C3.2.13 Antibiotics Notes

Antibiotics: Targeting Bacteria, Not Viruses

1. Antibiotics are chemicals that kill or inhibit the growth of bacteria by targeting processes that are unique to prokaryotic cells.
2. These processes include cell wall synthesis, protein synthesis, DNA replication, and metabolic pathways that do not exist in human (eukaryotic) cells.
3. However, antibiotics do not work against viruses because viruses have a completely different structure and method of reproduction compared to bacteria.

How Antibiotics Work

1. Bacteria are prokaryotic cells, which means they have unique structures and biochemical pathways that are different from human cells.
2. Antibiotics work by targeting these bacterial structures and processes without harming human cells.

Key Targets of Antibiotics

1. Cell Wall Synthesis
   1. Many bacteria have a rigid cell wall made of peptidoglycan.
   2. Antibiotics like penicillin block the enzymes that build this wall, causing the bacteria to lyse.
2. Protein Synthesis
   1. Bacterial ribosomes (70S) differ from eukaryotic ribosomes (80S).
   2. Antibiotics like tetracycline and streptomycin bind to bacterial ribosomes, preventing protein production.
3. DNA Replication and Transcription
   1. Antibiotics such as ciprofloxacin inhibit bacterial enzymes like DNA gyrase, which are essential for DNA replication.
4. Metabolic Pathways
   1. Some antibiotics, like sulfonamides, block bacterial enzymes involved in folic acid synthesis, a process not found in humans.

Why Antibiotics Don’t Work on Viruses

Viruses are not cells, they are non-living particles that require a host cell to reproduce. Unlike bacteria, viruses:

1. Do not have cell walls, ribosomes, or their own metabolism.
2. Do not carry out protein synthesis or DNA replication independently.
3. Hijack human cells to replicate instead of growing and dividing on their own.

Because viruses use human cell machinery to reproduce, antibiotics cannot target them without also harming human cells.

The Discovery of Penicillin: The First Antibiotic

1. Alexander Fleming was studying Staphylococcus bacteria when he noticed that a mold (Penicillium notatum) was killing the bacteria on one of his petri dishes.
2. The mold was producing a substance that destroyed bacterial cell walls.
3. He named this substance penicillin, which became the first antibiotic.