Antibiotic Susceptibility Zone: Diameter Interpretation
Clinical Microbiology C453
Task 3 Antibiotic Sensitivity Lab Report- Revision
After growing different kinds of bacteria in task 2, how do we kill the bacteria if we become infected? One way of killing or slowing the growth of bacteria is by using medications called antibiotics. One classification of antibiotics are called broad spectrum, which will treat a wide range of bacteria’s such as gram positive and gram negative. The problem with this is that the antibiotic can destroy our own normal bacteria that help us fight off invaders . So its important to know the level of selective toxicity of the antibiotic. Selective toxicity is the ability to kill or harm an organism without harming other cells in the body. If the wrong antibiotic or wrong dose is given this can lead to organisms being resistant to the antibiotics and can lead to ineffective antibiotics and even death if bacteria remains growing. Another group is referred to as narrow spectrum antibiotics, which are only affective for certain types of bacteria. Narrow spectrum antibiotics do not work well if there are multiple types of bacteria involved in the infection. Since the doctors don’t use narrow spectrum meds very often there is a less resistance to this group. Resistance to antibiotics is a real problem and concern. The best way to avoid resistance is to stop taking antibiotics for everything and to finish your prescription as ordered. The goals of antibiotics are to kill the bacteria that make you sick while keeping our own good bacteria and prevent resistance. For this report we will be looking at penicillin, gentamicin and novobiocin and how they respond to the staph epidermidis bacteria.
Methods and Materials
Before starting this test, new agar plate is brought to room temperature. While the agar plate is warming up, supplies were gathered. Supplies needed for this test are the 3 antibiotics discs; personal protective equipment (gown, gloves, mask and apron) Staph epidermidis culture vial, alcohol , sterile cotton swab, tea light, lysol wipes and a marker. The workstation was wiped with Lysol wipes, hands were washed with soap and water and personal protective equipment were worn. First step is to divide the agar plate into 3 equal parts using the marker and then labeling the sections with penicillin, gentamicin, and then novobiocin. Now, its time to inoculate the agar with the staph epidermidis using a sterile swab. But first the staph epidermidis culture tube will be placed over the lit tea light for sterilization. The cap from the vial is protected from contaminates as well. Take the sterile cotton swab and place it carefully into the staph epidermidis vial without touching the rim and then remove and wipe the whole surface of the petri dish. After rubbing the cotton swab over the agar, the agar is turned about 90 degrees and repeated. Make sure all the edges are swabbed well. Place the petri dish lid on and wait for the bacteria to absorb into the agar. Place the shaph epidermidis vial over the flame and return cap to the vial. The cotton swab will be placed in undiluted bleach and then discarded at the end of procedure. Now that the bacteria are absorbed, take the little tweezers and place in alcohol to disinfect, shake remaining alcohol off and make sure tweezers are dry. Take the penicillin disc and place in the section labeled penicillin; Repeat these steps for gentamicin and then the novobiocin. After discs have been placed in appropriate sections, lid will be replaced and dish will be left in dinning room for 48hrs to incubate. The workstation was wiped down with Lysol wipes and personal protective equipment removed and a 5 min hand wash was completed. On return 48hrs later, I wiped work area down with Lysol wipes and placed my personal protective equipment on so I could examine the area around the antibiotic disc. This area around the disc is known as the zone of inhibition. The diameter around the disc with no growth is measured in millimeters (mm) with your ruler and results are placed in task 3 data table 1. Once the measurement was completed the agar dish was put in undiluted bleach for about and hour and then taken out to the garbage. Work area wiped down with Lysol wipes and 5 minute hand washing was complete.
Antibiotic Susceptibility Zone: Diameter Interpretation
Zone Diameter Standards (mm)
Microbiology Task 3 Tables
Task 3 Data Table 1
Zone of inhibition measurement
The penicillin measured 17mm in diameter with no growth, which makes penicillin resistant to the staph epidermidis bacteria. Penicillin would not be effective in killing staph epidermidis. If penicillin was used the bacteria can still grow and cause an individual to remain sick even while taking the antibiotic.
Gentamicin measured 33mm in diameter with no growth which makes this antibiotics the most effective in killing the bacteria. Gentamicin would be the best antibiotics to treat the staph epidermidis bacteria because of its susceptabity.
Novobiocin measured only 5mm in diameter. This puts novobiocin as a resistant drug against the staph epidermidis bacteria. If novobiocin was used the bacteria can still grow and cause an individual to remain sick even while taking the antibiotic.
When we talk about selective toxicity, it means the drug can kill the bacteria and not harm other bacteria around them. So the drug of choice would act on the bacteria only and leave our good immune cells to do their work. It is so important for doctors to know what kind of bacteria is present and which antibiotic is the best choice of eliminating the bacteria. This is why the Kirby-Bauer test is preformed in labs to determine which antibiotic is sensitive and which one is resistant so we can eliminate the bacteria without destroying our good bacteria.
Hands on Labs (2015) Antibiotic Sensitivity-Kirby Bauer Diffusion Test
Hands on labs, Inc Version 42-0238-00-02.
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