Disk Diffusion Method Quiz
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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?

  • Chloramphenicol
  • Penicillin (correct)
  • Amikacin
  • Tetracycline

What is the disk content of Amoxicillin-clavulanic acid used for susceptibility testing?

  • 20 µg
  • 10 units
  • 20/10 µg (correct)
  • 30 µg

For which pathogenic ATCC strain is Kanamycin's susceptibility not determined due to missing data?

  • ATCC 49619 (correct)
  • ATCC 25922
  • ATCC 27853
  • ATCC 25923

Which antimicrobial agent shows a zone diameter interpretative standard ranging from 23-29 for ATCC 25922?

<p>Cefoxitin (A)</p> Signup and view all the answers

What is the minimum susceptibility zone diameter for Rifampin against ATCC 25923?

<p>26 mm (D)</p> Signup and view all the answers

What is the recommended turbidity level for a standardized bacterial inoculum?

<p>0.5 MacFarland standard (C)</p> Signup and view all the answers

Which of the following steps is not part of the preparation of inoculum?

<p>Inoculate plates with sterile saline (C)</p> Signup and view all the answers

When should antibiotic disks be applied to the inoculated agar plate?

<p>Immediately after inoculating and allowing a short drying period (D)</p> Signup and view all the answers

How should antimicrobial disks be stored prior to use?

<p>In a tightly sealed container with desiccant at 2-8°C (D)</p> Signup and view all the answers

What is the maximum number of antimicrobial disks that can be placed in a 9-cm petri dish?

<p>6 disks (C)</p> Signup and view all the answers

Which procedure helps ensure an even distribution of the inoculum on the agar plate?

<p>Rotating the plate by 60° and repeating the streaking (B)</p> Signup and view all the answers

What is the purpose of pressing the antibiotic disk onto the agar surface?

<p>To ensure complete contact for effective diffusion (B)</p> Signup and view all the answers

What is the concentration of a standardized inoculum?

<p>1-2x10^8 cfu/ml (D)</p> Signup and view all the answers

What should be done if overlapping zones of inhibition are encountered?

<p>Reduce the number of disks applied per plate (B)</p> Signup and view all the answers

At what temperature should the plates be incubated?

<p>30°C (D)</p> Signup and view all the answers

How should the diameter of the zones of inhibition be recorded?

<p>Round up to the nearest millimeter (C)</p> Signup and view all the answers

What is the result category if test isolates show a zone of inhibition?

<p>Susceptible (S) (D)</p> Signup and view all the answers

When should test plates be rejected?

<p>If the zones show distortion from circular shape (C)</p> Signup and view all the answers

What should be done with data collected from a set if the control strain zones of inhibition are not within tolerance limits?

<p>Reject all data from that set (C)</p> Signup and view all the answers

What is meant by interpreting susceptibility results as 'qualitative' until established zones are provided?

<p>Results only indicate presence or absence of inhibition (C)</p> Signup and view all the answers

What must be observed to ensure that the zone of inhibition measurement is valid?

<p>Ensure overlaps do not affect adjacent disks (C)</p> Signup and view all the answers

What principle does the disk diffusion method rely on?

<p>The antibiotic diffuses radially, creating a gradient that affects bacterial growth. (B)</p> Signup and view all the answers

Which medium is recommended for routine susceptibility tests?

<p>Mueller-Hinton Agar (A)</p> Signup and view all the answers

What is the correct temperature and time for autoclaving agar medium?

<p>121°C for 15 minutes (B)</p> Signup and view all the answers

How should the pH of the agar medium be checked?

<p>By using a pH meter with a small sample of the medium in water (A)</p> Signup and view all the answers

What is the proper storage condition for the agar plates if not used immediately?

<p>In airtight plastic bags at 2-8°C (B)</p> Signup and view all the answers

What happens if water droplets are present on the surface of the agar plates?

<p>They can lead to swarming bacterial growth, causing inaccurate results. (D)</p> Signup and view all the answers

Which broth can be used to prepare the inoculum for disk diffusion?

<p>Tryptic Soy Broth (B)</p> Signup and view all the answers

What is the recommended depth for pouring agar into petri dishes?

<p>4 mm (B)</p> Signup and view all the answers

Which antimicrobial agent has the highest zone diameter for Staphylococcus aureus?

<p>Ampicillin (B)</p> Signup and view all the answers

What does a zone diameter of ≤ 14 mm indicate for Amikacin against Escherichia coli?

<p>Resistant (C)</p> Signup and view all the answers

Which of the following antimicrobial agents is effective against Pseudomonas aeruginosa based on its zone diameter response?

<p>Imipenem (D)</p> Signup and view all the answers

Which antimicrobial agent is indicated as resistant when the zone diameter is < 10 mm for Staphylococcus aureus?

<p>Spectinomycin (A)</p> Signup and view all the answers

Which agent shows a significant susceptibility zone diameter for Streptococcus pneumoniae?

<p>Ampicillin (D)</p> Signup and view all the answers

For which antimicrobial agent does a zone diameter of < 13 mm suggest ineffective treatment for Escherichia coli?

<p>Kanamycin (A)</p> Signup and view all the answers

Which of these antimicrobial agents provides an intermediate zone diameter for Pseudomonas aeruginosa?

<p>Cefoxitin (A)</p> Signup and view all the answers

What is the indicated susceptibility level for Trimethoprim-sulfamethoxazole against Staphylococcus aureus based on the zone diameter?

<p>Resistant (A)</p> Signup and view all the answers

Which of the following agents is susceptible with the highest zone diameter against Streptococcus pneumoniae?

<p>Ampicillin (B)</p> Signup and view all the answers

Which antimicrobial agent shows resistance against Klebsiella pneumoniae based on a zone diameter of < 12 mm?

<p>Chloramphenicol (D)</p> Signup and view all the answers

Flashcards

Disk Diffusion Method

A method used to determine the susceptibility of bacteria to antibiotics.

Mueller-Hinton Agar

The recommended agar for routine antibiotic susceptibility tests.

Antibiotic Concentration Gradient

The gradual decrease in antibiotic concentration from the disk outward.

Clear Zone

The area of inhibited bacterial growth around an antibiotic disk.

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Inoculum Preparation

Creating a bacterial suspension for spreading on the agar.

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Storage of Plates

Properly storing agar plates to maintain sterility and quality

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Agar Depth

The uniform thickness maintained when pouring media, typically 4 mm.

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Sterilization of Media

The process to remove all microorganisms from media.

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Control Plate

A plate with a standard strain of bacteria, used to check if the test is working correctly.

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Fuzzy Zone

An area around the antibiotic disk with a less defined boundary, which should be ignored when measuring the zone of inhibition.

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Zone of Inhibition

The area around an antibiotic disk where bacterial growth is inhibited.

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Reject Criteria

Reasons why a disk diffusion test might need to be repeated, ensuring reliable results.

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Overlapping Zones

When two antibiotic disks are placed too close together, their zones of inhibition merge.

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Reading Plates

Examining the agar plates to assess bacterial growth and zone sizes.

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Interpreting Results

Analyzing the zone sizes to determine if a bacteria is resistant, intermediate, or susceptible to an antibiotic.

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Control Strain Limits

The acceptable range for the zone of inhibition on the control plate for a specific bacterium.

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MacFarland Standard

A solution used to standardize the turbidity of a bacterial suspension, ensuring a consistent concentration of bacteria for antibiotic testing.

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Inoculum Turbidity

The cloudiness of the bacterial suspension, which should match the 0.5 MacFarland Standard for accurate testing.

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Disk Diffusion

A method to test antibiotic effectiveness by observing the zone of bacterial growth inhibition around antibiotic disks on an agar plate.

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Antibiotic Disk Placement

Disks should be spaced 24 mm apart on the agar plate to prevent overlap of zones of inhibition and inaccurate results.

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Sterile Technique

Using sterile equipment and procedures to prevent contamination during inoculum preparation and antibiotic disk application.

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Representative Antibiotics

Selecting a limited number of antibiotics from different drug groups to represent the range of potential treatments.

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Antibiotic Disk Storage

Antibiotic disks should be stored properly in a sealed container with desiccant at 2-8°C to maintain their effectiveness.

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Inoculation Technique

Swabbing the agar plate with a standardized bacterial suspension to ensure even distribution and accurate antibiotic testing.

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Antibiotic Susceptibility Testing

A laboratory test that determines whether bacteria are sensitive to specific antibiotics, used to guide treatment.

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Zone Diameter Interpretation

Analyzing the size of the clear area around an antibiotic disk to determine if a bacteria is susceptible, intermediate or resistant.

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ATCC Reference Strains

Standard bacterial cultures used for quality control in susceptibility tests, ensuring the test is reliable.

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Antimicrobial Agent

A drug or compound that inhibits the growth or kills microorganisms.

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Disk Content

The specific amount of antibiotic contained in a disc used in susceptibility testing.

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Susceptibility Testing

A laboratory method used to determine if a specific bacteria is resistant, intermediate, or susceptible to an antibiotic.

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Resistant

A bacteria is resistant to an antibiotic if its growth is not significantly inhibited by the antibiotic.

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Susceptible

A bacteria is susceptible to an antibiotic if its growth is significantly inhibited by the antibiotic.

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Intermediate

A bacteria is considered intermediate when its growth is partially inhibited by the antibiotic, suggesting a possible therapeutic response with increased dosage.

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Control Strain

A known bacterial strain with a defined susceptibility profile used to validate the accuracy of the disk diffusion test.

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Interpretation of Results

Analyzing the zone of inhibition measurements against standardized guidelines to determine the bacteria's antibiotic susceptibility.

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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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Disk Diffusion Method | Kirby-Bauer Susceptibility Testing PDF

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.

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