Science 1
State a question about the natural world and outline how to develop it into a specific
inquiry that answers the question in whole or in part.
Evidence suggestions:
How effective are common household cleaners at sterilizing bacteria?
To test the effectiveness of a disinfectant and develop an inquiry for the question, one
must be aware of and be capable of employing the following: sterilization, spreading
techniques, and zone of inhibition.
Sterilization is the process of destroying any microscopic organisms that may be on the
surface of an object. For the purposes of this inquiry, the sterilization techniques
students will be using are flaming, alcohol disinfectant, and minimizing contamination.
Flaming is a technique used to sterilize small glass or metal objects, by dipping the
object into a beaker of alcohol and passing it through a flame. To minimize
contamination by airborne contaminants or human skin or secretions, one would need
to reduce the number of times the agar plate is exposed to air, wear gloves, avoid
talking, sneezing, and tie back long hair.
Spreading plates evenly distributes the cells on the surface of the agar plate. This even
distributing is accomplished by dispensing the bacterial sample on the agar plate, which
is placed on a spinner. While turning the spinner, hold a sterilized spreader on the agar
surface to evenly distribute the sample, continue to spin until the surface has been
covered with the sample.
A zone of inhibition is the diameter of the halo surrounding a circular piece of filter
paper soaked in disinfectant, it represents the effectiveness of the disinfectant. The
larger the diameter, the more effective the disinfectant is at sterilizing the bacteria.
With these in mind, an inquiry can be developed to measure the effectiveness of
common household cleaners, with the students working in pairs.
1. Sterilize workbench using 70% alcohol
2. Label agar plates as control, +Clorox, and +Lysol around the very edge of the
bottom of the bottom agar plate
3. Place agar plate labeled ‘Control’ on to spinner
4. Dip a metal spreader into a beaker of 70% alcohol
5. Pass the metal spreader through an open flame
6. Allow metal spreader to cool by allowing it to rest across top of beaker
7. Obtain 1mL sample of bacterial culture using sterilized pipette
8. Open the agar plate and keep the lid in one hand
9. Partner will dispense bacterial sample on to agar plate
10. Take the cooled metal spreader and gently place it on to the agar surface
11. Slowly turn the spinner and hold the metal spreader still
12. Continue to turn the spinner until the agar place has been evenly coated by the
bacterial sample
13. Repeat steps 3-12 for agar plates labeled ‘+Clorox’ and ‘+Lysol’
14. Incubate bacteria at 37 degrees C for 48 hours
15. After incubation period, take out agar plates and observe bacterial colony
growth
a. Record observations
16. Use a hole puncher to hole punch 2 pieces of filter paper
17. Flame metal tweezers
18. Use sterilized metal tweezers to pick up piece of filter paper
19. Dip filter paper into Clorox solution
20. Partner will open the agar plate labeled ‘+Clorox’ and hold lid in hand
21. Place the soaked piece of filter paper on agar place as close to the middle as
possible
a. Use another flamed metal tweezers to assist in placement if needed
22. Repeat steps 17-21 for Lysol on agar plate labeled ‘+Lysol’
23. Replace all three agar plates into incubator at 37 degrees C for 48 hours
24. After incubation period, take out agar plates, and observe zone of inhibition, if
any
a. Record observations and compare the zone of inhibition of Clorox and
Lysol