Lab Report On Testing An Aerobe Or Facultative Anaerobes Sensitivity To Different Antibiotics


Recently, Dr. Menard introduced the class to the Kirby Bauer method. In this lab we worked in groups of 2 and we tested 6 different antibiotics with one bacteria. However, 2 bacteria were used in total with some students using the latter bacteria. The bacteria my partner and I used was named Proteus mirabilis while the other bacteria used by students was named Staphylococcus aureus. These two bacteria are excellent to become familiar with because these are two common bacteria that cause infection. The first of these bacteria, P. Mirabilis, is often found in the human gastrointestinal tract. However, if this bacteria does find its way into a urinal tract it commonly causes problems such as UTIs and kidney stones, that’s a lot of damage! Less commonly it can also be found in the lungs and cause problems as pneumonia. P. Mirabilis is a gram negative bacteria and also a facultative anaerobe, meaning that it can grow with or without oxygen but if oxygen is present it will use it. It also has the ability to elongate itself and moves in swarms.

Similarly, S. Aureus is another common bacteria. Some would even call it the most dangerous of all the common staphylococcal bacteria. This bacteria has been known to cause skin infections but has also been reported to cause pneumonia, heart valve infections, and bone infections. Opposite of the previous bacteria S. Aureus is a gram positive bacteria, and is a cocci shape but does share the fact that it is a facultative anaerobe. They are found in 30% of healthy adults and about 20% on the skin of healthy adults. These are commonly spread from direct contact to a carrier, with common carriers being hospital patients and hospital workers, so always take proper precautions at the hospital and be careful who you shake hands with!

After learning what bacteria we would use my lab partner and I began the Kirby Bauer method. The Kirby Bauer method, also known as the disk diffusion test, is a 5 step process and was a common method used for many years. Although it was developed back in the 1950s, it was refined by W. Kirby and A. Bauer and became the standard in 1961 by the world health organizations. In modern day, the KB method has become a bit dated thanks to automated tests being more standardized in clinical labs. However, it is still used for some bacteria which is why I it is still relevant and helpful to know this method for the bacteria that automated tests do not work properly with. The test is to determine the sensitivity of aerobes and facultative anaerobes to certain antibiotics. The reason it’s helpful is because this can help clinicians decide the best form of treatment for patients with bacterial infections. How it works is a bacteria is added to a growth plate media and spread over the comets surface. Next white disks of antibiotics are added and after incubating the plate at a temperature of 37° C you view the results of the test by measuring the clear area around a disk where no bacteria grows, more commonly known as the zone of inhibition, around the disks. The presence or absence of growth around the disk will reveal the sensitivity of the drug to the bacteria.


After the Kirby Bauer method Lab class I feel as though I’ve walked away with a great amount of helpful information that will me along my medical career. Before even starting Dr. Menard walked us through the steps carefully to avoid any confusion and explained where to find the name of our bacteria. He also informed us on what pages in our lab manual to read through to find the steps of the process. The steps me and my lab partner took are discussed as followed.

First we labeled our plates with our initials, date, initials of the bacteria, and incubation time the disc’s were also labeled to differentiate the antibiotics from one another. We then dipped a cotton swab into the culture broth to spread on the agar plate listed in the materials earlier. Dr. Menard dispensed the disks into the plate for us so there would be no errors in the process and from there we just taped it and left it on the incubation plate where Dr. Menard handled the rest of the process. The lab was very simple and I never felt confused during any of the steps.

The next lab we just recorded the results and entered them into the table above. All in all it was a fun experience and I’m glad to have learned the Kirby Bauer method. The results of the test for my partner and I’s bacteria, P. Mirabilis, are that it is sensitive to the following antibiotics: Ciprofloxacin, Streptomycin, Chloramphenicol, and Trimethoprim. It is resistant to the following: Penicillin G, and Tetracycline. The results for the other bacteria, S. Aureus, are that it is sensitive to the following antibiotics: Penicillin G, Streptomycin, Tetracycline, Chloramphenicol, Trimethoprim, and Ciprofloxacin. This bacteria had no resistance to any of these antibiotics. What does this tell me? It shows me that if I were to have a patient with a UTI or any other infection caused by P. Mirabilis the drugs it’s sensitive to are the antibiotics that will be best to treat it and not give the patient an antibiotic that would have no effect. On the other hand if a patient were to have pneumonia I would want to use an antibiotic that is effective on both of these bacteria since both are able to cause this that way I won’t need to guess which bacteria is causing the problem and would be able to stop the infection. This would be most helpful for a nurse practitioner so they don’t give a patient the wrong drug to treat their infection. It’s interesting to see that S. Aureus did not have any resistance to these antibiotics and teaches me that there is a wide variety of antibiotics to treat this kind of infection and not just variants of penicillin as I had originally thought. I also know now that if someone has pneumonia penicillin may not be the best antibiotic to use because it could be caused by P. Mirabilis which is resistant to the drug. This is something I’d like to read more about and I’m curious to see if in the past this was a common mistake to misdiagnose the proper drug. I will definitely look back at what I learned during this lab in the future if I ever need to prescribe a drug to a patient.

Thanks to this lab I’ve also become aware of two bacteria that are commonly causing noscomial infections and can take the proper measures to avoid these bacterial infections for patients and myself. However if I ever were to see symptoms of there bacteria I will know the proper techniques to test the cause and will be well prepared for the results because I’ve researched these bacteria for the report and have become more familiar with them. This will also help me with the unknown project because I will know what to look for with these bacteria.

18 March 2020
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