Biology high-school May 24, 2026

How Do Antibiotics Kill Bacteria?

Tiny drugs target bacterial machinery

Diagram of several bacteria being affected by antibiotics that target cell walls, ribosomes, and DNA copying.

Antibiotics kill bacteria by blocking jobs that bacterial cells need to stay alive. Some weaken the outer wall, while others stop bacteria from making proteins or copying genetic material. Antibiotics do not kill viruses because viruses do not have the same cell parts or life processes.

Big Idea. NGSS HS-LS1 connects antibiotics to how cell structures carry out essential functions in living systems.

Antibiotics changed medicine because they gave doctors a way to treat many bacterial infections. They work because bacteria are living cells with parts that can be targeted. A bacterial cell must build a strong wall, make proteins, copy DNA, and control what enters and leaves. If a drug blocks one of these jobs, the cell may stop growing or die. Human cells share some chemistry with bacteria, but they are not the same. That difference is why an antibiotic can harm bacteria more than it harms the person taking it. Viruses are different again. They are not cells, and they use host cells to reproduce. That is why an antibiotic for strep throat will not treat the flu. The same biology also explains resistance. When bacteria change in ways that let them survive a drug, the resistant cells can multiply.

Bacteria Have Targets

Labeled cutaway diagram of a bacterial cell showing the cell wall, membrane, ribosomes, and DNA as possible antibiotic targets. — Antibiotics target bacterial cell parts and processes.

A bacterium is a living cell. It has a cell membrane, cytoplasm, DNA, and ribosomes. Many bacteria also have a stiff cell wall outside the membrane. Each part does a job. The wall helps the cell keep its shape. Ribosomes build proteins. DNA holds instructions for growth and repair. Antibiotics work when they interfere with one of these jobs. The best targets are parts that bacteria need but human cells do not have, or parts that are different enough in bacteria to be blocked safely. This selectivity matters. A drug that damaged every kind of cell would not be useful as medicine. Different antibiotic classes aim at different bacterial processes, so doctors choose drugs based on the likely bacterium and the infection site.

A useful antibiotic target is important to bacteria and different from human cells.

Penicillin Weakens the Wall

Diagram showing penicillin blocking bacterial cell wall construction and a weakened bacterium swelling. — Penicillin blocks cell wall construction.

Penicillin is part of a group called beta-lactam antibiotics. These drugs block enzymes that bacteria use to link molecules in the cell wall. The wall is like a pressure suit for the cell. Water tends to move into many bacteria from their surroundings. A strong wall helps the cell resist swelling. When penicillin blocks wall building, new wall sections are weak. Growing bacteria cannot repair the damage fast enough. The membrane can bulge, tear, and burst. Human cells do not have bacterial cell walls, so this target gives penicillin its useful selectivity. Penicillin works best on bacteria that are actively growing and building new wall material. It will not help against viruses because viruses do not build cell walls.

Penicillin can kill growing bacteria by making their cell walls fail.

Tetracyclines Block Protein Making

Diagram of a bacterial ribosome with tetracycline blocking the path of protein building blocks. — Tetracyclines interfere with bacterial ribosomes.

Proteins do most of the daily work inside a cell. Bacteria need proteins to copy DNA, use energy, move materials, and repair damage. Ribosomes are the cell structures that build proteins from instructions. Tetracycline antibiotics bind to bacterial ribosomes and block a key step in protein assembly. The ribosome cannot add new building blocks in the right way. Without enough working proteins, the bacterium may stop growing. Some antibiotics that target ribosomes kill bacteria directly, while others mainly slow growth so the immune system can clear the infection. Human cells also have ribosomes, but bacterial ribosomes are smaller and shaped differently. That difference lets tetracyclines affect bacteria more strongly than human cells.

Blocking bacterial ribosomes stops the cell from making proteins it needs.

Viruses Are Not Bacteria

Side-by-side comparison of a bacterium with cell structures and a virus particle without ribosomes or a cell wall. — Bacteria are cells, but viruses are not cells.

Antibiotics do not work on viruses because viruses are built differently. A bacterium is a cell that can grow, use energy, and reproduce by dividing. A virus is a small package of genetic material inside a protein coat, sometimes with a membrane envelope. It does not have ribosomes. It does not build a cell wall. It does not carry out most cell functions on its own. Instead, a virus enters a host cell and uses the host cell machinery to make more viruses. Because antibiotics target bacterial machinery, those targets are missing in viruses. Viral infections may need vaccines, antiviral medicines, rest, or immune protection, depending on the disease. Taking antibiotics for a viral infection can add side effects and can help resistant bacteria spread.

Antibiotics miss viruses because viruses lack bacterial targets.

Resistance Can Spread

Sequence showing susceptible bacteria dying after antibiotic exposure while resistant bacteria survive and multiply. — Selection can make resistant bacteria more common.

Antibiotic resistance happens when bacteria survive a drug that used to stop them. Resistance can begin with random mutations in bacterial DNA. It can also spread when bacteria share small DNA circles called plasmids. A resistance trait may let a bacterium break down the drug, pump it out, or change the target so the drug no longer fits. Antibiotics do not make bacteria decide to change. They create strong selection. Susceptible bacteria die or stop growing, while resistant bacteria survive and reproduce. Misusing antibiotics can increase this selection. Examples include taking antibiotics when they are not needed, stopping treatment early without medical guidance, or using leftover pills. Good prescribing, vaccination, sanitation, and infection control all help slow resistance.

Resistance spreads when surviving bacteria pass on traits that protect them.

Vocabulary

Antibiotic: A medicine that kills bacteria or slows their growth.
Cell wall: A stiff outer layer that helps many bacteria keep their shape and resist bursting.
Ribosome: A cell structure that builds proteins from genetic instructions.
Virus: A noncellular infectious particle that must use a host cell to make more copies.
Resistance: The ability of bacteria to survive an antibiotic that would normally stop or kill them.

In the Classroom

Model a Cell Wall Failure

20 minutes | Grades 9-12

Students use a sealed plastic bag inside a mesh or paper sleeve to model a bacterium with a wall. They predict what happens when the sleeve is weakened and connect the model to penicillin and osmotic pressure.

Sort the Targets

25 minutes | Grades 9-12

Students sort cards showing bacterial structures, human cell structures, and virus features. They identify which antibiotic targets are present in bacteria but missing in viruses.

Simulate Resistance Selection

30 minutes | Grades 9-12

Students use colored beads or paper dots to represent susceptible and resistant bacteria. After each antibiotic round, survivors reproduce, making selection visible over several generations.

Key Takeaways

• Antibiotics work by blocking essential processes in bacterial cells.
• Penicillin weakens bacterial cell walls, which can cause growing bacteria to burst.
• Tetracyclines block bacterial ribosomes and slow or stop protein production.
• Antibiotics do not work on viruses because viruses do not have bacterial cell machinery.
• Resistance spreads when bacteria with protective traits survive and reproduce.

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