Antibiotic Susceptibility Testing - Evaluating Antibiotic Resistance!

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Antibiotic Susceptibility Testing - Evaluating Antibiotic Resistance!

When you take your medicine for an infection or virus it is important to take a drug that will attack the cause of the illness. Antibiotics are medications that destroy or slow down the growth of bacteria. How knowledgeable are you when it comes to understanding antibiotic susceptibility? Take up the quick quiz below to find out.


Questions and Answers
  • 1. 
    The figure below shows the results after incubation of erythromycin microdilution testing of an isolate of Escherichia coli. Filled wells denote growth, empty wells denote no growth. After incubation, the contents of each well were plated on non-selective media, and the numbers of colonies growing from each are written in the appropriate well. Which of the following is the best interpretation of these results?
    • A. 

      The minimum inhibitory concentration is 1 mg/L and the drug is bacteriostatic

    • B. 

      The minimum inhibitory concentration is 1 mg/L and the drug is bactericidal.

    • C. 

      The minimum inhibitory concentration is 2 mg/L and the drug is bacteriostatic

    • D. 

      The minimum inhibitory concentration is 2 mg/L and the drug is bactericidal.

  • 2. 
    The diagram below shows the results of microdilution testing of a second isolate of Escherichia coli. Filled wells denote growth, empty wells denote no growth. After incubation, samples of each well were plated on non-selective media, and the numbers of colonies growing from each sample are written in the appropriate well. Which of the following is the best interpretation of these results?
    • A. 

      The MIC is 0.25 mg/L and the drug is bacteriostatic

    • B. 

      The MIC is 0.25 mg/L and the drug is bactericidal

    • C. 

      The MIC is 0.5 mg/L and the drug is bacteriostatic

    • D. 

      The MIC is 0.5 mg/L and the drug is bactericidal

    • E. 

      The MIC is 0.25 mg/L but nothing else can be determined.

  • 3. 
    Which of the following is an advantage of manual broth microdilution testing versus other methods of antimicrobial susceptibility testing?
    • A. 

      It can be used to determine antibiotic resistance as well as antibiotic susceptibility

    • B. 

      It produces results more rapidly than other methods

    • C. 

      It does not require initial isolation of the organism.

    • D. 

      It is more precise than other methods

    • E. 

      It can be used to determine the minimum bactericidal concentration of the drug

  • 4. 
    You receive the antibiotic susceptibility testing results for a Streptococcus pneunomiae strain isolated from the cerebrospinal fluid of a patient suffering from meningitis. The results state that the strain has reduced susceptibility to penicillin and that the drug acted in a bacteriostatic manner against the isolate. This is a strange result, because members of this antibiotic class are generally bactericidal. Which of the following phenomena is most likely occurring in this isolate, and which method of AST would have allowed this to be known?
    • A. 

      Resistance; Etest

    • B. 

      Tolerance; Etest

    • C. 

      Point mutation; Etest

    • D. 

      Resistance; microdilution

    • E. 

      Tolerance; microdilution

    • F. 

      Point mutation; microdilution

  • 5. 
    The diagnostic laboratory isolates a Haemophilus influenzae strain from a clinical specimen from one of your patients and proceeds to carry out AST by Etest. The following results are obtained: ampicillin, 16 mg/L; ceftriaxone, 3 mg/L; clarithromycin, 20 mg/L; moxifloxacin, 0.5 mg/L; trimethoprim/sulfamethoxazole, 0.33 mg/L. Which drug(s) might you select to treat your patient? (MORE THAN 1 ANSWER)
    • A. 

      Ampicillin

    • B. 

      Ceftriaxone

    • C. 

      Clarithromycin

    • D. 

      Moxifloxacin

    • E. 

      Trimethoprim/sulfamethoxazole

  • 6. 
    1. A 19-year-old man presents with a large abscess on his left thigh. It is painful upon palpation, erythematous, and has pus draining from several openings. After incision and drainage, a sample of the pus is sent to the laboratory for identification and antibiotic susceptibility testing. A Gram-positive, catalase-positive, coagulase-positive coccus is isolated. The following MIC results are obtained by Etest:
      Penicillin G: 256 mg/L (S0.12, R0.25) Oxacillin: 32 mg/L (S2, R4) Linezolid: 2 mg/L (S4, R4) Vancomycin: 3 mg/L (S≤2, I=4-8, R≥16)   What mechanisms of resistance are most likely responsible for some of the results listed?
    • A. 

      A. Antibiotic efflux and antibiotic modification

    • B. 

      B. Antibiotic modification and target point mutation

    • C. 

      C. Antibiotic degradation and target enzyme replacement

    • D. 

      D. Decreased permeability and target structure replacement

    • E. 

      E. Antibiotic degradation and target structure replacement

  • 7. 
    In a monomicrobial infection, successive isolates can be used to monitor the progress of therapy. Occasionally, antibiotic susceptibility levels can be seen to change over time in these series of isolates, particularly in response to antibiotic treatment. Why might this happen?
    • A. 

      A. The patient becomes desensitized to the drug, thereby allowing the bacteria to survive

    • B. 

      B. The antibiotic becomes inactivated over time, allowing the bacteria to survive

    • C. 

      C. Bacterial cells with random mutations providing resistance are selected for by treatment

    • D. 

      D. The bacteria acquire resistance genes over time, allowing them to survive

    • E. 

      E. Bacteria become less tolerant of the antibiotic, and start to develop resistance mechanisms

  • 8. 
    Which method of antibiotic susceptibility testing involves measuring diameters of zones of inhibition and interpreting them via standardized tables?
    • A. 

      A. Etest

    • B. 

      B. Broth microdilution

    • C. 

      C. Agar microdilution

    • D. 

      D. Kirby Bauer

    • E. 

      E. MIC testing

  • 9. 
    What element is NOT important in determination of susceptibility breakpoints by the Clinical and Laboratory Standards Institute?
    • A. 

      A. Pharmacokinetics

    • B. 

      B. Clinical outcomes

    • C. 

      C. MIC distributions

    • D. 

      D. Type of infection

    • E. 

      E. Minimum bactericidal concentrations

  • 10. 
      What is shown in the image?
    • A. 

      A. MIC interpretation table

    • B. 

      B. Susceptibility table

    • C. 

      C. Antibiogram

    • D. 

      D. Treatment table

    • E. 

      E. Research result

  • 11. 
     A 54-year-old man presents with endocarditis. The causative agent is a Gram positive coccus that is catalase-negative and alpha-hemolytic. Based only on this image, why would a competent physician be unlikely to prescribe a macrolide for treatment?
    • A. 

      A. 63% of similar isolates are resistant to a member of this antibiotic class

    • B. 

      B. Only 84% of similar isolates are susceptible to a member of this antibiotic class

    • C. 

      C. 9% of similar isolates are resistant to a member of this antibiotic class

    • D. 

      D. 37% of similar isolates are resistant to a member of this antibiotic class

    • E. 

      E. Only 87% of similar isolates are susceptible to a member of this antibiotic class

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