Read through this lab and use the information provided to answer the questions. Some
questions are thinking questions. Some questions refer to the tables that are provided.
First lab day:
This activity is a preparatory step for the lab work that will be carried out on the second lab day.
Each student grew a culture from the bacteria in his/her own mouth. They swabbed their mouths with a
sterile cotton-tipped swab, then dropped the swab into a labeled tube containing Tryptic Soy Broth
(Enriched broth). These tubes are not leak-proof! One must be careful not tip them or you may spill
some of the broth! If an inoculated broth is spilled, it must be treated with disinfectant! The tube was be
capped and incubated at 30° C for 24 to 48 hours. The student carefully pressed onto the tube a mailing
label listing his/her name. These bacterial cultures were inspected for growth on the second lab day and
used to inoculate the surface of a blood agar plate.
Second Lab Day:
We tested the effectiveness of different mouthwashes, household soaps, and disinfectants against bacterial
Mouthwashes–Each student checked his/her tube (from the first day) for growth. Growth was
recognized by cloudy broth. Students were given a blood agar plate, and with a wax pencil,
carefully marked the BOTTOM (why??) of the plate with their name, and with a mark to identify
where each of the four different mouthwash disks would be placed. (The four disks were to be
separated from each other as much as possible.). Students used a sterile disposable pipette to
place 2 or 3 drops of their bacterial culture onto the center of the plate. The method demonstrated
by the instructor was used to spread the bacteria carefully so that the entire surface of the plate was
covered with bacteria. Students soaked one filter paper disk in each of the four different
mouthwashes provided. They blotted the disks on the large piece of filter paper to remove excess
mouthwash, then placed each disk in its marked spot on the plate. The plates were incubated at
30° C for 48 hours. The larger kill zone was measured in mm. The larger the kill zone, the better
killing power of the mouthwash.
1. Why do you want to cover the entire plate surface with your bacterial culture?
2. Why must you place the mouthwash disks apart from each other? You don’t want them to
touch and start mixing with the different bacteria. If they mix you will not get accurate
3. Do you think the inoculated tubes have an odor?
4. What other experiments could be conducted?
5. When one swabs the mouth, would only one type of bacterium be picked up and transferred to
6. Will people have the same bacterial flora in their mouth? Why?
no, The bacterial flora is different because what people eat and what types of products each person
uses. If they use mouthwash or not and also each mouthwash is different so this would change the
bacterial flora in each person.
Soaps–Each student was assigned a specific soap. They labeled a blood agar contact plate with
their name, and the name of the soap assigned. The student washed his/her hands with the soap
and pressed the hand against a blood agar control plate. A template could have been used to press
the palm against the plate. If a template were used, it would allow a certain part of the hand to
contact the plate. The plate was incubated 48 hours at 30⁰ C. Some students were assigned to
press their hands against contact plates without washing their hands first (in this case, label your
plate with your name, and “no soap”).
8. Why might it be important to use a template to guide the placement of the hand on the plate?
9. What would be the control in this experiment?
10. What other experiments might one try?
Disinfectants–Each student was assigned a specific disinfectant. They labeled two contact plates
with their name. Also, they were to label one plate “untreated”, and the other plate with the name of
their disinfectant. Each person was to choose a different surface (example: desk, counter, or table)
and test the effectiveness of this disinfectant. The student pressed the plate labeled “untreated”
against the table surface, and left it in place. The student then sprayed or applied the disinfectant
on the surface next to the first plate, waited the specified amount of time, and then gently blotted the
surface dry using a paper towel. The student then pressed the second contact plate labeled with the
name of the disinfectant against the surface near the first contact plate, but in the area where
disinfectant had been applied. (They were to clean the area with a paper towel before leaving). If
the medium fell from the plate, they were to use the lid to put it back on the plate, or throw it in the
trash. The plates were to be incubated for 48 hours at 30°C.
11. Why would a student press the two contact plates in two different surfaces near each other? Why
not press the plates in the same place?
12. What is the purpose of the first plate?
Look at the Antimicrobial Results below there are three charts Read the tables with results for
Mouthwashes, Soaps/handcleaners, and Disinfectants.
Mouthwashes are measured by their kill zones.
Soaps and disinfectants are measured by the number of bacterial colonies growing. More colonies
mean less ability to disinfect.
Use the above results to:
13. Which mouthwash appears to kill more “saliva” bacteria on the blood agar plate?
14. Which hand cleaner inhibited the most bacterial growth?
15. Which hand cleaner allowed the most bacterial growth?
16. How could both bleach and Lysol producers claim their product was the best disinfectant? Explain!
16. It should be noted that you have been given numbers of bacterial colonies grown on the contact
plates. You have not been told what types of bacteria are present. Would the type of bacteria be an