Bacterial Structure, Classification and Growth Quiz

Option C is correct. Selective toxicity means harming a microorganism more than its human host. Peptidoglycan, a mesh of sugar chains joined by short peptide chains, strengthens many bacterial cell walls but is absent from human cells. Blocking its construction therefore attacks a bacterial structure without blocking an equivalent wall-building process in the patient. Beta-lactam antibiotics, including penicillins, interfere with the enzymes that join the peptide links. This selectivity explains a useful drug target, but it does not mean these antibiotics are free of adverse effects or effective against bacteria that lack a peptidoglycan wall.

The mitochondrial wall proposed in E does not exist: human mitochondria have no peptidoglycan for these drugs to attack. Nor is the bacterial wall hidden in a nucleus (D); it lies outside the plasma membrane, and bacteria lack membrane-bound nuclei. A mistakes selectivity for universality. Mycoplasma, for example, lacks peptidoglycan, yet the absence of this target from human cells still explains selective toxicity when a bacterium does possess it. The beta-lactamases in B are drug-breaking enzymes, so they inactivate beta-lactam antibiotics and produce resistance rather than activate treatment.