In this exercise, the Kirby-Bauer diffusion test was used to test the sensitivity of S. epidermidis to penicillin, novobiocin, and gentamicin. What results would you predict if the sensitivity of E. coli was tested. Explain your answer.

Penicillin:

Staphylococcus epidermidis is more sensitive to Penicillin as compared to E. coli

Novobiocin:

Staphylococcus epidermidis is more sensitive to Novobiocin as compared to E. coli.

Gentamicin:

E. coli isolates showed high rates of resistance to gentamicin as compared to Staphylococcus epidermidis

Explanation:

Kirby-Bauer diffusion test:

This test is also known as disk diffusion antibiotic sensitivity testing. This test uses antibiotic containing disks to test its inhibition against particular bacteria.

Staphylococcus epidermidis:

Gram-positive bacterium, which is part of the normal human flora and found mostly in skin flora but less commonly in mucosal flora.

E. coli

Gram-negative bacteria because its cell wall have an additional layer. During the staining process, it stains negative as compared to gram positive bacteria.

Penicillin:

This antibiotic kills bacteria by inhibiting the final step in cell wall biosynthesis, hence penicillin is most effective against gram-positive bacteria. Hence Staphylococcus epidermidis is more sensitive to Penicillin as compared to E. coli

Novobiocin:

Antibiotics which inhibits bacterial DNA synthesis. Its targets are the bacterial DNA gyrase and associated enzyme DNA topoisomerase. Due to extra membrane present outside gram negative bacteria this antibiotic is less effective against E. coli. Hence Staphylococcus epidermidis is more sensitive to Novobiocin as compared to E. coli.

Gentamicin:

Gentamicin is the broad spectrum antibiotic Its mode of action involves bacterial protein synthesis inhibition by binding to 30S ribosomes. E. coli isolates showed high rates of resistance to gentamicin as compared to Staphylococcus epidermidis