What is the difference between a bacterium and an archaeon?
Question
Last week during a study group, someone asked what’s the actual difference between bacteria and archaea. I remembered reading a detailed breakdown in a review article, so I looked it up again. Turns out, while they might look similar under a microscope, they’re worlds apart at the molecular level—especially in their cell walls, genetics, and metabolism.
Answer ( 1 )
Main idea
Bacteria and archaea both looks like small prokaryotic cells under the microscope, but they actually sit in two totally separate domains of life. They diverged billions of years ago so their core biology is pretty different.
Cell wall chemistry
Most bacteria build their wall from peptidoglycan (murein). Archaea almost never do that, instead they use pseudo‑peptidoglycan or an S‑layer made of proteins and glycoproteins. Because of that lysozyme and beta‑lactam antibiotics that break peptidoglycan just do not work on archaea.
Membrane lipids
Bacterial membranes are made of ester‑linked fatty acids. Archaeal membranes are ether‑linked isoprenoid chains and some species even fuse the two leaflets into a single monolayer, that trick helps them survive crazy heat or acidity.
Genetic machinery
When you look at DNA replication and transcription proteins, archaea resembles eukaryotes much more than bacteria. Their RNA polymerase is multi‑subunit like ours, they use TATA binding protein and transcription factors that are not present in bacteria.
Metabolism
Archaea can be methanogens, extreme halophiles or thermoacidophiles, but they never do oxygenic photosynthesis. Bacteria cover almost every metabolic style you can imagine including cyanobacterial photosynthesis and true pathogens of animals and plants.
Antibiotic sensitivity
Many classic antibiotics target steps that exist only in bacteria, for example ribosome inhibitors like chloramphenicol bind poorly to archaeal ribosomes. That is one more proof they are quite different.
Bottom line
So a bacterium and an archaeon might share the prokaryote label, still they are as far apart from each other as either group is from us. Calling them both “bacteria” is really outdated now.