BIOD171_Microbiology_Lab_Notebook

LECTSTEPHEN




Lab 5 Notebook

BS05 Antibiotic Sensitivity: Testing bacteria for antibiotic sensitivity

Objective: To determine the threshold of antibiotic sensitivity across bacterial strains using the
Kirby-Bauer method. (Aka Standardized Disc Susceptibility Test)

Procedure:

1. Streak bacteria A across an LB agar plate for confluent growth using a
sterile L-spreader.
a. Dispose of L-spreader in a biohazardous waste bin.
2. Evenly place the paper antibiotic discs on the plate.
3. Invert plate and incubate at 37°C overnight.
4. Measure zones of inhibition (diameter).
a. White zone around disk indicates that the bacteria are
susceptible to the antibiotic and that the bacteria has been
killed.
b. Disks that do not have a white zone indicates that the bacteria are resistant to
the antibiotic disks.
5. Compare results with sensitivity chart.

Materials needed:

1. Non-selective LB agar plate.
2. Bacterial cultures that are growing in liquid media.
3. Sterile L-spreader.
4. Paper antibiotic discs.
5. Tweezers.
6. All materials are disposable, place all waste in
biohazardous waste bin.

Results:

 List diameter of zone of inhibition for each disc.
o Letters on disks correspond with each antibiotic
 Ex: E=Erythromycin
 Reference chart to determine sensitivity level:
o Resistant >Intermediate>Susceptible
o Charts are specific for different types of bacteria. This chart is used when using a
STAPH strain of bacteria.
 E=Erythromycin is 31mm: Susceptible
 G=Gentamicin 9mm: Resistant


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