Unit 10: Antibiotic sensitivity of microorganism - Practice Quiz

1 What is antibiotic sensitivity testing used to determine?

2 Which method is commonly used to test antibiotic sensitivity on an agar plate?

3 What is placed on the agar surface in the disk diffusion test?

4 What does a clear area around an antibiotic disk indicate?

5 What is the zone of inhibition?

6 How is the zone of inhibition usually measured?

7 Which agar medium is commonly used for the Kirby–Bauer test?

8 A microorganism described as susceptible to an antibiotic is expected to:

9 A microorganism described as resistant to an antibiotic will generally:

10 What is the usual purpose of comparing a zone diameter with a standard chart?

11 Why should the bacterial inoculum be prepared uniformly?

12 What type of culture is generally preferred for antibiotic sensitivity testing?

13 What may happen if antibiotic disks are placed too close together?

14 Why are antibiotic disks pressed gently onto the agar surface?

15 What does antibiotic diffusion mean in the disk method?

16 What is an antibiotic sensitivity pattern?

17 Which result usually indicates greater antibiotic activity in a disk diffusion test?

18 Why is incubation necessary after placing antibiotic disks on the plate?

19 What is the minimum inhibitory concentration, or MIC?

20 Which practice helps prevent contamination during sensitivity testing?

21 A bacterial isolate produces a 24 mm zone around an antibiotic disk. The interpretive chart classifies zones of 21 mm or more as susceptible. How should the isolate be reported?

22 Why is a standardized inoculum important in the Kirby–Bauer disk diffusion test?

23 A student prepares a bacterial suspension much denser than the recommended turbidity before disk diffusion testing. What result is most likely?

24 Which reference standard is commonly used to adjust a bacterial suspension for disk diffusion testing?

25 An agar plate used for disk diffusion is noticeably thicker than the recommended depth. How can this affect the test?

26 Why should antibiotic disks be pressed gently onto the agar surface after placement?

27 Two antibiotic disks are placed too close together, and their inhibition zones overlap. What is the main problem?

28 A culture shows no visible growth on the entire antibiotic test plate, including areas far from the disk. Which explanation should be investigated first?

29 An inhibition zone measures 15 mm, while the susceptibility chart lists 16–20 mm as intermediate and 21 mm or more as susceptible. How should the result be classified?

30 Why should inhibition zones be measured from the underside of a plate without opening the lid whenever possible?

31 A test strain gives unusually large zones for every antibiotic tested, including those normally showing resistance. Which procedural issue is most likely?

32 Why are control strains included periodically in antimicrobial susceptibility testing?

33 A disk diffusion test is incubated at a temperature substantially below the recommended temperature. Which outcome is most concerning?

34 An isolate is susceptible to antibiotic X in vitro, but the drug does not reach effective concentrations at the infection site. What does this illustrate?

35 A culture produces a distinct inner growth ring close to an antibiotic disk and a clear outer zone. What should the laboratory do before reporting the result?

36 A mixed bacterial culture is tested instead of a pure isolate. Why might the susceptibility pattern be misleading?

37 A laboratory compares zone diameters obtained on two different media types. Why may direct comparison be inappropriate?

38 A disk has passed its expiration date and was stored at room temperature. What is the most appropriate action?

39 An antibiotic has a zone diameter in the intermediate category. What does this result generally indicate?

40 A laboratory obtains a resistant result for an antibiotic, but the control strain is outside its acceptable range. What should happen next?

41 A Kirby–Bauer test produces zones that are uniformly 3–4 mm smaller than expected for several antibiotics, while the control strain gives similarly reduced zones. Which error most likely explains the pattern?

42 A bacterial suspension appears equivalent to 0.5 McFarland by visual comparison, but viable counting shows an inoculum approximately three times higher. Which result is most likely?

43 In disk diffusion testing, two antibiotic disks are placed so close that their inhibition zones overlap. What is the main interpretive problem?

44 An isolate has a clear zone around erythromycin but grows within the zone adjacent to a clindamycin disk, producing a flattened D-shaped edge. What is the appropriate interpretation?

45 A Staphylococcus aureus isolate is resistant to cefoxitin but susceptible to erythromycin and clindamycin. Which conclusion is most appropriate?

46 An Enterobacterales isolate shows a cefotaxime zone of 21 mm, a ceftazidime zone of 19 mm, and a clavulanate combination zone 6 mm larger than the corresponding cephalosporin zone. What does this support?

47 A Gram-negative isolate is susceptible to cefotaxime initially, but resistance emerges during therapy and the isolate shows reduced susceptibility to cefoxitin. Which mechanism is most plausible?

48 A broth microdilution series contains antibiotic concentrations of 0.25, 0.5, 1, 2, and 4 µg/mL. Visible growth is absent at 2 and 4 µg/mL but present at 1 µg/mL. What is the MIC?

49 A minimum inhibitory concentration is measured as 8 µg/mL, but the clinical breakpoint for susceptibility is 4 µg/mL. How should the isolate be categorized?

50 A control strain produces inhibition zones outside the accepted quality-control range for three disks, although the test isolate's zones appear typical. What should the laboratory do first?

51 A culture plate has a faint haze extending inside an otherwise clear inhibition zone. Which factor is most important before calling this resistance?

52 An isolate gives a susceptible result by disk diffusion but an MIC above the resistance breakpoint by broth microdilution. Which response is most scientifically appropriate?

53 A swarming Proteus isolate obscures the edges of several inhibition zones on Mueller–Hinton agar. What is the best laboratory approach?

54 An isolate has a large inhibition zone around tetracycline but a very small zone around doxycycline. Which conclusion is justified without additional testing?

55 A mixed culture is mistakenly used for susceptibility testing, and the plate shows colonies with two distinct morphologies inside and outside antibiotic zones. What is the principal consequence?

56 A laboratory incubates disk diffusion plates at 25°C overnight instead of using the validated temperature for the organism. Why can this invalidate interpretation?

57 A disk is applied to agar 30 minutes after inoculation, but the plate surface has already dried completely. Which bias is most likely?

58 A urine isolate is susceptible to an antibiotic at the serum breakpoint but resistant at the urinary breakpoint. Why might the laboratory use a specimen-specific breakpoint?

59 An isolate is resistant to ciprofloxacin and levofloxacin but susceptible to nalidixic acid. Which interpretation is most defensible?

60 A bacterial population produces two colony types, one susceptible and one highly resistant, but the overall disk-diffusion zone appears susceptible. What phenomenon could explain this finding?