Question 1

Acid-fast smears should be examined carefully by scanning at least ________ oil immersion fields.

Accepted Answer

Smears for AFB should be examined by scanning at least 300 oil immersion fields before reporting a smear as negative.

Question 2

The first test that is always performed on organisms growing on solid or liquid mycobacterial media is a(n):

Accepted Answer

Regardless of the identification methods used,the first test that is always performed on organisms growing on solid or liquid mycobacterial media is an acid-fast stain to confirm that the organisms are,indeed,mycobacteria.

Question 3

Quality control is performed to test the validity of catalase production at 68°C.M.fortuitum and M.tuberculosis are used,which yield a positive and negative reaction,respectively.Which one of the following statements explains this discrepancy?

Accepted Answer

Because of the ability of the catalase enzyme to remain active after heating,a heat-stable catalase test that measures the heat stability of the enzyme is performed.When heated to 68°C for 20 minutes,the catalase of M.tuberculosis,M.bovis,Mycobacterium gastri,and M.haemophilum becomes inactivated.M.fortuitum remains positive.

Question 4

The stain that requires heating the slide for better penetration of stain into the mycobacterial cell wall is known as the:

Accepted Answer

The classic carbolfuchsin (Ziehl-Neelsen)stain requires heating the slide for better penetration of stain into the mycobacterial cell wall.

Question 5

The nonpathogenic Mycobacterium commonly found in tap water,which has been a problem when tap water or deionized water has been used in the preparation of smears,is:

Accepted Answer

M.gordonae,a nonpathogenic scotochromogen commonly found in tap water,has been a problem when tap water or deionized water has been used in the preparation of smears or even when patients have rinsed their mouths with tap water before using aerosolized saline solution for inducing sputum.

Question 6

One drawback associated with the fluorochrome stain method for detecting Mycobacterium is that:

Accepted Answer

One drawback associated with the fluorochrome stains is that many rapid-growers may not appear fluorescent with these reagents.

Question 7

For reliable results,the niacin test should be performed only from cultures that are at least:

Accepted Answer

Niacin (nicotinic acid)plays an important role in the oxidation-reduction reactions that occur during mycobacterial metabolism.For reliable results,the niacin test should be performed only from cultures that are at least 3 weeks old and show at least 50 colonies on Löwenstein-Jensen medium;otherwise,adequate niacin may not be produced to be detected.

Question 8

Optimal conditions for the recovery of M.haemophilum from specimens include:

Accepted Answer

A chocolate agar plate or the placement of an X-factor (hemin)disk on conventional media and incubation at 25°C to 33°C are needed for the recovery of M.haemophilum.

Question 9

The type of specimen collected for mycobacterial studies that must be neutralized with 10% sodium carbonate is:

Accepted Answer

Many specimen types,other than respiratory samples,contain normal flora and require decontamination and concentration.Gastric lavages should be processed within 4 hours of collection or neutralized with 10% sodium carbonate and refrigerated until processed as for sputum.

Question 10

Optimal conditions for most mycobacterial cultures are:

Accepted Answer

For optimal cultivation of Mycobacterium spp.from most specimens,cultures are incubated at 35°C in the dark in an atmosphere of 5% to 10% CO₂ and high humidity.If specimens are obtained from skin or superficial lesions suspected to contain Mycobacterium marinum or Mycobacterium ulcerans,then an additional set of solid media should be inoculated and incubated at 25°C to 30°C.In addition,a chocolate agar plate or the placement of an X-factor (hemin)disk on conventional media and incubation at 25°C to 33°C are needed for the recovery of Mycobacterium haemophilum.

Question 11

Negative mycobacterial cultures are reported final and the cultures are discarded after what period?

Accepted Answer

Most isolates appear between 3 and 6 weeks;a few isolates appear after 7 or 8 weeks of incubation.After 8 weeks of incubation,negative cultures (i.e. ,those showing no growth)are reported and the cultures are discarded.

Question 12

Sodium hydroxide (NaOH),Zephiran-trisodium phosphate,N-acetyl-L-cysteine-NaOH (NALC-NaOH),and oxalic acid are associated with:

Accepted Answer

Commonly used decontamination agents are NaOH and NALC-NaOH.Oxalic acid is useful for treating specimens known to harbor gram-negative rods,particularly Pseudomonas and Proteus,which are extremely troublesome contaminants.

Question 13

Fluorescent staining is more sensitive than conventional carbolfuchsin stains.What counterstain is used during the procedure?

Accepted Answer

Potassium permanganate is the counterstain used in fluorescent staining,which is more sensitive than conventional carbolfuchsin stains.

Question 14

The fluorochrome method of staining Mycobacterium:

Accepted Answer

The fluorochrome stain is more sensitive than the conventional carbolfuchsin stains;the fluorescent bacilli stand out brightly against the background and can be initially examined at lower magnifications.More fields can be visualized in a short period.In addition,a positive fluorescent smear may be restained by the conventional Ziehl-Neelsen or Kinyoun's procedure,thereby saving the time needed to make a fresh smear.

Question 15

In the identification of mycobacterial isolates,the niacin test involves the:

Accepted Answer

Niacin (nicotinic acid)plays an important role in the oxidation-reduction reactions that occur during mycobacterial metabolism.Niacin therefore accumulates in the medium in which these organisms are growing.A positive niacin test is preliminary evidence that the organism that exhibits a buff-colored,slow-growing,rough colony may be M.tuberculosis.

Question 16

When culturing for Mycobacterium,rapid-growers usually produce colonies within how many days after subculture?

Accepted Answer

The rate of growth is an important criterion for determining the initial category of an isolate.Rapid-growers usually produce colonies within 3 to 4 days after subculture.Even a rapid-grower,however,may take longer than 7 days to produce colonies initially because of the inhibition by a harsh decontaminating procedure.

Question 17

Under what circumstance might slow-growing mycobacteria appear to produce colonies in a few days?

Accepted Answer

The rate of growth is an important criterion for determining the initial category of an isolate.The dilution of the organism used to assess the growth rate is critical.Slow-growing mycobacteria appear to produce colonies in less than 7 days if the inoculum is too heavy.Rapid-growers usually produce colonies within 3 to 4 days after subculture.Even a rapid-grower,however,may take longer than 7 days to produce colonies initially because of the inhibition by a harsh decontaminating procedure.Therefore the subculture must determine the growth rate and pigment production.

Question 18

Specimens for Mycobacterium studies that require concentration,decontamination,or liquefaction include:

Accepted Answer

The majority of specimens submitted for mycobacterial culture consist of organic debris,such as mucin,tissue,serum,and other proteinaceous material,that is contaminated with organisms.A typical example of such a specimen is sputum.

Question 19

On Gram stain,mycobacteria appear as:

Accepted Answer

Because of their cell wall makeup,mycobacteria usually appear as slender,poorly stained,beaded,gram-positive bacilli on Gram stain.Sometimes they appear as "gram-neutral" or "gram-ghosts" by failing to take up either crystal violet or safranin.

Question 20

The stain that does not require heating the slide for better penetration of stain into the mycobacterial cell wall is known as the:

Accepted Answer

The Kinyoun's acid-fast stain is similar to the Ziehl-Neelsen stain but without heat,hence the term cold stain.

Quiz 41: Mycobacteria