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Questions and Answers
Which antimicrobial agent has a mechanism of action that differs from others due to being resistant to enzymatic inactivation by bacterial enzymes?
Which antimicrobial agent has a mechanism of action that differs from others due to being resistant to enzymatic inactivation by bacterial enzymes?
What is the disk content of Amoxicillin-clavulanic acid used for susceptibility testing?
What is the disk content of Amoxicillin-clavulanic acid used for susceptibility testing?
For which pathogenic ATCC strain is Kanamycin's susceptibility not determined due to missing data?
For which pathogenic ATCC strain is Kanamycin's susceptibility not determined due to missing data?
Which antimicrobial agent shows a zone diameter interpretative standard ranging from 23-29 for ATCC 25922?
Which antimicrobial agent shows a zone diameter interpretative standard ranging from 23-29 for ATCC 25922?
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What is the minimum susceptibility zone diameter for Rifampin against ATCC 25923?
What is the minimum susceptibility zone diameter for Rifampin against ATCC 25923?
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What is the recommended turbidity level for a standardized bacterial inoculum?
What is the recommended turbidity level for a standardized bacterial inoculum?
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Which of the following steps is not part of the preparation of inoculum?
Which of the following steps is not part of the preparation of inoculum?
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When should antibiotic disks be applied to the inoculated agar plate?
When should antibiotic disks be applied to the inoculated agar plate?
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How should antimicrobial disks be stored prior to use?
How should antimicrobial disks be stored prior to use?
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What is the maximum number of antimicrobial disks that can be placed in a 9-cm petri dish?
What is the maximum number of antimicrobial disks that can be placed in a 9-cm petri dish?
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Which procedure helps ensure an even distribution of the inoculum on the agar plate?
Which procedure helps ensure an even distribution of the inoculum on the agar plate?
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What is the purpose of pressing the antibiotic disk onto the agar surface?
What is the purpose of pressing the antibiotic disk onto the agar surface?
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What is the concentration of a standardized inoculum?
What is the concentration of a standardized inoculum?
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What should be done if overlapping zones of inhibition are encountered?
What should be done if overlapping zones of inhibition are encountered?
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At what temperature should the plates be incubated?
At what temperature should the plates be incubated?
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How should the diameter of the zones of inhibition be recorded?
How should the diameter of the zones of inhibition be recorded?
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What is the result category if test isolates show a zone of inhibition?
What is the result category if test isolates show a zone of inhibition?
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When should test plates be rejected?
When should test plates be rejected?
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What should be done with data collected from a set if the control strain zones of inhibition are not within tolerance limits?
What should be done with data collected from a set if the control strain zones of inhibition are not within tolerance limits?
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What is meant by interpreting susceptibility results as 'qualitative' until established zones are provided?
What is meant by interpreting susceptibility results as 'qualitative' until established zones are provided?
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What must be observed to ensure that the zone of inhibition measurement is valid?
What must be observed to ensure that the zone of inhibition measurement is valid?
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What principle does the disk diffusion method rely on?
What principle does the disk diffusion method rely on?
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Which medium is recommended for routine susceptibility tests?
Which medium is recommended for routine susceptibility tests?
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What is the correct temperature and time for autoclaving agar medium?
What is the correct temperature and time for autoclaving agar medium?
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How should the pH of the agar medium be checked?
How should the pH of the agar medium be checked?
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What is the proper storage condition for the agar plates if not used immediately?
What is the proper storage condition for the agar plates if not used immediately?
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What happens if water droplets are present on the surface of the agar plates?
What happens if water droplets are present on the surface of the agar plates?
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Which broth can be used to prepare the inoculum for disk diffusion?
Which broth can be used to prepare the inoculum for disk diffusion?
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What is the recommended depth for pouring agar into petri dishes?
What is the recommended depth for pouring agar into petri dishes?
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Which antimicrobial agent has the highest zone diameter for Staphylococcus aureus?
Which antimicrobial agent has the highest zone diameter for Staphylococcus aureus?
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What does a zone diameter of ≤ 14 mm indicate for Amikacin against Escherichia coli?
What does a zone diameter of ≤ 14 mm indicate for Amikacin against Escherichia coli?
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Which of the following antimicrobial agents is effective against Pseudomonas aeruginosa based on its zone diameter response?
Which of the following antimicrobial agents is effective against Pseudomonas aeruginosa based on its zone diameter response?
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Which antimicrobial agent is indicated as resistant when the zone diameter is < 10 mm for Staphylococcus aureus?
Which antimicrobial agent is indicated as resistant when the zone diameter is < 10 mm for Staphylococcus aureus?
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Which agent shows a significant susceptibility zone diameter for Streptococcus pneumoniae?
Which agent shows a significant susceptibility zone diameter for Streptococcus pneumoniae?
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For which antimicrobial agent does a zone diameter of < 13 mm suggest ineffective treatment for Escherichia coli?
For which antimicrobial agent does a zone diameter of < 13 mm suggest ineffective treatment for Escherichia coli?
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Which of these antimicrobial agents provides an intermediate zone diameter for Pseudomonas aeruginosa?
Which of these antimicrobial agents provides an intermediate zone diameter for Pseudomonas aeruginosa?
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What is the indicated susceptibility level for Trimethoprim-sulfamethoxazole against Staphylococcus aureus based on the zone diameter?
What is the indicated susceptibility level for Trimethoprim-sulfamethoxazole against Staphylococcus aureus based on the zone diameter?
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Which of the following agents is susceptible with the highest zone diameter against Streptococcus pneumoniae?
Which of the following agents is susceptible with the highest zone diameter against Streptococcus pneumoniae?
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Which antimicrobial agent shows resistance against Klebsiella pneumoniae based on a zone diameter of < 12 mm?
Which antimicrobial agent shows resistance against Klebsiella pneumoniae based on a zone diameter of < 12 mm?
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Study Notes
Disk Diffusion Method (Kirby-Bauer)
- Principle: Antibiotic disks placed on inoculated agar diffuse outward, creating a concentration gradient. A clear zone (zone of inhibition) around the disk indicates the antibiotic's effectiveness.
- Media: Mueller-Hinton Agar (MHA) is used. It provides good reproducibility and supports most bacterial growth while minimizing interference from other substances.
- Inoculum Preparation: A standardized bacterial suspension is prepared, typically a tryptic soy broth or saline solution. The turbidity of the suspension is crucial, ideally matching 0.5 McFarland standards.
- Agar Preparation: MHA is prepared according to manufacturer's instructions and sterilized. pH checks between 7.2 and 7.4 are essential. The cooled agar (40-50°C) is poured into sterile Petri dishes to a depth of 4mm. Plates can be stored refrigerated (2-8°C) for up to 4 weeks before use, but need to be dried before inoculation for optimal results and to avoid swarming.
- Inoculation: A sterile swab is dipped in the standardized bacterial suspension. The swab evenly distributes the inoculum onto the surface in a streaking pattern to achieve a well established lawn (uniform spread) of bacteria growth. Excess moisture needs to be removed (3-5 minutes - dry but not completely).
- Antimicrobial Disks: Select representative antimicrobial agents for the tests based on the organisms, veterinary needs, research or epidemiological purposes. Use disks from a reputable manufacturer that are stored at 2 - 8°C in a tightly sealed container with desiccant.
- Disk Placement: Sterile forceps or disk dispenser are used to carefully place the disks onto the agar surface, avoiding movement after contact to ensure even diffusion.
- Control Plates: Control strains should be included for each plate set.
- Incubation: Plates are incubated upside down at a specific temperature for a predefined time (16-18 hrs minimum).
- Zone of Inhibition: Observe the zone of inhibition formed around the disks after incubation. The zone diameter measurement, in millimeter, is crucial.
- Interpretation: Compare measured zone diameters of the isolates to interpretative standards or charts specific for the tested pathogens. Results can be; Susceptible (S), Intermediate (I), or Resistant (R). There may be different interpretative standards for different organisms.
- Rejection Criteria: Do not interpret plates with isolated colonies, sub-confluent growth, overlapping zones, or distorted zones. Any deviations from standard conditions invalidate the test.
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Description
Test your knowledge on the Kirby-Bauer disk diffusion method used for antibiotic susceptibility testing. This quiz covers principles, media preparation, inoculum standards, and agar preparation techniques. Perfect for microbiology students and practitioners looking to reinforce their understanding of bacterial resistance testing.
