Effects of Different Antibiotics on Bacteria

Because of their small size, rapid rate of reproduction, and relatively simple structure, bacteria are excellent subjects for laboratory studies. This investigation will introduce you to techniques of microbiology that will be of use to you throughout your biological work.


  1. Heat the test tubes of sterile agar medium in the water bath until the agar melts.
  2. Remove the test tubes from the water bath. Let them cool enough to hold in your hand, but not so much that the agar becomes solid again. Perform the following transfer as quickly as possible. You must work rapidly so that the liquid agar will not cool and solidify before the transfer is completed.

  3. Hold both a test tube of warm agar medium and the test tube containing the culture of bacteria in your left hand. Remove the cotton plugs and pass the mouth of each test tube through the burner flame twice. Flame the loop and cool it.

    Pick up a loopful of the bacterial culture and transfer it to the warm agar in the second test tube. Shake the loop in the liquid a few times and then remove the transfer loop. Flame the loop and the mouth of each test tube and replace the cotton plugs.

  4. Roll the test tube of warm agar between the palms of your hands ti mix the bacteria with the agar. Remove the cotton plug and flame the mouth of the test tube. Lift the cover of the sterile petri dish at an angle of slightly more than 45 degrees. Keep the lifted cover directly over the bottom half to the petri dish to avoid contamination as much as possible. Pour the agar into the bottom half of the dish. Be certain that the mouth of the test tube does not touch either half of the petri dish. Remove the test tube and lower the cover.

    Move the covered petri dish gently along the table top in a figure-eight pattern to distribute the agar evenly. Allow the agar to cool until it becomes firm.

  5. Remove the forceps from the 70% alcohol and pass the tips very quickly through the burner flame. Use the forceps to remove the penicillin disks from the container.

    Try to avoid contaminating the remaining disks with the forceps. Place the penicillin disks on one side of the agar medium by barely raising the top of the petri dish. Then rapidly pass the forceps through the flame again and pick up one of the aureomycin disks.

    Place this disk on the other side of the agar surface. Flame the forceps and replace them in the alcohol.

  6. After a few minutes turn the dish upside down. Let it stand at room temperature, or in an incubator, to allow the bacteria to grow.

    Observe the dish the next day and on several following days. Describe the color and shape of any bacterial colonies and other features you observe.

Data and Observations

While this picture that I have included on the back of this report may not be very clear, it shows the diameter of the lack of bacteria surrounding the different antibiotics.With the Erythromycin, there was 4.3 cm of a complete lack of bacteria around it. With the Streptomycin, there was 2.5 cm of a complete lack of bacteria around it. Finally, the Penicillin had 1.

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8 cm of a faint amount of bacteria, but still had traces unlike the Erythromycin and Streptomycin.


The Erythromycin had the highest diameter of complete lack of bacteria around it, followed by Streptomycin, and lastly, the Penicillin. The Erythromycin obviously was able to fight against the bacteria the best, which could be because of a variety of different reasons.

One of them is that possibly the bacteria had built up more of an immunity against the Streptomycin and Penicillin because it had already fought against it before. Also, quite simply, the Erythromycin may have worked the best against the bacteria because it had the best particular skill set to fight against it. Analysis Questions

  1. I do think that more than one species of bacteria is present in my petri dish, simply because if there wasn’t more than one species, the Penicillin would have completely wiped out the bacteria surrounding it. Since there was slight traces of bacteria still around the Penicillin, I’ve come to the conclusion that there must be more than one species of bacteria.
  2. The areas surrounding the Erythromycin and Streptomycin are completely free of bacterial colonies.

    The Erythromycin has a diameter of 4.3 cm that is free of bacterial colonies. The Streptomycin has a diameter of 2.

    5 that is free of bacterial colonies. This is because Erythromycin and Streptomycin are antibodies that are designed to completely wipe out the surrounding bacteria. In a sense, they were just doing their jo

  3. The clear area around the Erythromycin is 2.5 cm larger than the clear area around the Penicillin, because the area around the Erythromycin is 4.3 cm, while the area around the Penicillin is 1.8 cm. It can even be argued that the Penicillin has no clear area at all, because there was still traces of bacterial colonies around the Penicillin.

  4. One area might be larger than the other because the bacteria may have built up an immunity against the other antibiotics, because it may have already faced that particular antibiotic. Also, one antibiotic may have worked the best against the bacteria because it simply had the best particular skill set to fight against it.
  5. These discs may be deemed useful in medical research because they can be tested to check if they can fight against certain bacteria that can be harmful to us humans. The same experiment that we just did can be conducted with other diseases to see if the antibiotics can fight and kill diseases that can be potentially harmful.
  6. After examining the clear area around each disc, I have only found a colony of bacteria in the area surrounding the Penicillin, as a matter of fact, I have discovered more than one colony. To account for it in further detail, a microscope can be used.
  7. The mutant bacteria were not noticed until after the experiment because until the experiment was conducted, they blended in with the other bacteria, after the experiment was conducted, it became quite obvious that they were present just by looking at the petri disc.

    They appear in the clear area because they are resistant to the antibiotics.

  8. You can determine whether this colony may have developed because the antibiotic lost its potency by conducting the same experiment, on the already used bacteria, with an entirely new antibiotic, but the same type. If the colony still develops, it is obvious that it was not because the antibiotic lost its potency.
  9. The experiment that can be done to determine whether the strain resistant to one antibiotic is resistant to the other, is to use one antibiotic on the strain, and to determine from your results, and conduct the same experiment with the other antibiotic, with the exact same type of strain, but not the exact one, as the prior experiment could effect your results.
  10. I would explain that Penicillin and Erythromycin are completely different types of antibacterial, and that they work differently against different types of bacteria. 11.

    An experiment that would be a logical attempt at producing a strain of bacteria resistant to both antibiotics would have to be an experiment where a type of bacteria is exposed to both antibiotics repeatedly until it builds up some sort of immunity.


I have concluded that after my experiment, I have found that the Erythromycin works the best against this particular bacteria. While the Streptomycin worked well against the bacteria, it just didn’t work as well as the Erythromycin. I would also like to say that the Penicillin was easily the worst working antibiotic against this particular bacteria as it didn’t even fully get rid of the surrounding bacteria.

Sources of Error

There are many sources of error in this particular experiment, for example one mistake in the placement of the discs in the petri disc could have effected your results. Also, the measurements that I took could have easily been slightly off as my eye may not have successfully gotten the absolutely correct diameter of the clear area around the different discs.


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