Study Questions: 1. Which antibiotic seems to be most effective in inhibiting E. aerogenes? Which antibiotic seems to be least effective? Justify your choices. 2. Which organism was penicillin most effective against? Least effective against? How can you explain this difference? 3. If the zones of inhibition of two antibiotic disks (A and B) on a Sarcina lutea dish measure 17 and 18 mm respectively, which antibiotic is more effective against Sarcina lutea. Why? 4. Do the antibiotics kill the bacteria or only inhibit the growth? Design a method to determine whether the antibiotics are bacteriostatic (inhibit) or bactericidal (kill). 5. If the antibiotic concentration is doubled, will the growth zone be twice as large? Explain. 6. A doctor is prescribing medicine for a person with a systemic E. coli infection (a systemic infection is disseminated throughout the body by the circulatory system). Which antibiotic might a doctor choose? Hint: E. coli belongs to the same class of bacteria as Enterobacter aerogenes. Dish Diagram Tetracycline (red) "Student's Name" "Date" "Organism" Penicillin (blue) Chloramphenicol (black) Organism | Chloramph. | Penicillin | Tetra. E. aerogenes | 30 | 8 | 24 B. cereus | 5 | 5 | 5 S. lutea | 5 | 5 | 5 Chart 1: Zones of Inhibition (diameter in mm)
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We need to find the most and least effective antibiotics against Enterobacter aerogenes. To do this, we will look at the zones of inhibition for each antibiotic in the table provided. Organism | Chloramphenicol | Penicillin | Tetracycline --- | --- | --- | Show more…
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QUESTIONS: 1. Which two antibiotics were the most effective overall? What are their mechanisms of action? Amoxicillin as well as Tetracycline were most effective overall. Tetracycline's mechanism of action is that it ultimately inhibits protein synthesis, leading to a bacteriostatic effect. Amoxicillin's mechanism of action is unknown. 2. Which two were the least effective overall? What are their mechanisms of action? 3. Why would one antibiotic be effective against a Gram-positive cell but not a Gram-negative? Give an example from the results. 4. Would the spread plate technique or pour plate be best to seed the lawn of bacteria? Why?
Dominador T.
Suppose you do this test on a hypothetical Staphylococcus species with the antibiotics Oxacillin (OX) and clindamycin (CC). You record zone diameters of 14 mm for both the oxacillin and the clindamycin disks. Refer to the interpretation table above. a. Which antibiotic would be more effective against this organism? b. What does this tell you about comparing zone diameters to each other and the importance of a zone diameter interpretive table? 6. Why are Gram-negative bacteria resistant to certain antibiotics as compared to certain Gram-positive bacteria? 7. What accounts for the confluent growth of the microorganism in the MHA plate impregnated with antibiotic disks. 8. The antibiotic susceptibility testing result of Staphylococcus aureus showed growth of bacteria around the following antibiotic disks: penicillin, ampicillin, tetracycline, and chloramphenicol. What is the interpretation of this result? 9. Discuss the zone of inhibition and the minimum inhibitory concentration. Are they the same or different? 10. Why is Mueller-Hinton agar the most preferred for antibiotic susceptibility testing?
Adi S.
Zone measurement standards for specific antibiotics to determine Zones of Inhibitions. Antibiotic Sensitive Resistant Penicillin > 29mm < 28mm Tetracycline > 12mm < 9mm Streptomycin > 19mm < 14mm Chloramphenicol > 18mm < 12mm Gentamicin > 22mm < 15mm 37. Refer to the above chart. Patient B is in the burn unit and has an infection caused by Pseudomonas. The Antimicrobial Susceptibility Test determined a zone of 11 mm for Chloramphenicol, 25 mm for Gentamicin, 20mm for Penicillin, and 0mm for Tetracycline. Which Antibiotic would be the best to treat this patient? a. Gentamicin b. Penicillin c. Streptomycin d. Chloramphenicol e. None of the above antibiotics 38. Refer to the above chart. Patient S is in the hospital with a gastrointestinal infection caused by Escherichia Coli. The Antimicrobial Susceptibility Test determined a zone of 28mm for Penicillin, 20mm for Streptomycin, 12mm for Chloramphenicol, and 15mm for Gentamicin. The data concludes _______ would be best to treat this patient? a. Penicillin b. Streptomycin c. Chloramphenicol d. None of the above antibiotics e. All of the above antibiotics 39. Refer to the chart above. Which of the infections would be difficult to treat if the zone of inhibition treated with Tetracycline was a. Salmonellosis 9 mm b. Staph infection 6 mm c. Urinary Tract Infections 0 mm d. All of the above would be difficult to treat with Tetracycline e. All of the above would be suitable to treat with Tetracycline 40. Outline the first steps in identifying your unknown bacteria.
Madhur L.
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