Culture and Sensitivity Flashcards
Agar Diffusion Method
Most commonly performed sensitivity test
What does the Agar Diffusion method use?
Paper disks impregnated with antibiotics
What does the Agar Diffusion method measure?
Inhibitory zones to estimate antimicrobial sensitivity
Bauer-Kirby Technique
A standardized disk susceptibility method
Minimum inhibitory concentration
Lowest concentration of the specific antimicrobial that can inhibit the growth of a given bacteria
Antimicrobial disks
Paper with known concentrations of antibiotics
Indirect Sensitivity Testing
Colony samples are take from the culture plate and subcultured in broth media
Direct Sensitivity Testing
Undiluted samples are added directly to Mueller Hinton plate
Indirect Sensitivity vs Direct Sensitivity testing
InDirect is not as precise as direct.
Zone of inhibition
Area of no bacterial growth around in inhibitory disk that indicates some sensitivity of the particular organism to an antimicrobial
Reading the Zone of inhibition
Should be read after a constant period, most satisfactorily after overnight incubation (18 to 24 hours)
When do rapid results need to be read?
After 6 to 8 hours of incubation
How do you measure a zone of inhibition
On the underside of the plate using calipers to the nearest millimeter
Interpreting Zone of Inhibition
- Resistant
- Susceptible
- Intermediate susceptibility
Control organisms
Can purchase examples of cultures to use as quality control for stains
What are common control organisms that can be purchased?
Staphylococcus aureus
Streptococcus spp
Escherichia coli
What bacteria will not grow on Mueller-Hinton plates?
Anaerobic bacteria
What strain of bacteria do routine tests may not identify?
MRSP
May needs to be identified at 30 degrees celsius
How to do a colony count on urine?
Streak blood agar with calibrated loop that contains 10mcl of urine
What happens after incubation of the urine?
Count all the colonies and multiply by 100 to determine the number of colony forming units per milliliter of urine
