Question
Alexander Fleming discovered penicillin in 1928. Which of the following best describes the mechanism by which penicillin kills bacteria?
(a) It disrupts the bacterial cell membrane (b) It inhibits bacterial cell wall synthesis (c) It blocks bacterial protein synthesis (d) It inhibits bacterial DNA replication
Solution — Step by Step
Penicillin belongs to the β-lactam class of antibiotics. Its target is the bacterial cell wall — specifically, it interferes with the synthesis of peptidoglycan, the structural polymer that gives bacterial cell walls their rigidity.
Bacteria maintain a high internal osmotic pressure. The peptidoglycan cell wall acts like a rigid cage that holds the cell together against this pressure. Without it, the bacterium literally bursts — a process called osmotic lysis.
Penicillin binds to and inactivates penicillin-binding proteins (PBPs), which are the enzymes responsible for cross-linking peptidoglycan chains during cell wall formation. New bacteria trying to grow and divide can’t build a proper wall, so they rupture.
The correct answer is (b) — penicillin inhibits bacterial cell wall synthesis. Options (a), (c), and (d) describe mechanisms of other antibiotic classes (polymyxins, chloramphenicol/erythromycin, and fluoroquinolones respectively).
Why This Works
Fleming noticed in 1928 that a Penicillium notatum mold had contaminated his Staphylococcus culture plates — and the bacteria around the mold were dying. What he observed was the mold secreting penicillin to kill competing bacteria, a classic example of antibiotic activity in nature.
The reason penicillin is so effective against gram-positive bacteria (like Staphylococcus and Streptococcus) is that these bacteria have a thick peptidoglycan layer as their outermost structure. Penicillin has direct access to it. Gram-negative bacteria have an outer lipid membrane shielding their thinner peptidoglycan layer, making many of them naturally resistant.
Human cells have no cell wall at all — we use a cell membrane for structure. So penicillin’s target simply doesn’t exist in our cells. This is why it’s relatively non-toxic to humans, which is the fundamental criterion for a useful antibiotic.
Alternative Method — Process of Elimination
If you blank on the mechanism in the exam hall, use what you know about other antibiotic classes to eliminate:
| Antibiotic Class | Target |
|---|---|
| Penicillin, Cephalosporins | Cell wall synthesis |
| Polymyxins | Cell membrane integrity |
| Chloramphenicol, Erythromycin | Protein synthesis (50S ribosome) |
| Tetracyclines | Protein synthesis (30S ribosome) |
| Fluoroquinolones | DNA gyrase (DNA replication) |
Once you remember that penicillin is a β-lactam and β-lactams = cell wall, options (a), (c), (d) are gone immediately.
NEET loves asking about antibiotic mechanisms in the context of “why can’t we use antibiotics against viruses?” — the answer is that viruses have no cell wall, no ribosomes, and no bacterial enzymes. Penicillin has nothing to bind to in a virus.
Common Mistake
Students frequently confuse cell wall with cell membrane and mark option (a). The cell wall is the rigid outer layer made of peptidoglycan. The cell membrane is the inner phospholipid bilayer. Penicillin targets the wall, not the membrane. Drawing a quick gram-positive bacterial diagram in your rough sheet — wall outside, membrane inside — takes five seconds and prevents this error every time.
A related trap: some students remember “penicillin disrupts bacterial structure” and vaguely pick (a) thinking membrane and wall are the same thing. They are not — and NEET options are specifically designed to exploit this confusion.
Final answer: (b) Penicillin inhibits bacterial cell wall synthesis.