This experiment was performed to determine which antibiotic provides the largest zone of inhibition and to determine which antibiotic is more effective to inhibit or completely kill the bacteria successfully. Eight antibiotics (penicillin, cefotaxime, chloramphenicol, erythromycin, SxT, nalidixic acid, polymyxin B, and tetracycline) were used to determine which would inhibit growth of the pure cultures of Staphylococcus epidermidis and Escherichia coli. We measured the zone of inhibition and concluded that Erythromycin was considered resistant because it had the largest zone of inhibition (31mm) for Staphylococcus epidermidis and Cefotaxime was also considered resistant with the second largest (30mm) for Escherichia coli. Behind those followed SxT for Escherichia coli with an inhibition of 29mm which was also resistant.
Introduction
When microorganisms cause disease in humans and animals, the infection is treated with the chemotherapeutic agents called antimicrobial drugs (1). There are about 73,000 cases of E. coli per year just in the United States alone with Staphylococcus epidermidis eliminating that number with 1.2 million cases per year. We compared the effectiveness of the eight antibiotics used on Staphylococcus epidermidis and Escherichia coli in order to determine how well they would inhibit the growth or decrease the amount of bacteria present. The eight antibiotics used included penicillin, cefotaxime, chloramphenicol, erythromycin, SxT, nalidixic acid, polymyxin B, and tetracycline. Considering we only tested two bacteria that are the most common, it was easily determined which antibiotics successfully inhibited growth.
S. epidermidis creates a biofilm. This allows them to adhere to various surfaces. These biofilms make S. epidermidis strains and some other biofilm forming organisms begin to encase themselves. This makes it impossible for the immune system or any antimicrobial agents, to destroy the biofilm that may be associated with the infection (2).
Penicillin is used for many different infections including ear infections, scarlet fever, and respiratory infections such as pneumonia. It also prevents peptidoglycan from cross linking in the last stages of the cell wall synthesis. This will eventually cause the bacterium to lyse and die due to the osmotic pressure (4).
Cefotaxime is mostly used for preventing people from having any infections during or after surgery such as bacterial infections in the blood. It can also help with gonorrhea. Cefotaxime works by stopping the ability of bacteria to form cell walls. It keeps the bonds from holding the bacterial cell wall together (3).
Chloramphenicol is a broad-spectrum agent that is used for severe life-threatening infections. It works by stopping the bacteria from producing proteins that are important to them. The antibiotic will take charge of the infection and the remaining bacteria will be killed by the body’s immune system (5).
Erythromycin is a broad-spectrum agent which means it can be used to treat a number of infections including any bacterial infections that may occur after burns, surgery, or trauma. It is also useful in treating acne. It is mostly taken by mouth. Just like Chloramphenicol, it stops the bacteria from producing proteins which stops them from growing and replicating (6).
SxT is known for treating urinary tract infections and it is suitable for adults and children. It is also suitable to treat chest infections like bronchitis or pneumonia. The antibiotic works in a way where it stops a substance called folate and without that, the DNA from bacteria cannot be produced which does not allow to grow and multiply. It disrupts the production of dihydrofolic acid while trimethoprim disrupts the production of tetrahydrofolic acid (9).
Nalidixic Acid is very similar to SxT in the way that it is also used for urinary tract infections and it does the same thing to the bacteria. One thing that is different is that you are not allowed to take any medicines that contain calcium, iron, or zinc as it can mess up the effects of the medicine. You are also told to not drink milk as well a couple hours before taking the medication (10).
For Polymyxin B, it is actually from the strain of Bacillus polymexa. It is a mixture of Polymyxins B1 and B2. It can be used as an injection or topically. It is considered toxic if taken in high doses, so caution must be taken. It is used to treat pink eye, so it is in many medications (11).
Now for Tetracycline, many young adults use this antibiotic without realizing because it is known to treat acne. The antibiotic stops the production of proteins of the bacteria which stops the growth and reproduction. This antibiotic is also common in treating chest infections.
Methods
Broths with Staphylococcus Epidermidis and Escherichia Coli were spread on the MHA plates (2 plates/bacteria) and incubated overnight at 37oC. The following day, filter paper disks of each antibiotic (penicillin, cefotaxime, chloramphenicol, erythromycin, SxT, nalidixic acid, polymyxin B, and tetracycline) were placed precisely in the middle of the inoculated plates. Sterilized metal forceps dipped in alcohol and flamed were used to eliminate any cross contamination that could occur. After each antibiotic was used for each plate, it was then incubated aerobically at 37ºC for 24 hours (7).
The following day, measurements (mm) of each zone of inhibition took place for the antibiotics . The resistance was also checked with the Antimicrobial Sensitivity Evaluation table in Appendix C in the West Texas A&M University Microbiology Lab Manual (13).
Results
This experiment took place in order to compare and analyze how well each antibiotic reacts with Staphylococcus epidermidis and Esechieria coli. Tests were conducted to observer how effective each antibiotic is in inhibiting the growth or completely killing the bacteria used.
Table 1 shows each zone of inhibition measured. Compared to the others, Erythromycin, Cefotaxime, and Chloramphenicol were the antibiotics with the largest zone of inhibition for Staphylococcus Epidermidis. For Escherichia Coli, Cefotaxime, Chloramphenicol, and SxT had the largest zone of inhibition and were considered sensitive according to the Antimicrobial Sensitivity Evaluation table along with Erythromycin, Cefotaxime, and Chloramphenicol being considered sensitive as well for Staphylococcus Epidermidis (13).
These results were expected from the successful antibiotics because they are broad spectrum agents which cover a wide range of bacteria. Penicillin is not a broad spectrum and it performed poorly for E. Coli and Poly B. also performed poorly for Staph. Epi. considering it is a topical antibiotic. Chloramphenicol is considered potentially toxic so taking cautious measures is a must when prescribing this medication in high doses.
Discussion
The purpose of this experiment was to determine which antibiotics are the most effective on inhibiting the growth of the following bacteria: Staphylococcus Epidermidis and Esechieria Coli. Table 1shows the results of each antibiotic and how each of the bacteria reacted to it. Notice how the results show the three strongest antibiotics against both bacterial growths are, Cefotaxime, Chloramphenicol, and SxT. Both Staph. Epi and E. Coli were the most sensitive to those antibiotics. Throughout the year, antibiotics are usually given but there seem to be more cases of Staphylococcus Epidermidis and Esechieria Coli where it leads to resistant mutants and many of the treatments begin to fail (8).
Sources of error were noted during the experiment. Considering each antibiotic needed to be placed once onto the plate, one disk was dropped and picked back up and moved to the middle where it was intended to be. That interrupted the zone of inhibition and how well the antibiotic was supposed to work. Another source of error could be that the forceps may have not been sterilized before grabbing the antibiotic disks which possibly resulted in contamination.
This method is used out in the science field due to the amount of poorly prescribed medications. There are now many bacteria that are becoming resistant to antibiotics which is resulting in patients not being properly prescribed medications. This tested how susceptible each bacterium is to the antibiotics used. Better therapy will be available to the public once more tests are done to figure out which bacteria are becoming “superbugs.”
...(download the rest of the essay above)