Ideally, an antibiotic should focus on a microbial target not found in mammalian cells. By this standard, which of the following antibiotic agents would be expected to be most toxic to humans?

Ideally, antibiotics should attack a microbial structure or function not found in human cells. Except for mitomycin, all the antibiotics listed in the question interfere with cell-wall synthesis in bacteria. Mitomycin inhibits DNA synthesis in both mammalian and microbial systems; viral DNA synthesis, however, is relatively resistant to mitomycin.