Laboratory Science Microbiology Techniques 2 — Questions and Answers
Question 1: What does MIC stand for in antimicrobial susceptibility testing?
- Minimum Inoculation Concentration
- Minimum Inhibitory Concentration (Correct answer)
- Maximum Inhibitory Colony
- Microbial Identification Code
Correct answer: Minimum Inhibitory Concentration
MIC (Minimum Inhibitory Concentration) is the lowest concentration of an antibiotic that inhibits visible bacterial growth, guiding antibiotic selection in clinical therapy.
Question 2: Which staining technique is used to identify Mycobacterium tuberculosis?
- Gram stain
- India ink stain
- Ziehl-Neelsen (acid-fast) stain (Correct answer)
- Giemsa stain
Correct answer: Ziehl-Neelsen (acid-fast) stain
The Ziehl-Neelsen stain uses carbol fuchsin that binds mycobacterial cell wall mycolic acids; acid-fast bacilli resist decolorization with acid-alcohol, appearing red.
Question 3: Blood cultures are typically incubated for how many days before being reported as negative?
- 1 day
- 3 days
- 5 days (Correct answer)
- 14 days
Correct answer: 5 days
Most clinical blood cultures are incubated for 5 days in automated systems (e.g., BACTEC), which is sufficient to detect the vast majority of clinically significant bacteremia.
Question 4: What is a presumptive identification when a beta-hemolytic Streptococcus is sensitive to bacitracin?
- Streptococcus pneumoniae
- Streptococcus agalactiae (Group B)
- Streptococcus pyogenes (Group A) (Correct answer)
- Enterococcus faecalis
Correct answer: Streptococcus pyogenes (Group A)
Group A Streptococcus (S. pyogenes) is sensitive to bacitracin (A disk), while other beta-hemolytic streptococci are resistant, providing a presumptive identification.
Question 5: Which organism produces a characteristic 'school of fish' arrangement on gram stain?
- Streptococcus pneumoniae
- Haemophilus influenzae
- Corynebacterium diphtheriae (Correct answer)
- Listeria monocytogenes
Correct answer: Corynebacterium diphtheriae
Corynebacterium diphtheriae cells arrange in palisades (rows) or V/L shapes resembling a 'school of fish' or 'Chinese letters' on Gram stain.
Question 6: What is the purpose of the indole test in identifying Enterobacteriaceae?
- Detect urease production
- Detect tryptophan degradation to indole by tryptophanase (Correct answer)
- Detect hydrogen sulfide production
- Detect nitrate reduction
Correct answer: Detect tryptophan degradation to indole by tryptophanase
The indole test detects tryptophanase activity: positive organisms (like E. coli) degrade tryptophan to indole, producing a red ring with Kovac's reagent.
Laboratory Science Microbiology Techniques 2
What does MIC stand for in antimicrobial susceptibility testing?