Exam 6 - (CH 26) Mycobacterium tuberculosis and Nontuberculous Mycobacteria Textbook Review Questions

Question 1

If sputum or bronchial washings are cultured during the primary tuberculosis infection, the positivity rate is
A. 15% to 20%.
B. 25% to 30%.
C. 45% to 55%.
D. 70% to 80%.

Answer 1

B. 25% to 30%.

Question 2

Skeletal tuberculosis of the spine is
A. scurvy.
B. Hansen disease.
C. Pott disease.
D. Whipple’s disease.

Answer 2

C. Pott disease.

Question 3

Which of the following colony description best describes Mycobacterium bovis?
A. Pinpoint, smooth, concave, and buff-colored
B. Larger, irregular, flat, and translucent
C. Small, irregular, rounded with yellow-orange pigment
D. Small, granular, rounded, and nonpigmented

Answer 3

D. Small, granular, rounded, and nonpigmented

Question 4

Long, thin, beaded bacilli resembling Nocardia spp. may be seen in the young cultures of
A. Mycobacterium intracellulare.
B. Mycobacterium tuberculosis.
C. Mycobacterium bovis.
D. Mycobacterium kansasii.

Answer 4

A. Mycobacterium intracellulare.

Question 5

Cervical lymphadenitis in children is most associated with
A. Mycobacterium marinum.
B. Mycobacterium ulcerans.
C. Mycobacterium kansasii.
D. Mycobacterium scrofulaceum.

Answer 5

D. Mycobacterium scrofulaceum.

Question 6

Mycobacterium ulcerans grows best under these conditions:
A. 28°C to 30°C
B. Microaerophilic at 37°C
C. Capnophilic at 25°C
D. 37°C to 42°C

Answer 6

A. 28°C to 30°C

Question 7

Which set of reactions is consistent with Mycobacterium chelonae?
A. Arylsulfatase and sodium citrate negative
B. Arylsulfatase and mannitol positive
C. Arylsulfatase positive and nitrate reduction negative
D. Arylsulfatase negative and inositol positive

Answer 7

C. Arylsulfatase positive and nitrate reduction negative

Question 8

The current therapy recommended for lepromatous leprosy consists of
A. diaminodiphenylsulfone and rifampin for 6 months.
B. clofazimine for 12 months.
C. quinolones of 3 months.
D. triple therapy of a azithromycin, tetracycline, and bismuth salt for 12 months.

Answer 8

A. diaminodiphenylsulfone and rifampin for 6 months.

Question 9

This is a liquid broth used for subculturing stock strains and preparing inoculum for in vitro testing of Mycobacterium spp.
A. MGIT
B. Middlebrook 7H11
C. Middlebrook 7H9
D. Mycobactosel

Answer 9

Middlebrook 7H9

Question 10

Some bacteria are able to hydrolyze Tween 80 into
A. pyrazinamide and ammonia.
B. ferric ammonium citrate and iron oxide.
C. cyanogen bromide and NaOH.
D. oleic acid and polyoxyethylated sorbitol.

Answer 10

D. oleic acid and polyoxyethylated sorbitol.

Question 11

Which species is able to grow in the presence of high salt concentration in egg-based media?
A. Mycobacterium tuberculosis
B. Mycobacterium scrofulaceum
C. Mycobacterium szulgai
D. Mycobacterium flavescens

Answer 11

D. Mycobacterium flavescens

Question 12

Which statement is true regarding the Quantiferon-TB Gold assay?
A. It is an interferon-alpha inhibition assay.
B. It detects a patients’ cell-mediated immune response to the bacterial antigens in a type IV hypersensitivity reaction.
C. It will be positive if the patient has been vaccinated with the BCG vaccination.
D. It is not affected by the BCG vaccination.

Answer 12

D. It is not affected by the BCG vaccination.

Question 13

Describe the current recommendations for the identification of M. tuberculosis in the clinical laboratory.

Answer 13

Many mycobacterial species, saprophytes and potential pathogens, may be isolated from humans. Historically, mycobacteria have been identified by growth characteristics and biochemical testing. More recently, molecular biology assays have been developed. These assays include mycolic acid analysis of bacterial cell walls by high-pressure liquid chromatography, DNA probe technology, DNA sequencing, and MALDI-TOF MS. Genetic probe technology offers tremendous promise in microbial identification at a variety of levels— family, genus, species, and subspecies. The most common probe technology is the commercially available, single-stranded, acridinium ester– labeled DNA probe for the detection of rRNA (Gen-Probe, San Diego, CA). Probes specific for the M. tuberculosis complex (M. tuberculosis, M. bovis, M. africanum, M. canettii, and M. microti), M. kansasii, and M. gordonae, and separate probes for M. avium and M. intracellulare, are available. Laboratories should perform identification according to the level of service for which they are qualified. All isolates should be identified to the species level.

Question 14

Explain why mycobacterial infections should be treated for 6 months or longer and the need to use multiple drugs when treating M. tuberculosis infections.

Answer 14

Slowly growing M. tuberculosis has the extraordinary ability to persist and replicate in the harsh environment of the alveolar macrophage. The pathogenic mycobacteria are slow growers, which also lends to drug resistance. Antimicrobial agents are more active against rapidly growing bacteria. The American Thoracic Society, Centers for Disease Control and Prevention, and the International Union Against Tuberculosis and Lung Disease recommend a regimen of isoniazid, rifampin, pyrazinamide, and ethambutol for the first 8 weeks, for the treatment of tuberculosis. This is followed by isoniazid and rifampin for 18 weeks. Multidrug-resistant tuberculosis, defined as an isolate resistant to at least isoniazid and rifampin, is more complicated to treat.

Question 15

Compare the different levels of mycobacterial laboratory testing, and explain why smaller-volume laboratories should consider not performing full identification and susceptibility testing on mycobacterial isolates.

Answer 15

Mycobacteriologic services are available in many laboratories. Clinical laboratory functions that contribute to the diagnosis and management of tuberculosis have been divided into the following three major categories of service offered. * Level 1: Collection and transport of specimens, preparation and examination of smears for acid-fast bacilli. * Level 2: Procedures of level 1, plus isolation and identification of M. tuberculosis. * Level 3: All procedures of level 2, plus identification of mycobacteria other than M. tuberculosis.

The determination of drug susceptibility may be performed at level 2 and should be performed at level 3. However, antimicrobial susceptibility testing of the mycobacteria is difficult and should be attempted only by laboratories with experience in this assay. A laboratory may choose to develop or maintain the skills defined under one of the above levels, depending on the frequency with which specimens are received for isolation of mycobacteria, the nature of the clinical community being served, and the availability of a specialized referral service. All laboratories that perform clinical mycobacteriology should participate in recognized proficiency testing programs, and levels of service should be established and limited by the quality of performance demonstrated in these examinations.

Question 16

Discuss the methods used to process clinical specimens for mycobacterial culture and the reasons for the need for specimens to be decontaminated and digested before culture.

Answer 16

Most clinical specimens contain an abundance of nonmycobacterial microorganisms. Unless an attempt is made to inhibit these usually fast-growing contaminants, they can quickly overgrow the more slowly growing mycobacteria. Organic debris (e.g., tissue, serum, other proteinaceous material) surrounding the organism in the specimen must also be liquefied so that decontaminating agents will kill undesirable microbes, allowing surviving mycobacteria to gain access to the growth media. Mycobacteria are slightly more refractory to harsh chemicals; therefore chemical digestion and decontamination procedures have been used with success to enhance the recovery of acid-fast bacteria from clinical specimens. NaOH digests and decontaminates specimens, whereas N-acetyl-L-cysteine effectively digests the specimen but does not decontaminate.

Question 17

With respect to laboratory technique in the isolation and identification of mycobacteria, discuss some causes of false-negative and false-positive results.

Answer 17

Because low levels of mycobacteria organism may be present in clinical samples, meticulous care must be practiced during the processing of specimens for the detection and isolation of mycobacterial organisms. False-negative results may occur if processing protocols are not followed appropriately. Prolonged decontamination can have a negative effect by killing the mycobacteria. Improper centrifugation force may also lead to false-negative results. If insufficient force is applied, the mycobacteria may remain in the interface of the processed specimen and can be inadvertently discarded during processing. Care must be applied during culturing of the specimen. False-positive results may occur through carryover from the mouths of containers used to transfer processing agents, introduction of the organism from environmental sources, such as water, or introduction of organisms from aerosols produced when specimen containers are opened. When making slides, care must be taken to prevent carryover from one side to another. Slides should never come into contact with one another. Cross-contamination between specimens may occur if instruments are not properly maintained.

Question 18

Describe important safety considerations for laboratories attempting mycobacterial isolation and identification.

Answer 18

The mycobacteria are easily spread by the airborne route; therefore, it is important to avoid aerosol-generating procedures. In addition, mycobacterial laboratories should be under negative air pressure, so that when doors to the laboratory are opened, air flows inward. Specimen processing should be conducted in a biological safety cabinet; laboratory scientists must wear laboratory coats, eye protection, and a respirator (not a surgical mask). When disposable inoculating loops and needles are not used, electric incinerators should be used instead of open flames for sterilizing metal loops and needles. When specimens are centrifuged, they must be in a screw-capped tube in a secondary screw-capped container.

Question 19

Which of the following is (are) fluorescent stain( s) used in the detection of the mycobacteria?

a. Auramine-rhodamine
b. Kinyoun
c. Ziehl-Neelsen
d. Both b and c

Answer 19

a. Auramine-rhodamine

Question 20

A nonpigmented mycobacterium is isolated that reduces nitrate to nitrite and is niacin positive. You should suspect:

a. M. kansasii
b. M. xenopi
c. M. tuberculosis
d. M. avium complex (MAC)

Answer 20

c. M. tuberculosis

Question 21

The causative agent of Hansen disease:

a. Is highly contagious
b. Readily grows on most mycobacterial media
c. Grows best at core body temperature (37 ° C)
d. None of the above

Answer 21

d. None of the above

Question 22

The skin test for tuberculosis:

a. Detects antibodies to mycobacterial antigens
b. Detects a cell-mediated immune response to mycobacterial antigens
c. Uses the bacillus Calmette-Guérin (BCG) strain as the antigen source
d. Both a and b

Answer 22

b. Detects a cell-mediated immune response to mycobacterial antigens

Question 23

Points to Remember

Answer 23

■ Mycobacteria are important causes of human diseases, such as TB and Hansen disease.
■ Mycobacteria have a unique cell wall, and special stains are required to visualize the microorganisms. ■ Many Mycobacterium spp. are environmental microorganisms infrequently isolated from clinical specimens.
■ Isolation of mycobacteria requires specific safety precautions, including laboratories with negative air pressure, biological safety cabinets, the use of respirators and other PPE, and electric incinerators instead of flame incinerators.
■Most pathogenic mycobacteria are slow growers, taking up to several weeks for isolation on artificial media.
■ Some Mycobacterium spp. produce a pigment, which can be a helpful feature in the identification of mycobacteria.
■Other key tests for identification of mycobacteria include rate of growth, nitrate reduction, niacin production, presence of heat-stable catalase, and sensitivity to T2H.
■ Antimicrobial susceptibility testing of the mycobacteria, while important, is a technically demanding assay and should be performed only by experienced personnel.