10 Jun MBG 353 Laboratory – Antibiotic Sensitivity Testing
Question
Microbiology MBG 353 Laboratory
Part A: Antibiotic Sensitivity Testing
Measure the diameters of the zones of no growth on your plates and record the data in the charts below. Compare your results to the ‘Guide to Interpreting Antibiotic Disc Assays’ and report the sensitivity of the bacteria to each antibiotic. (6 marks)
Table 1: Antibiotic Sensitivity of Staphylococcus epidermidis to Various Antibiotics
Antibiotic Zone Diameter (mm) Sensitivity*
Ampicillin 7mm Resistant
Chloramphenicol 25mm Susceptible
Gentamycin 27mm Susceptible
Penicillin G 7mm Resistant
Polymyxin B 9mm Intermediate
Tetracycline 8mm Resistant
*R = resistant; I = intermediate; S = susceptible
Table 2: Antibiotic Sensitivity of Escherichia coli to Various Antibiotics
Antibiotic Zone Diameter (mm) Sensitivity*
Ampicillin 15mm Susceptible
Chloramphenicol 22mm Susceptible
Gentamycin 23mm Susceptible
Penicillin G 6mm Resistant
Polymyxin B 13mm Susceptible
Tetracycline 16mm Intermediate
*R = resistant; I = intermediate; S = susceptible
Part B: Evaluation of Disinfectants and Antiseptics
Measure the diameters of the zones of no growth on your plates and record the data in the chart below (6 marks)
Antimicrobial agent Diameter of Zone of Inhibition (mm)
B. thuringeinsis E. coli S. epidermidis
Sterile saline 6mm 6mm 6mm
Listerine 7mm 8mm 9mm
70% Isopropanol 10mm 10mm 10mm
Mr. Clean 15mm 7mm 6mm
3% H2O2 8mm 10mm 17mm
Lysol 14mm 9mm 21mm
Diluted bleach 15mm 19mm 15mm
Questions:
1. According to your test results, which antibiotic(s) were effective against Gram negative bacteria? (2 marks)
2. Which antibiotic(s) were effective against Gram positive bacteria? (2 marks)
3. According to your results, which antibiotic(s) can be classified as broad spectrum? Why? (2 marks)
4. In terms of ring diameter, which antimicrobial agent was the most effective and which was the least effective? (2 marks)
5. In terms of spectrum (number and kinds of bacteria) which antimicrobial agent was the most effective and which was the least effective? (2 marks)
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4. a) What is the experimental control in the antiseptic/disinfectant lab?. (2 marks)
b) Which antibacterial agent would you use to clean a kitchen counter and why?. (2 marks)
5. Which antibacterial agent (you may not select listerine) would you use to rinse your mouth and why? (2 marks)
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