Kirby Bauer Test – Term Paper

Antibiotics are agents used to inhibit growth of bacteria or destroy bacterial cells in different ways. This doesn’t mean that a particular antibiotic can destroy all kinds of bacterial cells. Selective toxicity refers to the effectiveness of a particular antibiotic to a particular type of bacteria with minimal harm to the human. Certain antibiotics are extremely effective against certain antibiotics while there is no harm to the human, this is measured as therapeutic index where the higher the index, the safer the drug (Kenneth T, 2012). In this lab experiment we studied the toxicity of penicillin, novobiocin and gentamicin against the bacteria Staphylococcus epidermis. The relationship between antibiotic and selective toxicity is that both are highly considered in order for the antibiotic to be used, the higher the selective toxicity level, and the better the antibiotic.

Kirby-Bauer method

The spectrum refers to the range of bacteria that are being effected by a particular antibiotic when used. Spectrum is being divided in broad-spectrum and narrow-spectrum so as to distinguish the range of bacteria/micro-organisms. Broad-spectrum antibiotics are effective against gram-positive and gram-negative bacteria, this makes it effective on a wide range of bacteria/micro-organism. The benefits of broad-spectrum antibiotics are (Hopkins S.J., 1997):

1. It can be used primarily on infections that are unidentified.

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2. It can be used for drug resistance bacteria.

3. It can be used for conditions where infection is caused by several different types of bacteria at ones. 

Narrow-spectrum antibiotics are only effective for a particular small group of bacteria which can be either gram-negative or gram-positive, its effectivity is limited due to this (Kenneth T, 2012). The advantage of narrow-spectrum antibiotic are:

1. Since it only effects a limited number of bacteria types, it will not cause antibiotic resistance.

2. Since it is very particular on the bacteria type which will be effected, it will not destroy the normal micro-organisms in the body, therefore doesn’t cause super infection.

;Materials and Methods

We completed the Kirby-Bauer Diffusion Test using the methods and materials below:

1. An agar plate was prepared.

2. Everything was cleared from work area and hand were washed thoroughly.

3. The work area was disinfected with bleach.

4. Agar plate was divided into 3 sections using a marker on the plate which were labelled as Penicillin, Gentamicin, Novobiocin.

5. A swab was flame sterilized.

6. Sterilized swab was dipped into the S.epidermis broth.

7. The broth was transferred to the agar plate.

8. The entire agar plate was swabbed.

9. After the broth was absorbed, the lid was sterilized again.

10. A tweezer was sterilized in bleach and the antibiotic disks were transferred into the labelled sections accordingly.

11. The disks was pressed so that it stick with the agar on the surface.

12. After closing the agar plate with the sterilized lid, it was incubates up-side down for 48 hourS.

13. After 48 hours the plate were checked.

14. The plate was bought to the sterilized surface and a metric ruler was used to measure the diameter of the zone of inhibition around each disk.

15. Data was recorded and the agar plate was placed in bleach before disposing.

16 Hands were washed thoroughly with soap.

Kirby-Bauer Test;


The results of the Kirby-Bauer Diffusion Test is below:


Penicillin is known to be a narrow-spectrum antibiotic that is effective against gram-positive bacteria, hence with reference to the table above, it is very effective as its target is limited but its effectiveness is high. According to this test the most effective antibiotic when compared to gentamicin and novobiocin is Penicillin as its resistant rate is 28 which is higher than the rest. The susceptible rate (;29) and intermediate rate (28) are also comparatively high for Penicillin against S.epidermis.;This reveals that its effectivity against this bacteria us very high and if we increase the dosage there will be better results for Penicillin.

Novobiocin is also a narrow-spectrum antibiotic and targets gram-positive bacteria, novobiocin is not as effective as penicillin but it does inhibit growth of the bacteria as its resistant rate according to however it can be improved with higher dosage as its intermediate range is high. ;This antibiotic does inhibit growth of S.epidermis to a moderate level, it can be better with higher dosage.

Gentamicin is the least effective against S.epidermis when compared to the other two antibiotic but it doesn;t inhibit the growth of S.epidermis, the inhibition can be improved with higher dosage. ;Since gentamicin has very little resistance (;12), it;s not very effective. Gentamicin is also a narrow spectrum antibiotic which is effective again gram-negative bacteria, this can be the reason why is it not very effective against S.epidermis since it;s gram-positive.


S. epidermis




















Based on the KB lab test (Kirby-Bauer antibiotic testing), the susceptibility of the S. epidermidis is observable from the reaction of the S. epidermidis to the Penicillin, Gentamicin and Novobiocin. For example, when the isolated S. epidermidis was grown in agar plates with different antibiotics, the S. epidermidis reacted differently in the different antibiotics. The S. epidermidis was mainly susceptible to the penicillin more than gentamicin and Novobiocin. Considering the size of the areas or the zone diameters, the S. epidermidis are more susceptible to the penicillin than the other two antibiotics because the zone diameter of ;29 indicates that the S. epidermidis is more susceptible to the penicillin while the zone diameter for resistances is ;28 and the intermediate is zero. This indicates that the penicillin can be more effective against Staphs, ;-Hemolytic Streps and enteric. ;Additionally, one can also argue that the infection due to the S. epidermidis can be effectively treated with the concentration of the penicillin if the disk potency is 10 ;g or more. The larger zone of inhibition around the S. epidermidis is penicillin indicates that the S. epidermidis is more sensitive to the penicillin.

Gentamycin has a smaller zone of inhibition estimated at ;15 in diameter which indicates that the bacteria is less susceptible. Higher doses may be recommended but not advisable. Gentamycin in higher concentration is a susceptible antibiotic against the S. epidermidis but it is important to note that there are emerging mechanisms of resistances that needs continuous caution when it comes to relying on the test method to determine the resistance of the antibiotics. In conclusion, penicillin is the most effective treatment against S. epidermidis infection followed by Novobiocin, while gentamicin treatment may require higher concentration.



Kenneth T. (2012). Online Textbook of Bacteriology.

Hopkins S.;J. (1997). Drugs and Pharmacology for Nurses (12th ed.).