosis
A. Mycobacterium fortuitum and Mycobacterium tuberculosis
Note: Growth rates of mycobacteria are used along with biochemical tests as an aid to identification. M. fortuitum grows within 3–5 days at 37°C and is used as the control for rapid growers. M. tuberculosis grows in 12–25 days at 37°C and is a control organism for slow growers. In addition to M. fortuitum, M. chelonei is a rapid grower (3–5 days at 28°C–35°C). In addition to M. tuberculosis, M. avium and M. kansasii are slow growers (10–21 days at 37°C).
Which of the following Mycobacterium spp. produce(s) pigmented colonies in the dark (is a scotochromogen)?
A. M. szulgai
B. M. kansasii
C. M. tuberculosis
D. All of these options
A. M. szulgai
Note: M. tuberculosis does not produce pigmentation in the dark or after exposure to light (photochromogen). A common tapwater scotochromogen is Mycobacterium gordonae. The pathogenic scotochromogens are Mycobacterium szulgai, Mycobacterium scrofulaceum, and Mycobacterium xenopi. M. kansasii is a photochromogen producing a yellow pigment following exposure to light and red β-carotene crystals after long incubation periods.
All of the following mycobacteria are associated with skin infections except:
A. Mycobacterium marinum
B. Mycobacterium haemophilum
C. Mycobacterium ulcerans
D. Mycobacterium kansasii
D. Mycobacterium kansasii
Note: M. kansasii is a photochromogen that causes chronic pulmonary disease (classic tuberculosis). The other three species cause cutaneous or subcutaneous disease. It is important to culture skin lesions at the correct temperature to facilitate growth.
All of the following Mycobacterium spp. produce the enzyme required to convert niacin to niacin ribonucleotide except:
A. M. kansasii
B. M. tuberculosis
C. M. avium–intracellulare complex
D. M. szulgai
B. M. tuberculosis
Note: Niacin production is common to all mycobacteria. However, the niacin accumulates as a watersoluble metabolite in the culture medium when the organism cannot form niacin ribonucleotide. M. tuberculosis, M. simiae, and some strains of M. marinum, M. chelonae, and M. bovis lack the enzyme and therefore are called niacin positive because of the accumulation of niacin detected in the test medium.
The catalase test for mycobacteria differs from that used for other types of bacteria by using:
A. 1% H2O2 and 10% Tween 80
B. 3% H2O2 and phosphate buffer, pH 6.8
C. 10% H2O2 and 0.85% saline
D. 30% H2O2 and 10% Tween 80
D. 30% H2O2 and 10% Tween 80
Note: One milliliter of an equal mixture of 30% H2O2 (Superoxol) and Tween 80 (a strong detergent) is added to a 2-week-old subculture on Löwenstein–Jensen medium and placed upright
for 5 minutes. Catalase activity is determined semiquantitatively by measuring the height of the column of bubbles produced above the culture surface.
Growth inhibition by thiophene-2-carboxylic hydrazide (T2H) is used to differentiate M. tuberculosis from which other Mycobacterium specie?
A. M. bovis
B. M. avium–intracellulare complex
C. M. kansasii
D. M. marinum
A. M. bovis
Note: M. bovis and M. tuberculosis are very similar biochemically, and some strains of M. bovis also accumulate niacin. The T2H test differentiates M. tuberculosis from M. bovis. M. tuberculosis is not inhibited by T2H.
Which of the following Mycobacterium spp. is best differentiated by the rapid hydrolysis of Tween 80?
A. M. fortuitum
B. M. chelonae
C. M. kansasii
D. M. gordonae
C. M. kansasii
Note: The hydrolysis of Tween 80 is usually positive when testing the clinically insignificant mycobacteria. M. fortuitum, M. chelonae, and M. gordonae are saprophytic (and opportunistic) species, but M. kansasii is a pathogen. M. kansasii hydrolyses Tween 80 more rapidly than the other species (within 3–6 hours). A positive reaction is indicated by a change in the color of neutral red from yellow to pink.
Mycobacteria isolated from the hot water system of a hospital grew at 42°C. Colonies on Löwenstein–Jensen medium were not pigmented after exposure to light and were negative for niacin accumulation and nitrate reduction. The most likely identification is:
A. Mycobacterium xenopi
B. Mycobacterium marinum
C. Mycobacterium ulcerans
D. Mycobacterium haemophilum
A. Mycobacterium xenopi
Note: M. xenopi causes a pulmonary infection resembling M. tuberculosis and is frequently isolated from patients with an underlying disease such as alcoholism, AIDS, diabetes, or malignancy. It is often recovered from hot water taps and contaminated water systems and is a possible source of nosocomial infection. The other three species cause skin infections and grow on artificial media at a much lower temperature than M. xenopi (below 32°C).
A Mycobacterium species recovered from a patient with AIDS gave the following results:
Niacin = Neg T2H = +
Tween 80 hydrolysis = Neg
Nitrate reduction = Neg
Heat-stable catalase (68°C) = ±
Nonphotochromogen
What is the most likely identification?
A. M. gordonae
B. M. bovis
C. M. avium–intracellulare complex
D. M. kansasii
C. M. avium–intracellulare complex
Note: With the exception of M. tuberculosis, M. avium–intracellulare (MAI) complex is the Mycobacterium species most often isolated from AIDS patients. It is biochemically inert, which is a distinguishing factor for identification. MAI complex is highly resistant to the antibiotics used to treat tuberculosis, including multidrug therapy. Treatment with streptomycin, rifampin, ethionamide, ethambutol with cycloserine, or kanamycin has shown little success.
