Testing the effectiveness of bacteria killing agents
In this lab, a petri dish is inoculated with bacteria then a paper disk
treated with an antiseptic agent is placed in the dish. After several days, a
halo develops around the paper disk indicating a zone of no growth.
Comparisons can be made between different antibacterial agents.
Materials
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Prepared petri dishes containing agar medium and nutrients.
E. coli bacteria.
Wax pencil or permanent marker for labeling dishes.
Masking tape.
Sterile swabs or inoculating loop.
Alcohol burner (source of flame to sterilize inoculating loop).
Antibacterial agent (soaps, disinfectants, etc.).
Sterile water.
Test tubes, 12 x 75mm.
Filter paper or paper towel.
Small containers in which to soak paper disks.
Hole punch.
Tweezers.
Ruler.
Bleach.
Method
1. Prepared petri dishes should be refrigerated until used and always
stored upside down (i.e media in upper dish, cover on bottom). This
keeps condensation, which forms in the lid from
dropping onto and disrupting the bacterial
growing surface.
2. When ready to use, let petri dishes come to
room temperature before taking samples
(about one hour).
3. Divide the petri dish into four quadrants
(see sample picture).
4. Swab bacteria from one single location
and inoculate the entire dish with source.
5. Collect bacteria from each location using
one swab for each new spot.
6. Prepare antiseptic disks by using a hole punch to
create paper disks out of a piece of filter paper or paper towel.
Soak one disk in each antibacterial agent to be tested.
7. Place a different pretreated antiseptic disk in each quadrant of the
inoculated petri dish. Make sure that the dish is labeled according
to which antiseptic is in each quadrant.
8. Replace cover on dish, tape closed, store upside down. Make sure
that your name or initials and period are on the dish.
9. Let grow in undisturbed warm location, ideally in an environment
around 100° F (37° C) - not in sunlight or on a heating register.
10. You should see growth within a couple of days. You should also see
a "halo" around each disk indicating a no growth zone. Measure
and compare the size of the kill zone to determine effectiveness of
each antibacterial agent.