(P) Lec 3.2: Bacterial Pathogenesis and Epidemiology, Antimicrobials and Mechanism of Bacterial Resistance, and Indigenous Microbial Flora Flashcards
Chapter 13 of Mahon's
Performed on bacteria and fungi isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections caused by these organisms
Antimicrobial Susceptibility Testing
Only organisms that are likely to be contributing to an infection should be tested
Antimicrobial Susceptibility Testing
Often performed by a disk diffusion or dilution (minimal inhibitory concentration [MIC]) method
Antimicrobial Susceptibility Testing
Inoculum concentration of bacteria to be tested must be ______
Standardized
____ results may occur if too few bacteria are tested
False-susceptible
____ results may be the outcome of testing too many bacteria
False-resistant
Name the standard
Standard Suspension
McFarland 0.5 standard: contains 99.5 mL of 1% sulfuric acid and 0.5 mL of 1.175% barium chloride.
Name the standard
Bacterial Inoculum Size
1.5 × 10^8 CFU/mL
Name the standard
Standard Medium
Mueller-Hinton Heated Agar
Name that standard
Standard Depth
4-6 mm
Name that standard
Standard pH
7.2-7.4
Name that standard
Atmosphere requirement
Aerobic, no CO2
Name that standard
Temperature
35 - 37°C
Disk Diffusion
1.5 x 10^8 CFU/mL
Broth Dilution
5 x 10^5 CFU/mL
Agar Dilution
1 x 10^4 CFU/mL
Composition of Mueller Hinton Agar
- Beef infusion solids
- Starch
- Casein hydrolysate agar
What are the storage guidelines of Mueller Hinton Agar?
- Store prepared media below 8°C
- Protect from direct light
- Store the dehydrated powder in a dry place and tightly sealed containers at 2 - 25°C
Main Purpose of 0.5 McFarland Standard
To adjust the turbidity of bacterial suspensions to a standardized level
Most common composition of 0.5 McFarland Standard
- 99.5 mL of 1% sulfuric acid
- 0.5 mL of 1.175% barium chloride
Can be prepared by adding specific volume of 1% sulfuric acid and 1.175% barium chloride to obtain a barium sulfate solution with a specific optical density
Storage of 0.5 McFarland Standard
Stored in an upright position at 4°C to 25°C and protected from light
Under the proper storage conditions, how long does the 0.5 McFarland Standard last?
12 weeks from date of manufacture
Used to determine the minimum inhibitory concentration (MIC) or the lowest concentration of antimicrobial agent required to inhibit the growth of the bacterium
Dilution Susceptibility Testing
Those that are attainable in vivo following standard dosing of the respective antimicrobial agent
Serial Two Fold-Dilution Concentrations
Once the MIC has been determined, the organism is interpreted as ____, ____, ____ or ____ to each agent
Nonsusceptible, susceptible, intermediate, or resistant
They described a test that measured the effectiveness with which penicillin in serum killed bacteria associated with endocarditis
Schlichter and MacLean
What year did Schlichter and MacLean describe the Serum Bactericidal Test?
1940s
Analogous to the MIC-MBC test except that the medium used is patient’s serum
Serum Bactericidal Test
Results relate to the amount of antimicrobial agent and any other antibacterial factors (e.g., antibody, opsonins, complement) present in the patient’s serum)
Serum Bactericidal Test
Also known as oxacillin resistance screening agar
Oxacillin Agar Screen
A specialized medium used for the screening of oxacillin-resistant microorganisms, primarily focusing on detecting MRSA
Oxacillin Agar Screen
What are the steps involving oxacillin agar screen?
- Allow medium to reach room temperature.
- Take the specimen swab and roll it near the edge of the plate.
- Then, use a sterile loop to perform a four-quadrant streak starting from that area to get isolated colonies.
- Incubate aerobically at 35°C.
- Examine after 24 hours for typical MRSA colonies. Re-incubate plates an additional 24 hours before discarding.
Used to detect VRSA and Enterococci resistant to VAN
Vancomycin Agar Screen
What are the steps involving vancomycin agar screen?
- Dip the swab into the inoculum (comparable to 0.5 McFarland std) to the plate medium (BHI w 6 mg/L of vancomycin) by streaking over the entire agar surface.
- Inc. 24 hrs
