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Antibiotic Sensitivity Lab Report

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Antibiotic Sensitivity Experiment
INTRODUCTION
Antibiotics are substances which are synthesized by one organism and exhibit toxicity to other organisms. The substance is said to have selective toxicity. This is the basis of differentiating substances which are useful and those which are harmful. Antibiotics which inhibit the growth of pathogenic organisms without causing harmful effects have therapeutic and economic benefits. A proposal by suggests that a selective inhibitor reacts with a substance from a specific sensitive organism. There is also a possibility that antibiotic inhibits a reaction which is atypical to the parasite.
Broad-spectrum antibiotics attack a wide range of bacteria. They are powerful and flexible when dealing with bacterial infections. A good example is Ampicillin which has the ability to destroy Gram-negative and Gram-positive bacteria. On the other hand, narrow-spectrum antibiotics attack specific bacteria. They can be used to prevent the treatment form damaging useful bacteria in the body. These antibiotics are less likely to cause re-infection to a patient with a resistant bacterium.
MATERIALS AND METHODS
This Kirby-Bauer test experiment uses Muller-Hinton medium which is high in protein. The experiment aims at determining the sensitivity of different bacteria species like S. epidermidis to different types of antibiotics.
Materials. The experiment needs two Mueller-Hinton agar plates, 24-hour cultures (S. epidermidis).

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Other materials needed are ethanol, sterile swabs, antibiotics (penicillin, novobiocin, and gentamicin), ethanol and forceps. A demonstration chart is also needed for interpreting antibiotic susceptibility.
Procedure
1. The Mueller-Hinton plate with bacteria is swabbed followed by dipping the sterile swab into the broth.
2. Moisture was reduced by pressing the swab against the tube wall. Thereafter, the surface of the agar was swabbed accordingly.
3. The plate was then turned 90 degrees and swabbing process repeated. The swab was run around the circumference of the plate before it was discarded.
4. Thereafter, the surface was allowed to dry for 5 minutes
5. Antibiotic discs were placed equidistant from each other. They were later removed from the antibiotic dispensers using sterile forceps. To make sure that each disk is in good contact with the agar surface, each disk was lightly touched using a sterile inoculating loop. They were then incubated at 37º C.
RESULTS AND ANALYSIS
Circular shapes of different diameters were observed in the two plates around antibiotic discs. The bacteria died in the area around the disks in different sizes.
The diameters of areas formed around Antibiotic disks P (Penicillin), GM (Gentamicin) and NB (Novobiocin) in S. epidermidis medium were given as;
Diameter of circle formed around antibiotic P = 3 mm, GM= 14 mm, NB= 23mm

Fig.1. Mueller-Hinton agar plate with S. epidermidis strain and three antibiotic discs
The interpretation was done using the disk diffusion zone diameter chart. The antibiotics provided circle diameters. This allowed the characterization of antibiotics sensitivity as sensitive, intermediate and resistant.
DISCUSSION AND CONCLUSION
The size of circular areas or antibiogram were different for the two antibiotics. The clear areas surrounding the disks represent zones of inhibition. Here substances with antimicrobial activity have diffused. The presence of these inhibition zones shows the effectiveness of the antibiotic. The diameters of zone of inhibition were measured with a ruler. S. epidermidis has shown resistance to penicillin, therefore, it cannot be controlled by the antibiotic. In the second disc. It has shown intermediate susceptibility to gentamicin. The bacteria can be used to control the bacteria but with reduced efficiency. S. epidermidis shows significant sensitivity to novobiocin. This makes the antibiotic effective in controlling the Gram-positive bacterium.
Works Cited
Gale, E. F. “The nature of the selective toxicity of antibiotics.” British medical bulletin 16.1 (1960): 11-15.

Appendix
Appendix A
Interpretation table for zones of inhibition in mm for Kirby-Bauer antibiotics susceptibility test

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