ANTIBIOTIC SUSCEPTIBILITY TESTING Flashcards
What is the purpose of dilution in microbiology, and what numerical values do MIC, MBC, and MLC represent?
• Dilution in microbiology is used to quantitatively determine the Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), and Minimum Lethal Concentration (MLC), with MIC, MBC, and MLC
• Representing numerical values as a result
What is the reporting format for disk diffusion testing using the Kirby Bauer method for antibiotic susceptibility?
• The results are reported qualitatively as either “Sensitive,” “Intermediate,” or “Resistant.”
What does MIC stand for, and what does it represent in microbiology?
• Minimum Inhibitory Concentration
• It represents the lowest concentration of a substance that inhibits bacterial growth.
What does MBC/MLC stand for in the context of antibiotic effectiveness?
• Minimum Bactericidal Concentration or Minimum Lethal Concentration
• Indicating the lowest concentration of antibiotic that can kill bacteria.
What factors can lead to a “LARGER ZONE - FALSE SENSITIVE” result in Kirby Bauer disk diffusion testing?
- too thin agar
*a very dry agar surface, - a too light inoculum
- Thin medium
- Delay before incubation
- Low temperature and old colonies
What are some factors that can lead to a FALSE RESISTANT result in a SMALLER ZONE when conducting microbiological testing?
- Too thick,
excessive moisture on the agar surface,
a heavy inoculum
extended incubation time,
and delay before disc application.
What are the two categories of antimicrobial agents based on their spectrum of activity?
• Can be categorized as broad spectrum or narrow based on their spectrum of activity.
What is the maximum number of antibiotic disks allowed on a 150 mm plate when swarming on Kirby Bauer?
• The maximum number allowed is 12 antibiotic disks.
What is the maximum number of antibiotic disks allowed on a 100 mm plate when swarming on Kirby Bauer?
• No more than 5 mm of disks
What is the recommended storage temperature for antibiotic disks in a working supply?
• The recommended storage temperature is 2-8°C.
Where should antibiotic disks in the working supply be placed for storage?
• They should be placed in the refrigerator (REF) for storage.
Name two classes of cell wall inhibitors in the context of antibiotic medications.
• beta lactams (including Penicillin, Cephalosporins, and Carbapenems)
• glycopeptide (Vancomycin).
Which antibiotics are classified as protein synthesis inhibitors?
• Aminoglycosides: Gentamycin Tetracycline
• MLS (macrolide lincosamide streptogramin) -erythromycin, Clarithromycin
What type of media is commonly used for the AST (Antibiotic Susceptibility Testing) of Streptococci, and why is it used?
• Typically performed using MHA (Mueller-Hinton Agar) supplemented with blood.
• This medium is used because Streptococci are fastidious organisms
What does “Broad Spectrum” mean in relation to antibiotics?
• “Broad Spectrum” antibiotics can act against many different types of bacteria.
What is the primary characteristic of “Bacteriostatic” antibiotics?
• “Bacteriostatic” antibiotics have an inhibitory effect on bacteria.
What is a limitation of “Narrow Spectrum” antibiotics?
• Limited range of action and typically target only gram-positive bacteria.
What is the defining feature of “Bactericidal” antibiotics?
• “Bactericidal” antibiotics have a killing effect on bacteria.
What is the Kirby Bauer Technique used for?
• The Kirby Bauer Technique is used for antimicrobial susceptibility testing.
What type of method is used in Kirby Bauer technique?
• The disk diffusion method is used in the Kirby Bauer Technique.
What media is prepared for this technique, and how is it applied in Kirby Bauer Technique?
• Media prepared is Mueller-Hinton Agar (MHA),
• Applied using a sterile cotton swab with overlapping streaking.
What should be the depth of the agar in this Kirby Bauer technique?
• The depth of agar should be 3-5mm.
In Kirby Bauer Technique, What is the size of the antibiotic disks used?
• The antibiotic disks used are 6mm in size.
In Kirby Bauer Technique, What is the recommended distance of the antibiotic disk from the center of the agar plate?
• The recommended distance of the disk from the center is 24mm.
What is the distance between two antibiotic disks on the same plate in Kirby Bauer Technique?
• The distance between two antibiotic disks on the same plate is 15mm.
In the Kirby Bauer Technique, What are the incubation conditions for the agar plates?
• Incubate the plates at 37°C for 16-18 hours.
How should the zone of inhibition be measured, and what’s the rule regarding numerical values in the context of Kirby Bauer technique?
• The zone of inhibition should be measured using a ruler or caliper.
What is the maximum number of plates allowed in the Kirby Bauer technique?
• The maximum number of plates allowed is 5.
How many antibiotics should be on each disk in the Kirby Bauer technique?
• Each disk should contain only 1 antibiotic.
Are numerical values allowed in the Kirby Bauer technique?
• No, numerical values are not allowed in this technique.
What does a zone size of 16 mm indicate in the context of Kirby Bauer technique?
• A zone size of 16 mm suggests an intermediate level of sensitivity or resistance.
In Kirby Bauer technique, If the zone size for inhibition is greater than 16 mm, what does it indicate?
• If the zone size is greater than 16 mm, it indicates sensitivity.
In the context of Kirby Bauer technique, What does a zone size less than 16 mm imply?
• A zone size less than 16 mm implies resistance to the substance being tested.
What is the primary purpose of antibiotic susceptibility testing?
- The primary purpose of antibiotic susceptibility testing is to detect the ability of antimicrobial agents to inhibit bacterial growth in vitro.
What does antibiotic susceptibility testing help determine?
- Helps determine the susceptibility or resistance of organisms against antimicrobial agents.
What is the Tube or Broth Dilution Method used for?
- Used for challenging the organism of interest with microbial agents in a broth environment
- To determine MIC and MLC concentrations.
How is the concentration of antibiotic prepared in the Tube or Broth Dilution Method?
- A specific amount of antibiotic is prepared in a decreasing concentration
- In the broth by using the serial dilution technique
What is the standard inoculum size used in this method?
- The standard inoculum size for the Tube or Broth Dilution Method is 5 x 10^5 CFU/mL.
What is the basic principle of the agar dilution method?
- Combining antimicrobial concentrations and test organisms on an agar-based medium.
What is the typical shelf life of an agar dilution plate for most antimicrobial agents?
- The shelf life of an agar dilution plate is usually only 1 week for most antimicrobial agents.
What is the agar medium used for testing anaerobes and Neisseria gonorrhoeae?
- For anaerobes, the agar medium used is Brucella agar with leaked blood and Vitamin K.
What is the standard inoculum size for the agar dilution method?
- The standard inoculum size for the agar dilution method is 1 x 10^4 CFU/mL.
What does MIC stand for, and what does it represent in a laboratory setting?
- MIC stands for Minimal Inhibitory Concentration
- It represents the lowest concentration of a substance that inhibits visible growth in vitro (in a laboratory environment).
What does MBC stand for in the context of antibiotic effectiveness?
- MBC stands for Minimal Bactericidal Concentration
- Which represents the lowest concentration of the antibiotic required to kill a bacterium.
What is the principle behind Disk Diffusion – Kirby Bauer (Qualitative) testing?
- Based on the inverse linear relationship between the diameter of the zone of inhibited growth around the antibiotic disk
- And the logarithm of the MIC (Minimum Inhibitory Concentration) of the antibiotic.
Which types of microorganisms is Disk Diffusion testing limited to?
- Disk Diffusion testing is limited to Aerobes and Facultative anaerobes
What is the procedure in Disk Diffusion testing?
- Filter paper disks impregnated with various antimicrobial agents of specific concentrations are carefully placed on an agar plate
- Previously inoculated with the bacteria being tested.
What is the primary purpose of Disk Diffusion testing?
- To determine the susceptibility or resistance of a pathogenic organism to various antimicrobial agents.
What is the purpose of using Muller Hinton Agar (MHA) in disk diffusion testing?
- MHA is used as the susceptibility standard medium
What is the standard inoculum size for the Kirby Bauer disk diffusion test?
- The standard inoculum size for the Kirby Bauer test is 1.5 x 10^8 colony-forming units per milliliter (CFU/mL).
How is the 0.5 MacFarland standard prepared for the disk diffusion test?
- The 0.5 MacFarland standard is prepared by mixing 99.5 mL of 1% H2SO4 (sulfuric acid)
- With 0.5 mL of 1.175% BaCl (barium chloride)
How does an acidic pH affect Tetracycline, Novobiocin, and Methiliccin?
- An acidic pH can decrease the effectiveness of Tetracycline, Novobiocin, and Methiliccin.
How does an alkaline pH affect the activity of Aminoglycosides, Clindamycin, and Erythromycin?
- Can reduce the effectiveness of Aminoglycosides, Clindamycin, and Erythromycin,
- Potentially leading to decreased antimicrobial activity.
What is the recommended storage temperature for antibiotic disks (working supply)?
- The recommended storage temperature for antibiotic disks (working supply) is 2-8°C.
