Effects of Antibiotics on Bacterial Growth

This experiment investigates the effect of different antibiotics on the growth of bacteria and the effect of different concentration of penicillin on the growth of staphylococcus aureus. The experiment aims to determine the best antibiotic against staphylococcus aureus and the antibiotic that is effective against a certain strain of bacteria. The experiment should be carried out under aseptic conditions from the start because contamination of the culture or the multidisc can affect the reliability of the results.

RUNNING HEAD: EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 1
Effects of Antibiotics on Bacterial growth
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EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 2
Effects of Antibiotics on Bacterial growth
AIM
The aim of this experiment is to investigate the effect of different antibiotics on the
growth of bacteria and the effect of different concentration of penicillin on the growth of
staphylococcus aureus.
INTRODUCTION
Antimicrobial drugs can basically be defined as those chemotherapeutic substances that
usually hinder the growth of different microorganisms that might be different in different tissues.
This substances differ from antiseptics as well as disinfectants in that they have a high
therapeutic index. This means that they have a high toxicity to the microorganism than the tissue
(Kong, Yu, Zhang, Li, & Wang, 2017). This antibiotics also possess a very distinct mode of
antimicrobial action. They can be introduced to the body either through oral, parenteral or topical
application.
Antibiotics are usually produced by different living organisms which kill or impair the
growth of other microorganisms and some are not toxic to be used like antimicrobial
drugs .Majority of the antimicrobial drugs that are utilized in both the human and veterinary
medicine are usually antibiotics that have a high therapeutic index.
Majority of the antibiotics will hinder the growth of an array of bacteria. These are often
referred to as broad spectrum antibiotics for example tetracycline. Other antibiotics are restricted
in nature and are known as antibacterial spectrum (Pál, Papp, & Lázár, 2015) An example is the
penicillin which is usually active against Gram Positive bacteria that include Neisseria and the

EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 3
spirochetes. Streptomycin on the other hand is most active against Gram negative and the acid
fast bacteria.
There have been important techniques that have been used of late to develop antibiotics
and chemotherapy. They include drug sensitivity tests which determine the drug that is effective
against a particular strain of bacteria and the streak tests that determines the antibacterial
spectrum (Urish et al., 2016).Finally the quantitative antibiotic assays which determines the
quantity of an antibiotic in solution.
Method.
Take 3 Nutrient agar plate label it with bacteria name.
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Take one molten agar tube from the water bath wipe it out with the tissue.
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Add 0.2 ml of bacterium culture in to the molten agar with aseptically between a flames.
And mix it well.
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Then pour that molten agar with culture on the given N. agar plate and cover it with that agar.
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For solidifying put this plate on a table for 10-15 mins.
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Take a force sterilize it with the help of ethanol and flame.
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Take Multidisc from the box with the help of this sterile forceps and put that disc on middle of
the agar.
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EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 4
Repeat this with other two bacterial culture and nutrient agar plate.
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Incubate all plate at 28 degree Celsius for 2 days then observe a result.
Result
AP Ampicillin 25 μg
C Chloramphenicol 50 μg
CO Colistin sulphate 100 μg
K Kanamycin 30 μg
NA Nalidixic acid 30 μg
NI Nitrofurantoin 50 μg
S Streptomycin 25 μg
T Tetramycin 100 μg
Table; 1 Name of antibiotics that used and concentration of drugs
Table: 2 Result of zone of inhibition on different microorganisms as an effect of
Antibiotics.
R-1; Reading 1, R-2; Reading 2, R-3; Reading3 and Ave; Average.
Antibioti
c
E. coli S. marcescens Staphylococcus
aureus
R-
1
R-
2
R-
3
av
e
R-1 R-
2
R-
3
av
e
R-
1
R-
2
R-
3
av
e
AP 15 13 14 14 3 3 4.5 3.5 8 8 8 8
C 9 11 9 9.6 2 3 2 5.6 10 9 9 9.3
CO 1 2 1 1.3 6 6 6 6 1 1 1 1
K 9 7 8 8 5 5 5 5 9 9 8 8.6
NA 2 2.5 2 5.1 7 7 8 7.3 0 0 0 0
NI 3 2.5 3 2.8 0 0 0 0 4 3 4 3.6
S 7 6 7 6.6 3 2.5 3 2.8 7 7 7 7
T 12 12 13 12 2 2 1.5 5.5 12 11.
5
11.
5
11.
6

EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 5
DISCUSSION
The rationale of this experiment was to determine the best antibiotic against
staphylococcus aureus. The multidisc that yields the largest diameter at a low concentration is
the drug of choice. In this experiment, Ampicillin at a concentration of 25 μg resulted into a
diameter of 14 mm when using serratia marcescens microorganism. This was the largest. Other
antibiotics like Tetramycine and colistin sulphate were used at very high concentration of 100
μg. Their diameter measurements were however low at 12 mm and 1.3.This therefore means that
the antibiotic that should be prescribed for an individual who is infected with seratia marcescens
would be Ampicillin (Haaber et al., 2015). Ampicillin is known to hinder the growth of this
microorganisms by inhibiting the function of cell wall penicillin binding proteins.
For staphylococcus aureus, the disc for Tetramycin is the one that yielded the highest
diameter of 11.6 mm. Colistin sulphate was second at 9.3mm, Ampicillin was at 8 mm while
Kanamycin was 8.6mm.Despite Tetramycin being the most sensitive as per the results, it cannot
be used as the drug of choice. This is because the concentration used was very high at 100 μg.
This means that the patient would need very high doses to hinder the growth of bacteria (Harkins
et al., 2017). This therefore means that for infections with staphylococcus aureus, Ampicillin and
Kanamycin would be the best since they yielded an average diameter despite being used at less
concentrations of 25 and 30 μg respectively. The experiment should however be carried out
under sterile conditions. This is because contamination may yield unreliable results that would
eventually lead to the prescription of wrong antibiotics.

EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 6
CONCLUSION
In order to determine the antibiotic that is effective against a certain strain of bacteria, it
is essential to carry out the sensitivity test. The antibiotic that yields the highest diameter under a
very little concentration is the best to be used .The test should however be carried out under
aseptic conditions from the start because contamination of the culture or the multidisc can affect
the reliability of the results. In this test, Ampicillin is the drug of choice against staphylococcus
aureus.

EFFECTS OF ANTIBIOTICS ON BACTERIAL GROWTH 7
References
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Reversible Antibiotic Tolerance Induced in Staphylococcus aureus by Concurrent
Drug Exposure. mBio, 6(1). doi:10.1128/mbio.02268-14
Harkins, C. P., Pichon, B., Doumith, M., Parkhill, J., Westh, H., Tomasz, A., …
Holden, M. T. (2017). Methicillin-resistant Staphylococcus aureus emerged long
before the introduction of methicillin into clinical practice. Genome Biology, 18(1).
doi:10.1186/s13059-017-1252-9
Kong, H., Yu, F., Zhang, W., Li, X., & Wang, H. (2017). Molecular Epidemiology and
Antibiotic Resistance Profiles of Methicillin-Resistant Staphylococcus aureus Strains
in a Tertiary Hospital in China. Frontiers in Microbiology, 8.
doi:10.3389/fmicb.2017.00838
Pál, C., Papp, B., & Lázár, V. (2015). Collateral sensitivity of antibiotic-resistant microbes.
Trends in Microbiology, 23(7), 401-407. doi:10.1016/j.tim.2015.02.009
Urish, K. L., DeMuth, P. W., Kwan, B. W., Craft, D. W., Ma, D., Haider, H., …
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