EFFECT OF ANTIBIOTICS ON THE GROWTH OF Staphylococcus epidermidis

EFFECT OF ANTIBIOTICS ON THE GROWTH OF Staphylococcus epidermidis Grace Schofield (Year 10) Science Faculty, The Illawarra Grammar School, Western Avenue, Mangerton, 2500

Abstract

In this experiment antibiotic disks consisting of tetracycline, penicillin, ampicillin, chloramphenicol, streptomycin and sulphatriad, were placed on agar plates spread with the bacteria Staphylococcus epidermidis, to determine the effectiveness of antibiotics proven through the zone of inhibition. S. epidermidis was spread on an agar plate and an antibiotic disk was placed on top. To ensure reliability and sanitation, tools used tweezers and inoculation loops were heated with a bunsen burner to rid of any unwanted bacterias. This was repeated another 2 times to ensure reliability. Agar plates were put in an incubator set at 30 degrees for 2 days. The diameter of the zone of inhibition was measured and results concluded which antibiotic was most effective against the growth of S. epidermidis. It was hypothesized that tetracycline, ampicillin, chloramphenicol and penicillin would have larger zones of inhibition as they fight against gram-positive bacteria. Results supported the hypothesis as tetracycline had the largest zone of inhibition (3.9cm) however, results did not support the hypothesis for penicillin as it was essentially non-effective with a zone of inhibition of 0.1cm.

Introduction

Bacteria are a small single celled organism found in all types of ecosystems and the human body. Pathogens and parasites are bacteria that cause diseases however, pathogens are very rare, whereas other bacterias are very helpful to the ecosystem and human body, such as bacterias found in a human gut. Staphylococcus epidermidis is a gram positive non-pathogenic bacteria, it is part of human flora typically skin flora, it is a facultative anaerobic bacteria. Bacteria that produces adenosine triphosphate (ATP) by anaerobic respiration if exposed to oxygen however, S. epidermidis is capable of switching to fermentation if oxygen is absent. The zone of inhibition is an area of media where bacteria colonies are unable to grow. The zone of inhibition is measured to determine the effectiveness of antibiotics against bacteria. Ampicillin, an extended spectrum of penicillin, and tetracycline antibiotics, are active against both gram-positive and gramnegative bacteria therefore, will be effective against S. epidermidis. Chloramphenicol is an antibiotic with a broad spectrum of activity against gram-positive, gramnegative and Rickettsia, hence effective against S. epidermidis. Penicillin works best on gram-positive bacteria through inhibiting peptidoglycan production, which is found in most bacterial cell walls including S. epidermidis. Penicillin fights against peptidoglycan making it effective against S. epidermidis. Streptomycin commonly fights against gram-negative bacteria however, also fights off a small group of bacteria including the staphylococci group, streptomycin is potentially effective against S. epidermidis. Sulphatriad fights against gram-negative bacterias and essentially cannot fight against gram-positive bacteria,

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consequently sulphatriad is non-effective against S. epidermidis. The aim of the experiment was to determine which antibiotic is most effective against the bacteria, Staphylococcus epidermidis. It is hypothesized that the more effective antibiotics, ampicillin, tetracycline, chloramphenicol and penicillin will have a larger zone of inhibition against S. epidermidis.

Method

Heat up the inoculation loop on the blue flame of a Bunsen burner. This will remove unwanted bacteria. Gather bacteria with pipette and put three drops of S. epidermidis on agar plate. Spread bacteria with inoculation loop until evenly and thinly spread out and immediately shut agar plate lid. Heat tweezers with the bunsen burner and let cool making sure the tweezers do not touch any surfaces. Once cool, pick up mastring antibiotics and gently place on agar plate. Gently press down mastring antibiotics with tweezers to ensure contact with agar plate. Immediately shut the agar plate lid and tape edges with masking tape. Repeat another 2 times to ensure reliability. Place agar plates in an incubator at 30 degrees for 2 days.

Results

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Discussion

Results proved that tetracycline was the most effective antibiotic against Staphylococcus epidermidis. This was proven by the average zone of inhibition of 3.9cm, followed by chloramphenicol (2.8cm), ampicillin (2.5cm), streptomycin (2.1cm), penicillin (0.1cm) and sulphatriad (0cm). Tetracycline was the most effective antibiotic due to its ability to fight against both gram-positive and gram-negative bacterias proven through extensive research. S. epidermidis is a gram-positive bacteria further proving why tetracycline was the most effective antibiotic against the bacteria S. epidermidis. The hypothesis was somewhat supported as penicillin was the second least effective antibiotic against S. epidermidis when it was predicted to be one of the most effective, however, the results supported the hypothesis as tetracycline, ampicillin, and chloramphenicol were the top most effective antibiotics against the bacteria. An answer to the aim was provided as results proved which antibiotic (tetracycline) was most effective against the bacteria S. epidermidis. Variables were well controlled as the incubator was kept at 30 degrees for 2 days, the same antibiotics disks were used for all 3 agar plates and the same bacteria (S. epidermidis) and amount (3 drops from pipette) was also used for all 3 agar plates. The control was an agar plate with just S. epidermidis spread on it, this plate was also kept at 30 degrees for 2 days in the incubator and proved that the zone of inhibition was due to the antibiotics proving that no other factors contributed to the inhibition. The experiment was reliable as it was successfully conducted and repeated 3 times for each antibiotic with no faults in the method resulting in consistent quantitative results of the zone of inhibition. All antibiotics averaged with a difference of 2mm at the largest however, the largest difference in results was for penicillin with one agar plate the zone of inhibition (0.4cm) differing to the other two agar plates that had a zone of inhibition of 0cm. To further evaluate the effectiveness of antibiotics against the growth of S. epidermidis, specifically penicillin, extensive research and an experiment on the concentration of penicillin could be conducted. An experiment will determine if the concentration of the penicillin had an effect on the bacteria. Research proved penicillin would fight against S. epidermidis as it is gram-positive; however, results showed lack of effectiveness against the bacteria (average zone of inhibition 0.1cm).

Conclusion

In conclusion, tetracycline was the most effective antibiotic proved through research as tetracycline fights against gram-positive and gram-negative bacterias. Research also concluded the bacteria S. epidermidis is a gram-positive bacteria, therefore, explaining why tetracycline was most effective against S. epidermidis. The hypothesis was somewhat supported as tetracycline was the most effective antibiotic (average zone of inhibition 3.9cm) however, penicillin was essentially non-effective against the bacteria (average zone of inhibition 0.4cm) indicating there was some kind of resistance against the bacteria.

References

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