Antibiotic mechanisms of action: chart and quiz

The five targets

1. Cell wall. Peptidoglycan keeps the cell from bursting. Human cells do not have it.
2. Ribosome (protein synthesis). Bacteria have 70S ribosomes and human cytoplasm has 80S, so drugs can tell them apart.
3. DNA and RNA synthesis. Bacterial gyrase, topoisomerase IV and RNA polymerase are the main targets.
4. Cell membrane. Some drugs disrupt bacterial membranes. Selectivity varies: polymyxins can also damage human cells, while daptomycin preferentially targets bacterial membranes.
5. Folate synthesis. Many bacteria make their own folate. Humans get it from food.

Antibiotic mechanism of action chart

Read across each row: where the drug acts, what it actually blocks, and whether it is usually taught as killing bacteria (bactericidal) or stopping their growth (bacteriostatic).

On a phone, swipe the chart sideways to see every column.

The "usual effect" column is a rule of thumb. The same drug can act differently at a different dose or against a different organism.

These five groups are a study aid, not an exhaustive list. Isoniazid blocks synthesis of mycolic acids in the mycobacterial cell envelope; bedaquiline targets mycobacterial ATP synthase.

30S vs 50S: one memory line

Use this memory line: "buy AT 30, CELL at 50."

The line tells you the subunit, not the action. Within 30S, remember that aminoglycosides cause misreading and tetracyclines block tRNA from attaching.

The folate pathway: two drugs, two steps

Sulfamethoxazole and trimethoprim are often paired because each blocks a different step of the same pathway. In susceptible bacteria, the combination can have a greater effect than the sum of the drugs used separately. That is called synergy.

Three questions that answer most exam items

When a question asks why a drug works or fails, go through these in order:

1. Is the target there? No cell wall means no cell wall drug will work.
2. Can the drug reach it? Size, the outer membrane, porins and oxygen use all matter.
3. Has the target or drug been changed? This is where acquired resistance comes in.

Worked examples

Example 1: why vancomycin does not work on E. coli

1. Is the target there? Yes. E. coli has peptidoglycan with D-Ala-D-Ala precursors.
2. Can the drug reach it? No. Vancomycin cannot cross the gram-negative outer membrane.
3. Answer: this is an access problem, not a target problem. It is also why vancomycin is a gram-positive drug.

Example 2: why gentamicin fails against Bacteroides

1. Is the target there? Yes. Bacteroides has 30S subunits.
2. Can the drug reach it? No. Aminoglycoside uptake across the inner membrane depends on respiratory energy. Obligate anaerobes such as Bacteroides do not take up enough drug through this route.
3. Answer: aminoglycosides lack activity against anaerobes. By contrast, susceptible anaerobes reduce metronidazole to reactive forms that damage DNA.

Example 3: a Pseudomonas that stops responding to imipenem

1. Is the target there? Yes. It still has PBPs.
2. Can the drug reach it? Less efficiently after OprD loss. This porin is an important entry route for imipenem, a carbapenem, through the outer membrane of P. aeruginosa.
3. Has anything changed? If the isolate lost OprD, less imipenem reaches its PBPs. The answer is reduced uptake, not a changed target; this does not mean every drug molecule is excluded.

Example 4: why sulfonamides selectively target bacteria

1. Is the target there? Susceptible bacteria use dihydropteroate synthase to make folate. Sulfonamides inhibit this enzyme.
2. What about humans? Humans lack dihydropteroate synthase and get folate from food, so this bacterial target is absent from our cells.
3. Answer: this is selective toxicity, not a guarantee of no side effects. Do not extend this explanation to every folate enzyme: humans do have dihydrofolate reductase, the enzyme trimethoprim targets more strongly in bacteria.

Resistance basics

Some resistance is built in. Mycoplasma has no wall to target, so that is called intrinsic resistance. Other resistance is acquired, through a mutation or a new gene. Bacteria use a short list of strategies.

How resistance spreads

• Vertical: existing resistance genes are passed from a parent cell to daughter cells. They may have arisen by mutation or been acquired earlier from another bacterium.
• Horizontal: genes move between bacteria, even between species. Plasmids moved by conjugation are the classic route. Transformation and transduction also move DNA.

Antibiotics do not teach bacteria to resist. They kill or slow the susceptible cells and leave the resistant ones to grow.

Drug sorter: pick the target

Tap the target for each drug. You get feedback right away, and you can keep trying until you get it. Every row has a solution if you get stuck.

1. Amoxicillin (penicillin)

   Solution: Amoxicillin

   Cell wall. A beta-lactam. It binds penicillin-binding proteins (PBPs), so formation of new peptidoglycan cross-links is inhibited.

2. Gentamicin (aminoglycoside)

   Solution: Gentamicin

   Protein synthesis. It binds the 30S subunit and causes misreading, so the cell builds faulty proteins.

3. Ciprofloxacin (fluoroquinolone)

   Solution: Ciprofloxacin

   Nucleic acid. It blocks DNA gyrase and topoisomerase IV, so DNA replication stalls.

4. Sulfamethoxazole (sulfonamide)

   Solution: Sulfamethoxazole

   Folate. It looks like PABA and blocks dihydropteroate synthase, an early step in making folate.

5. Colistin (polymyxin)

   Solution: Colistin

   Membrane. It binds LPS and disrupts the membranes of gram-negative bacteria.

6. Vancomycin (glycopeptide)

   Solution: Vancomycin

   Cell wall. It binds the D-Ala-D-Ala end of wall building blocks, so they cannot be linked in.

7. Azithromycin (macrolide)

   Solution: Azithromycin

   Protein synthesis. It binds the 50S subunit and blocks the growing protein chain from moving along.

8. Rifampin (rifamycin)

   Solution: Rifampin

   Nucleic acid. It blocks bacterial RNA polymerase, so transcription stops.

9. Trimethoprim (antifolate)

   Solution: Trimethoprim

   Folate. It blocks dihydrofolate reductase, a later step in the folate pathway.

10. Daptomycin (lipopeptide)

    Solution: Daptomycin

    Membrane. It inserts into the gram-positive cell membrane and disrupts it.

11. Doxycycline (tetracycline)

    Solution: Doxycycline

    Protein synthesis. It binds the 30S subunit and blocks tRNA from attaching to the ribosome.

12. Ceftriaxone (cephalosporin)

    Solution: Ceftriaxone

    Cell wall. Another beta-lactam, so it has the same PBP target as amoxicillin.

13. Metronidazole (nitroimidazole)

    Solution: Metronidazole

    Nucleic acid. It is activated inside anaerobes and damages their DNA.

14. Linezolid (oxazolidinone)

    Solution: Linezolid

    Protein synthesis. It binds the 50S subunit and interferes with the start of protein synthesis.

Start the sorter over

Scenario quiz: 10 questions

These test the reasoning, not just the names. Pick an answer and press Check answer. If you miss one, the feedback explains why and you can try again.

Frequently asked questions

What are the 5 mechanisms of action of antibiotics?

Most antibiotics do one of five things: block cell wall synthesis, block protein synthesis at the ribosome, block DNA or RNA synthesis, damage the cell membrane, or block folate synthesis. Each one hits something bacteria need and that human cells lack or build differently.

Which antibiotics work on the 30S subunit and which work on the 50S subunit?

Aminoglycosides and tetracyclines bind the 30S subunit. Macrolides, lincosamides, chloramphenicol and oxazolidinones such as linezolid bind the 50S subunit. One memory line is "buy AT 30, CELL at 50."

Which antibiotics are bactericidal and which are bacteriostatic?

Beta-lactams, vancomycin, aminoglycosides and fluoroquinolones are usually taught as bactericidal, meaning they kill. Tetracyclines, macrolides, linezolid and sulfonamides used alone are usually taught as bacteriostatic, meaning they stop growth. The label is a rule of thumb, because the result can change with the drug level and the organism.

Which antibiotics inhibit folic acid synthesis?

Sulfonamides such as sulfamethoxazole block dihydropteroate synthase, and trimethoprim blocks dihydrofolate reductase. The two are often given together because they block two steps of the same pathway.

How do bacteria become resistant to antibiotics?

Bacteria can destroy or change the drug, change or bypass its target, keep the drug out, or pump it back out. Resistance can arise through mutations or acquisition of genes from other bacteria, often on plasmids.