Biology - HOT Science Lab

Student
3. To inoculate your agar plate, obtain a sterile cotton swab. Wet the swab with sterile water
and gently roll the swab on one surface to obtain a sample of any bacteria present.
(Note: Agar is like Jello. If you press too hard on the agar with the swab, the agar will
tear.)
4. Raise the petri dish cover slightly. Starting at one edge, rub the cotton swab across the
entire surface. Use back-and-forth motion to ensure that the entire surface has been
covered with the culture. You are creating a lawn.
5. Turn the petri dish a quarter of a turn and re-streak the swab across the agar surface to
ensure that bacteria are on the entire surface. (Remember, bacteria are microscopic so
you won‘t be able to see them!)
6. Replace the cover of the petri dish. To dispose of the swab, place it in the Ziploc bag
containing bleach. Be careful not to place the swab on any surface or allow the swab to
come into contact with anyone.
7. To assay bacterial effectiveness, you will conduct a sensitivity test. You will place the 3
antibiotic disks one blank (plain) disk soaked in sterile water on your plate, one per
quadrant.
a. Raise the cover of the petri dish slightly and place one disk that has been soaked in
sterile water in quadrant #1 and record it in the data table. Follow the same procedure
of the other 3 disks placing them in quadrants 2 (penicillin), 3 (antiseptic), and 4
(Disinfectant). Press lightly on the antibiotic disks so that they are stuck to the agar.
Record this information in the data table.
8. Tape the petri dish shut and return it to your teacher.
Day 5: DO NOT OPEN YOUR PETRI DISH!!!!
1. Observe your petri dish after the incubation period. Hold your dish up to the light to see
the zones of inhibition more clearly.
2. Use a metric ruler to measure the zone of inhibition (the diameter of the clear zone).
Biology HSL Page 291
Curriculum and Instruction