1. Antibiotics, Antiseptics, and Disinfectants
Magen Wietzel
October 8, 2014
I. Purpose
The purpose of this experiment was to compare the effectiveness of antibiotics,
antiseptics, and disinfectants on gram negative and gram positive bacteria.
II. Procedure
Antibiotics Antiseptics Disinfectants
1. Penicillin 1. Hydrogen Peroxide 1. Bleach
2. Erythromycin 2. Listerine 2. Ammonia
3. Tetracycline
4. Chloramphenicol
III. Results
Table 1.1 E. coli and Antibiotics
Antibiotic Zone Size (mm) Susceptibility
Tetracyclin 28 sensitive
Penicillin 0 resistant
Erythromycin 22 sensitive
Chloramphenicol 30 sensitive
Table 1.2 E. coli and Antiseptics and Disinfectants
Antiseptic/Disinfectant Zone Size (mm)
Listerine 8
Hydrogen Peroxide 20
Ammonia 0
Bleach 7
Table 2.1 S. aureus and Antibiotics
Antibiotic Zone Size (mm) Susceptibility
Tetracyclin 30 sensitive
Penicillin 42 resistant
Erythromycin 25 sensitive
Chloramphenicol 22 sensitive

2. Table 2.2 S. aureus and Antiseptics and Disinfectants
Antiseptic/Disinfectant Zone Size (mm)
Listerine 9
Hydrogen Peroxide 39
Ammonia 16
Bleach 14
IV. Discussion
In this experiment, we were measuring zones of inhibition to determine the effectiveness
of antibiotics, disinfectants, and antiseptics on E. coli and S. aureus. An antibiotic is a product
made by microorganisms to inhibit the growth of other bacteria. An antiseptic is a substance that
can either kill or inactivate bacteria but is gentle enough to use on skin. A disinfectant is a
substance that can only be used on an inanimate object because it is very harsh. It kills most
microorganisms. A zone of inhibition is an area on a plate of no bacterial growth due to death
from an antibiotic, disinfectant, or antiseptic. If a zone of inhibition is observed it means that the
substance being tested is effective at killing the bacteria present on the plate. The zone of
inhibition is measured and then interpreted on a relative scale to determine susceptibility. If the
bacteria being studied is susceptible to the substance there is a relatively good chance that the
substance can be used to kill the bacteria. In this experiment, we soaked paper disks with
Tetracycline, Erythromycin, Chloramphenicol, Penicillin, Listerine, hydrogen peroxide,
ammonia, and bleach. Once we had growth on the plates, we measured the zones of inhibition in
mm for each substance listed above.
Another important aspect of this experiment was whether or not the bacteria was gram
positive or gram negative. If the bacteria is gram positive, it means that the cell wall has no outer
layer. It is composed of a large layer of peptidoglycan. In order from most external to most
internal it would be peptidoglycan, intermembrane space, and plasma membrane. If the bacteria
is gram negative, it means that the cell wall has an outer layer. Again, in order from most
external to most internal it would be outer layer, small layer of peptidoglycan, intermembrane
space, and plasma membrane. Antibiotics target peptidoglycan. Antibiotics are a very good drug
to use because human cells do not have peptidoglycan. In other words, the antibiotics can kill the
bacterial cells without harming the human cells. However, most antibiotics will not be as
effective against gram negative bacteria because their cell wall has an extra outer layer. It is hard
for the antibiotics to penetrate the outer layer and get to the peptidoglycan. E. coli is gram
negative and S. aureus is gram positive. It is important to consider the mode of action of each
antibiotic in order to compare the gram negative and gram positive bacteria. If the mode of
action is not peptidoglycan/cell wall then it doesn’t matter whether or not the bacteria is gram
negative or positive. The mode of action is what particular structure and or cellular process the
antibiotic targets.
The mode of action for penicillin is the bacteria’s cell wall. It prevents synthesis of the
cell wall because it breaks down peptidoglycan. When the cell wall breaks down, water can rush
in and lyse the cell (Ophardt, 2003). Since, S. aureus is gram positive we would expect it to be
highly susceptible to penicillin. Inversely, we would expect E. coli to be much less affected by
penicillin. There was no zone of inhibition for the plate with E. coli and penicillin. There was a
very large zone of inhibition for the plate with S. aureus and penicillin. The E. coli was

3. considered resistant and the S. aureus was considered sensitive to penicillin. Our actual results
supported what we expected.
The mode of action for erythromycin is the 50S subunit of the ribosome. The antibiotic
binds to the 23S rRNA molecule which is a part of the 50S subunit. When that molecule binds, it
prevents new polypeptide chains from exiting the ribosome. It is expected that gram positive
bacteria will be more sensitive to the drug because they accumulate about 100 times more
erythromycin than gram negative bacteria (Ophardt, 2003). Curiously, our plates showed
approximately equal zones of inhibition for E. coli and S. aureus. Both types of bacteria were
considered sensitive to erythromycin.
The mode of action for chloramphenicol is also the ribosome. It prevents peptide bonds
from forming between amino acids (Ophardt, 2003). We would expect that both E. coli and S.
aureus would have zones of inhibition because both types of bacterial cells would not be able to
make proteins that are needed for life. Our data supported this prediction because E. coli and S.
aureus had similar zones of inhibition. Both types of bacteria were sensitive to chloramphenicol.
The mode of action of tetracycline is also the ribosome. It prevents the tRNA-amino acid
complex from binding to the ribosome. Specifically, the drug acts as an inhibitor and prevents
the codon and anticodon interaction between the complex and the ribosome (Ophardt, 2003). We
would expect tetracycline to affect both the E. coli and the S. aureus. We had approximately
equal zones of inhibition for the E. coli and the S. aureus. Both types of bacteria were sensitive
to tetracycline.
We also tested the effectiveness of antiseptics and disinfectants. For E. coli, hydrogen
peroxide was the most effective. There was a small zone of inhibition for bleach and Listerine
and no zone of inhibition for ammonia. For S. aureus, hydrogen peroxide was also the most
effective. The bleach and ammonia was slightly effective. The Listerine was the least effective.
V. Conclusion
The purpose of the experiment was accomplished. Our data showed that penicillin was
effective only against S. aureus because it is gram positive. The other antibiotic had similar
effectiveness for both types of bacteria. The disinfectants and antiseptics had similar effects on
both types of bacteria except for the ammonia. There was a small zone of inhibition for S. aureus
and no zone of inhibition for the E. coli. Our data also showed that penicillin is more effective in
gram positive bacteria because it targets the cell wall. Since gram positive cell walls are made
from peptidoglycan and have no outer membrane, it is more easily destroyed by penicillin. We
had reasonable results and we would not change anything in the procedure.
VI. Works Cited
1. Ophardt, Charles. "Antibiotic-Penicillin." Virtual ChemBook. Elmhurst College, 1 Jan. 2003.
Web. 6 Oct. 2014. <exphttp://www.elmhurst.edu/~chm/vchembook/652penicillin.html>.
2. Ophardt, Charles. "Other Antibiotics." Virtual ChemBook. Elmhurst College, 1 Jan. 2003.
Web. 6 Oct. 2014. <http://www.elmhurst.edu/~chm/vchembook/654antibiotics.html>.
VII. Questions

4. 1. Yes, the E. coli had some growth within the bleach zone of inhibition. It is possible because
sometimes the liquid does not evenly diffuse. There probably wasn’t any or a low concentration
of bleach in that spot because it didn’t diffuse there properly.
2. The sensitivity and growth rate of the microorganism can also affect the size of the zone of
inhibition.
3. cell wall formation, ribosome 50S subunit (polypeptide can’t exit), and inhibition preventing
the codon on the tRNA from interacting with the anticodon of the ribosome.
4. No, penicillin is not effective for gram negative bacteria because it targets the cell wall. It will
not penetrate the outer layer of gram negative bacteria. It will destroy the peptidoglycan layer
and therefore destroy the cell wall of gram positive bacteria.