Question

3. Why would an antibiotic that inhibits bacteria by binding to the ribosome not have the same effect on us?

4. Hypothetically, you need to inject ampicillin to treat a blood infection caused by E. coli. How much ampicillin, in milligrams, would you need to inject to achieve the minimum inhibitory concentration in the blood? (Assume an average body contains about 5 liters in blood and that excretion and metabolism don’t matter).

Answer 1

3. Bacteria contain 70S ribosome (50S larger subunit and 30S smaller subunit) but humans or eukaryotes contain 80S subunit (60S larger subunit and 40S smaller subunit). Though the basic ribosome structure is comparable, the constituent rRNAs and the proteins differ which cause different binding site for the antibiotics. So, the specific tRNA or the protein, the antibiotic is targeting for the 70S bacterial ribosome might not target the 80S eukaryotic ribosome as that specific antibiotic binding motif will be absent in the 80S ribosome.

4. The MIC90 (or the minimum inhibitory concentration that kills 90% of bacteria) for ampicillin is 1 μg/ml.

The average human body contain 5 liter or 5000 ml of blood.

So, the total dose of ampicillin required = (5000 * 1) = 5000 μg = 5 g.