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Bacterial Structure, Classification and Growth Quiz
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Why can bacterial peptidoglycan synthesis be targeted selectively?
- A Peptidoglycan is needed during cell division, so human cells are spared because they divide slowly You picked ✕
- B Peptidoglycan lies inside the outer membrane in Gram-positive bacteria, so it is hidden from drugs You picked ✕
- C Human cells do not make peptidoglycan, so wall synthesis can be blocked without harming them You picked ✓
- D Human cells contain peptidoglycan within lysosomes, so antibiotics cannot reach the target there You picked ✕
- E Peptidoglycan is absent from Gram-negative bacteria, so Gram-negative organisms are unaffected by these drugs You picked ✕
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Incorrect
The answer is C: Human cells do not make peptidoglycan, so wall synthesis can be blocked without harming them
Explanation
Why C is correct
See Cell Envelope Recap. Option C is correct because this is the basis of selective toxicity: a drug is useful when it harms the microbe but not the patient. Peptidoglycan is the mesh of long sugar chains, cross-linked by short peptide bridges, that gives the bacterial cell wall its strength. It is found only in bacteria, and human cells make no peptidoglycan. Antibiotics that block wall synthesis, such as the beta-lactams, therefore act on a target that human cells simply do not have. Without a strong wall, the bacterium cannot withstand its internal osmotic pressure and dies. Gram-positive bacteria have a thick peptidoglycan layer. Gram-negative bacteria have a thinner layer beneath an outer membrane that contains lipopolysaccharide (LPS), and this outer membrane can limit how well some large or hydrophobic drugs reach their target. Susceptibility therefore also depends on whether the drug can get in, and on the physiological state of the bacterium. The core idea is simple: the best antibiotic targets are structures that bacteria have and humans lack.
- Principle: Selective toxicity means a drug harms the microbe but not the patient.
- Peptidoglycan: Peptidoglycan is found only in bacteria, and human cells make none.
- Drug action: Beta-lactams block wall synthesis, so the bacterium cannot withstand osmotic pressure and dies.
Why the other options are incorrect
- A. Peptidoglycan is needed during cell division, so human cells are spared because they divide slowly: Human cells do not make peptidoglycan whether or not they are dividing, and many human cells divide frequently. Division rate is not the basis of selectivity because the target does not exist in human cells. Your answer
- B. Peptidoglycan lies inside the outer membrane in Gram-positive bacteria, so it is hidden from drugs: Gram-positive bacteria do not have an outer membrane, so peptidoglycan cannot lie inside one. In Gram-negative bacteria the layer lies beneath the outer membrane, but this statement misplaces it in Gram-positive cells. Your answer
- D. Human cells contain peptidoglycan within lysosomes, so antibiotics cannot reach the target there: Human cells do not make peptidoglycan in lysosomes or anywhere else. The selective action of these drugs is based on the absence of the target, not on an unreachable intracellular location. Your answer
- E. Peptidoglycan is absent from Gram-negative bacteria, so Gram-negative organisms are unaffected by these drugs: Both Gram-positive and Gram-negative bacteria have peptidoglycan. Gram-negative cells have a thinner layer beneath an outer membrane, and many wall-active antibiotics still affect them. Peptidoglycan is therefore not absent from Gram-negative organisms. Your answer
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