`Harr 7.1: Specimen Collection, Media, and Methods

Question 1

The aseptic collection of blood cultures requires that the skin be cleansed with:
A. 2% iodine and then 70% alcohol solution
B. 70% alcohol and then 2% iodine or an iodophor
C. 70% alcohol and then 95% alcohol
D. 95% alcohol only

Answer 1

B. In order to attain asepsis of the skin, 70% alcohol followed by 2% iodine is used for obtaining blood cultures.

Question 2

When cleansing the skin with alcohol and then iodine for the collection of a blood culture, the iodine (or iodophor) should remain intact on the skin for at least:
A. 10 sec 
B. 30 sec
C. 60 sec
D. 5 mins

Answer 2

C. The iodine should remain on the skin for 1 min because instant antisepsis does not occur when cleansing the skin for a blood culture.

Question 3

What is the purpose of adding 0.025%-0.050% sodium polyethanolsulfonate (SPS) to nutrient broth media for the collection of blood cultures?
A. It inhibits phagocytosis and complement
B. It promotes formation of a blood clot
C. It enhances growth of anaerobes
D. It functions as a preservative

Answer 3

A. SPS is used in most commercial blood culture products because it functions as an anticoagulant and prevent phagocytosis and complement activation. In addition, SPS neutralizes aminoglycoside antibiotics.
Addition of SPS may inhibit some Neisseria and Peptostreptococcus, but this can be reversed with 1.2% gelatin.

Question 4

A flexible calcium alginate nasopharyngeal swab is the collection device of choice for recovery of which organism from the nasopharynx?
A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Corynebacterium diptheriae
D. Bacteroidis fragilis

Answer 4

C. C. diptheriae must be recovered from the deep layers of the pseudomembrane that forms in the nasopharyngeal area. A flexible calcium alginate nasopharyngeal swab is the best choice for collecting a specimen from the posterior nares and pharynx.

Question 5

Semisolid transport media such as Amies, Stuart, or Cary-Blair are suitable for the transport of swabs for culture of most pathogens except:
A. Neisseria gonorrhoeae
B. Enterobacteriaceae
C. Campylobacter fetus
D. Streptococcus pneumoniae

Answer 5

A. Specimens for culture of N. gonorrhoeae are best if plated immediately or transported in a medium containing activated charcoal to absorb inhibitory substances that hinder their recovery.

Question 6

Select the method of choice for recovery of anaerobic bacteria from a deep abscess.
A. Cotton fiber swab of the abscess area
B. Skin nip of the surface tissue
C. Needle aspirate after surface decontamination
D. Swab of the scalpel after debribement

Answer 6

C. Anaerobic specimens are easily contaminated with organisms present on the skin or mucosal surfaces when a swab is used. Needle aspiration of an abscess following surface decontamination provides the least exposure to ambient oxygen.

Question 7

Select the primary and differential media of choice for recovery of most fecal pathogens.
A. MacConkey, blood, birdseed, and Campylobacter (Campy) agars
B. Hektoen, MacConkey, Campy, colistin-nalidixic acid (CNA) agars
C. CNA and Christensen urea agars and thyoglycollate media
D. Blood, Campy, Mueller-Hinton agars, and thioglycollate media

Answer 7

B. Hektoen agar selectively isolates pathogenic coliforms, especially Salmonella and Shigella.
MacConkey agar differentiates lactose fermenters from non fermenters.
CNA agar contains antibiotics that prohibit growth of gram-negative coliforms but not gram-positive cocci.
Campy agar contains the antibiotics cephalotin, trimethoprim, vancomycin, polymyxin B, and amphotericin B to prevent growth of Enterobacteriaceae, Pseudomonas spp., and fungi.

Question 8

Select the media of choice for recovery of Vibrio cholerae from a stool specimen.
A. MacConkey agar and thioglycollate media
B. Thiosulfate-citrite-bile-sucrose (TCBS) agar and alkaline peptone water (APW) broth
C. Blood agar and selenite-F (SEL) broth
D. CNA agar

Answer 8

B. TCBS agar is used to grow vibrio cholerae, which appear as yellow colonies as a result of the use of both citrate and sucrose
APW is used as an enrichment broth and should be subcultured to TCBS agar to further evaluation Vibrio colonies

Question 9

Colistin-nalidixic-acid agar (CNA) is used primarily for the recovery of:
A. Neisseria species
B. Enterobacteriaceae
C. Pseudomonas aeruginosa
D. Staphylococcus aureus

Answer 9

D. CNA agar inhibits the growth of gram-negative bacteria and is used to isolate gram positive cocci from specimens. This medium is especially useful for stool and wound cultures because these may contain large numbers of gram-negative rods.

Question 10

In the United States, most blood agar plates are prepared with 5% or 10% red blood cells (RBC's) obtained from:
A. Sheep
B. Horses
C. Humans
D. Dogs

Answer 10

A. Sheep RBCs are used in agar plates because they are readily available and less inhibitory than cells of other species. The type of hemolysis is determined by the source of RBCs.
Sheep RBCs are chosen because of the characteristically clear hemolysis produced by B-hemolytic streptococci, Staphylococcus, and other pathogens producing B-hemolysins.
Sheep blood does not support the growth of Haemophilus haemolyticus, eliminating the possibility of confusing it with B-hemolytic cocci in throat cultures