Question
Lab Report 16: Control of Microbial Populations: Effect of Chemicals1. What was the purpose of this lab exercise?2. Complete the following table with your observations:Disinfectant / Antiseptic | Name of the bacterium | Zone of inhibition | Comments3. Name the chemical that inhibited both bacteria tested.Bacterial Cultures- Escherichia coli broth culture- Staphylococcus aureus broth cultureMaterials- Nutrient agar- Sterile petri plates- Forceps- Filter paper discs- Bunsen burner- Sterile cotton swab- Wax pencilSuggested Disinfectants / Antiseptics- Hydrogen peroxide- Rubbing alcohol- Lab disinfectant- Copper sulfate (1%)- Any home disinfectant/antiseptic productProcedure1. Boil water in a beaker and place four tubes of nutrient agar in it.2. When the medium is completely melted, transfer the tubes to a test tube rack and let them cool slightly.3. Before the medium solidifies, pour two tubes of agar into each sterile petri dish, swirling gently to obtain an even distribution. Allow the agar to solidify.4. With a wax pencil, label the bottom of two petri dishes with the name of each bacterium. Divide the petri dishes into four quadrants and label each with the name of the chemical agent.5. Disperse the bacterial cultures by rolling the tubes between the palms.6. Dip a sterile cotton swab into the culture and press it against the inner wall of the culture tube to remove excess liquid.7. Partially open the petri dishes and inoculate the entire surface of agar with the inoculum.8. Repeat steps 6 and 7 at least two more times to ensure full coverage of the surface area and even distribution of the bacteria.9. Using a pair of sterile forceps, pick a filter paper disc and touch it (do not dip to avoid excess chemical that may run on the agar surface) to one of the disinfectants or an antiseptic solution.10. When the chemical is soaked into the disc, place the disc in the center of the matching, labeled quadrant of the culture dish.11. Tap the disc gently against the agar so that it does not fall off when the plate is inverted during incubation.12. Repeat steps 9, 10, and 11 for other chemicals and for both agar plates.13. Incubate both agar plates at 37°C for 24 hours.14. Check for the presence/absence of a zone of inhibition around each disc.15. Complete the lab report.
Answers
1. Purpose of the exercise
The purpose was to compare how selected chemical agents affect two bacterial cultures, Escherichia coli and Staphylococcus aureus, using an agar disc-diffusion test. Each chemical diffuses from its paper disc into the agar. After incubation, a clear area around a disc indicates that bacterial growth was inhibited under the conditions of the test.
2. How to complete the observation table
The question supplies the procedure and possible chemicals, but it does not supply the observed zones of inhibition. Record the chemicals actually placed on the four quadrants, the bacterium on that plate, the measured zone diameter or an explicit “no zone,” and relevant observations such as an uneven lawn or confluent growth.
| Disinfectant / antiseptic used | Bacterium | Zone of inhibition | Comments |
|---|---|---|---|
| Hydrogen peroxide | Escherichia coli | ||
| Rubbing alcohol | Escherichia coli | ||
| Lab disinfectant | Escherichia coli | ||
| Copper sulfate (1%) | Escherichia coli | ||
| Hydrogen peroxide | Staphylococcus aureus | ||
| Rubbing alcohol | Staphylococcus aureus | ||
| Lab disinfectant | Staphylococcus aureus | ||
| Copper sulfate (1%) | Staphylococcus aureus |
The supplied record sheet identifies these four chemical agents and the two bacterial cultures, so the eight chemical–bacterium combinations are shown above. The suggested list alone is not a results table: enter a zone diameter or “no zone” and any plate observation only after checking the incubated plates.
3. Chemical that inhibited both bacteria
No observation results are supplied, so the report must not name a chemical as the answer. Identify the chemical whose disc produced a zone of inhibition on both the E. coli plate and the S. aureus plate. If no one chemical produced zones on both plates, the correct conclusion is that no tested chemical inhibited both bacteria under these experimental conditions.
How to interpret the result responsibly
A larger or smaller zone in this classroom assay is not, by itself, a universal ranking of disinfectants. Diffusion through agar, chemical concentration, contact time, the bacterial strain, inoculum density, and the experimental setup all affect the observed zone. In particular, disinfectants and antiseptics are not interchangeable labels: antiseptics are intended for living tissue, while disinfectants are used on inanimate objects.
Conclusion
This lab assesses inhibition of visible bacterial growth in a controlled disc-diffusion assay. The final conclusion should report the actual zone observations for each agent and culture, then name a chemical only if the recorded data show inhibition for both bacterial cultures.
Evidence boundary
The procedure and tested chemicals are supplied, but no zone-of-inhibition results are provided. Record the actual zone diameters and plate observations, and name a chemical that inhibited both bacteria only when the student's results support that conclusion.
Sources
These references support the concepts and methods used in the explanation above.