A 67-year-old woman with acute myeloid leukemia is on day 12 following allogeneic bone marrow transplantation. She has been receiving vancomycin and piperacillin-tazobactam for febrile neutropenia, but she remains febrile with a temperature of 38.9°C. Blood cultures drawn 3 days ago now grow Candida albicans. The transplant team discontinues the antibacterial regimen and initiates caspofungin. A medical student asks the attending physician: "Why didn't the vancomycin and piperacillin work against the Candida, and why is it safe to give caspofungin without worrying about harming the patient's own cells?"
Which of the following best explains both the failure of the antibacterial agents and the selective toxicity of caspofungin against Candida albicans?