Antibiotic susceptibility testing Flashcards
Antibiotics
-metabolic by-products of certain microorganisms that can inhibit or kill other microorganisms
Alexander Fleming
-was culturing staphylococcus aureus to study growth properties when he discovered an agar plate with culture was contaminated by a Penicillium fungal infection (penicillium notatum)
Penicillium
crude antimicrobial extract that was found to inhibit the growth of staphylococcus aureus
-it inhibits the formation of peptidoglycan cross links by inactivating the responsible enzyme (osmotic pressure –> cell death)
What other name is the diffusion susceptibility test called
Kirby Bauer Test
Kirby Bauer Test
-Test if a bacteria of interest is resistant or susceptible to antibiotic
-experiment involves culturing a bacteria species on a petri dish in the presence of an antibiotic
FIRST
- petri plate will be uniformly inoculated with a standardize amount of bacteria species of interest
SECOND
- a small disk is added with standard concentration of an antibiotic
THIRD
-after incubation, a lawn of bacteria will grow everywhere on the plate except regions around the disks
Do antibiotics with a large molecular weight diffuse faster or slower?
slower, travel less distance
Zone of inhibition
clear area where the bacteria did not grow
-the diameter of the zone is measured and compared to a standard table
The larger the diameter of the zone of inhibition means what?
-the more effective that antibiotic is against killing the bacteria species being tested
If there is no growth on the petri plate what do you measure?
- you measure the diameter of the disk
What does no zone of inhibition mean?
-means the bacteria is resistant to the antibiotic
Susceptible result on Kirby Bauer means?
-the antibiotic is killing the bacteria
Intermediate result on Kirby Bauer means?
-further testing required to determine if the antibiotic can be used as a treatment option against the bacterial species
Minimal inhibitory concentration (MIC)
the smallest amount of the antibiotic that can inhibit the growth of the bacteria in question
-are recognized by the presence of growth inhibitor zones
What is the chart used in the Kirby Bauer test called?
antibiogram
List what affects the size of growth inhibition zone
- type of culture
- dept of agar plate
- concentration of agar in plate
- rate of diffusion of the antibiotic
- the number of organisms on the plate
- the rate of growth of the organisms
- the degree of susceptibility of the organism to the drug
- the gram reaction of the organism
If the dept of the agar plate is too shallow what might happen?
-antibiotic will diffuse more laterally thereby increasing zone of inhibition
If the dept of the agar plate is too deep what might happen?
-the antibiotic will sink more below the disk and diffuse less laterally- thereby decreasing the zone of inhibition
Why does an antibiotic work better on gram positive bacteria than gram negative bacteria
the cell walls of gram positive bacteria are more permeable to antibiotics
True or false… A zone of inhibition for an antibiotic is absolutely indicative that the organism is susceptible to that antibiotic
False
A small zone of inhibition may still be indicative of clinical resistance of the bacteria to the antibiotic. While the antibiotic may inhibit the growth of the bacteria to some degree, it is not enough to correlate to susceptibility in a clinical (patient) setting
How does the antibiotic get from the disc to the agar?
the antibiotic is released from the disc into the agar, the antibiotic then diffuses into the agar (outward)
What is the principle of the agar disk diffusion method?
-there will be absence of bacterial growth across the surface of an agar plate if there is the presence of an effective antibiotic. Persistence of bacterial growth indicates resistance to the antibiotic
Susceptibility
a term used when microbes are unable to grow in the presence of antimicrobial drugs
