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Questions and Answers
What is the primary purpose of performing antibiotic susceptibility tests?
What is the primary purpose of performing antibiotic susceptibility tests?
Which of the following is NOT a factor that is standardized and controlled in antimicrobial susceptibility testing?
Which of the following is NOT a factor that is standardized and controlled in antimicrobial susceptibility testing?
What organization publishes guidelines and recommendations for antimicrobial susceptibility testing?
What organization publishes guidelines and recommendations for antimicrobial susceptibility testing?
What is the name of the system that uses a microtiter plate with a specific concentration of antimicrobial agents?
What is the name of the system that uses a microtiter plate with a specific concentration of antimicrobial agents?
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What is the purpose of controlling the bacterial inoculum size in antimicrobial susceptibility testing?
What is the purpose of controlling the bacterial inoculum size in antimicrobial susceptibility testing?
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What type of methods use instruments to measure the growth of bacteria in the presence of antimicrobial agents?
What type of methods use instruments to measure the growth of bacteria in the presence of antimicrobial agents?
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What is the primary difference between the macrobroth dilution method and the microbroth dilution method?
What is the primary difference between the macrobroth dilution method and the microbroth dilution method?
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What is the purpose of the 'alamar blue' indicator in the Alamar system?
What is the purpose of the 'alamar blue' indicator in the Alamar system?
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What is the primary step in the agar dilution method after adding the inoculum to the MHA?
What is the primary step in the agar dilution method after adding the inoculum to the MHA?
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What is the term used to describe the area where bacterial growth is inhibited in the diffusion method?
What is the term used to describe the area where bacterial growth is inhibited in the diffusion method?
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What is the primary difference between the E-test and the agar dilution method?
What is the primary difference between the E-test and the agar dilution method?
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What is the primary purpose of the Vitek system?
What is the primary purpose of the Vitek system?
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What is the primary purpose of incubating the inoculated surface with disks in disk diffusion testing?
What is the primary purpose of incubating the inoculated surface with disks in disk diffusion testing?
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What is the interpretation of a susceptible result in disk diffusion testing?
What is the interpretation of a susceptible result in disk diffusion testing?
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According to the CLSI protocol, what is the recommended temperature range for short-term storage of a working supply of disks?
According to the CLSI protocol, what is the recommended temperature range for short-term storage of a working supply of disks?
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What is the recommended timeframe for using inoculum suspensions after preparation?
What is the recommended timeframe for using inoculum suspensions after preparation?
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What is the term for the selection of antimicrobial agents to be tested against a particular bacterial isolate?
What is the term for the selection of antimicrobial agents to be tested against a particular bacterial isolate?
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What does the intermediate category in disk diffusion testing imply?
What does the intermediate category in disk diffusion testing imply?
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What is the purpose of pressing the disks firmly onto the agar surface?
What is the purpose of pressing the disks firmly onto the agar surface?
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How long should the inoculated plates be incubated in a 35°C ambient air incubator?
How long should the inoculated plates be incubated in a 35°C ambient air incubator?
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What is the main purpose of disk diffusion testing?
What is the main purpose of disk diffusion testing?
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Why are certain antimicrobials excluded from the test battery?
Why are certain antimicrobials excluded from the test battery?
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What is the recommended storage condition for long-term storage of disks?
What is the recommended storage condition for long-term storage of disks?
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What is the purpose of excluding certain antimicrobials from the test battery due to acquired resistance patterns?
What is the purpose of excluding certain antimicrobials from the test battery due to acquired resistance patterns?
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What is not a possible interpretation of an intermediate result in disk diffusion testing?
What is not a possible interpretation of an intermediate result in disk diffusion testing?
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Which of the following is not a category in disk diffusion testing?
Which of the following is not a category in disk diffusion testing?
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What is the purpose of using a sterile cotton swab to inoculate the Mueller-Hinton agar plate?
What is the purpose of using a sterile cotton swab to inoculate the Mueller-Hinton agar plate?
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Why is ceftazidime included in the test battery for Pseudomonas aeruginosa but not for Staphylococcus aureus?
Why is ceftazidime included in the test battery for Pseudomonas aeruginosa but not for Staphylococcus aureus?
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What is the significance of a susceptible result in disk diffusion testing?
What is the significance of a susceptible result in disk diffusion testing?
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What is the role of the CLSI in selecting antimicrobial agents for testing?
What is the role of the CLSI in selecting antimicrobial agents for testing?
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Study Notes
Inoculum Preparation
- Inoculum suspensions should be used within 15 minutes of preparation
- Suspensions may be diluted or supplemented with more organisms if they do not match the standard's turbidity initially
Selection of Antimicrobial Agents
- Antimicrobial agents chosen for testing against a particular bacterial isolate are referred to as the antimicrobial battery or panel
- The CLSI publishes up-to-date tables listing potential antimicrobial agents for testing against particular organisms or organism groups
Criteria for Antimicrobial Battery Content and Use
- Antimicrobials to which the organism is intrinsically resistant are excluded from the test battery (e.g., vancomycin vs. Gram-negative bacilli)
- Antimicrobials specifically developed for use against particular organisms but not others (e.g., ceftazidime for Pseudomonas aeruginosa but not Staphylococcus aureus) are only included in the appropriate battery
- Antimicrobials with resistance common to local microbial flora are excluded from routine testing, and only more potent antimicrobials are included in the test battery
Antibiotic Susceptibility Test Laboratory Activity
- Performed on isolated bacteria from clinical specimens to determine effective antimicrobial agents for treating infections
- Only bacteria likely to be contributing to an infection are tested
- Standard conditions for antimicrobial susceptibility testing methods have been established by the CLSI
Standardized and Controlled Components
- Bacterial inoculum size
- Growth medium
- pH
- Cation concentration
- Blood and serum supplements
- Thymidine content
- Incubation atmosphere
- Incubation temperature
- Incubation duration
- Antimicrobial concentrations tested
Methods
Dilution Methods
- Macrodilution
- Drug is diluted with MH broth in several tubes together with the inoculum
- Incubate for 24 hours
- Check for presence of growth
- Plate them by overlaying a MIC media with the broth
- Incubate plates for 24 hours
- Determine the MBC
- Microdilution
- Utilizes microtiter plates
- Utilizes minute volume of broth and inoculum
- Almost the same with macrodilution
- Incubate microtiter plates for 24 hours
- Check for growth
- Agar dilution
- Utilizes a type of tray where antibiotics are added first in the MHA, then just before it solidifies, you add the inoculum using a “replicator”
- Incubate for 24 hours
- Determine MIC by checking for presence or absence of colonies
Diffusion Method
- E-test or Epsilometer test
- Utilizes MHA plus a plastic strip impregnated with antibiotics on one side and the label of concentration on the other side
- Inoculate organism to MHA, then place the strip (antibiotic side, face-down)
- Incubate for 24 hours
- Observe for ZOI (MIC)
Alamar System
- Utilizes a type of tray with many wells and each row has a specific antibiotic, MHB, the inoculum + “alamar blue” indicator
- Initially, all wells will appear blue in color
- Incubate
- Presence of “pink” colored well means it is negative for growth
Automated Methods
- Vitek Systems
- Conventional Testing Methods
- Paper disks containing specific concentrations of antimicrobial agent are placed onto the inoculated surface
- After overnight incubation, the diameters of the zones produced by antimicrobial inhibition of bacterial growth are measured
- Result is interpreted as nonsusceptible, susceptible, intermediate, or resistant to a particular drug according to preset criteria
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Description
This quiz is based on a laboratory activity on antimicrobial susceptibility testing, which determines the effectiveness of antimicrobial agents against bacteria isolated from clinical specimens. It covers the standard conditions and methods used in this process. Test your knowledge of this important laboratory procedure!
