5. 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?
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Suppose you do this test on a hypothetical Staphylococcus species with the antibiotics penicillin (P 10) and chloramphenicol (C 30). You record zone diameters of 25 mm for the chloramphenicol and penicillin disks. Which antibiotic would be more effective against this organism? What does this tell you about comparing zone diameters to each other and the importance of the zone diameter interpretive table?
Madhur L.
Imagine that the zone you just measured was associated with a disc containing penicillin and a seeded lawn of Staphylococcus aureus. Consult table 7-4 in your lab text and interpret this result: a. This strain of S. aureus is susceptible to penicillin b. This strain of S. aureus has intermediate susceptibility to penicillin c. This strain of S. aureus is resistant to penicillin To further reinforce the concept that zones must be measured and interpreted, imagine that you had performed a Kirby-Bauer test in which ciprofloxacin, penicillin, and tetracycline were evaluated against a new strain of Staphylococcus aureus. Interestingly, all three drugs produced identical zones of inhibition with a diameter of 16 mm. How would you interpret these results? This strain of S. aureus is/has ________________ to ciprofloxacin. a. Resistant b. Intermediate susceptibility c. Susceptible This strain of S. aureus is/has ________________ to penicillin. a. Resistant b. Intermediate susceptibility c. Susceptible his strain of S. aureus is/has ________________ to trimethoprim. a. Resistant Intermediate susceptibility b. Intermediate susceptibility c. Susceptible
Dominador T.
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. 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) Dish Diagram
Adi S.
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