Three domains of life
Living cells are sorted into three domains: Bacteria, Archaea and Eukarya. Bacteria and archaea are both prokaryotes: their DNA is not enclosed in a nuclear membrane and they have no membrane-bound organelles. Eukaryotic microbes (yeasts, moulds, protozoa, algae) have a nucleus and organelles. Archaea look like bacteria under a microscope, yet their ribosomal RNA, membrane lipids and cell walls differ, and many live in extreme places such as hot springs and very salty lakes.
Inside a bacterial cell
- Nucleoid. The single, usually circular chromosome sits in a region with no membrane around it. Small extra DNA circles called plasmids often carry useful genes.
- Ribosomes. Prokaryotes have 70S ribosomes, smaller than the 80S ribosomes in the cytoplasm of eukaryotes. This difference matters later for antimicrobial action.
- Plasma membrane. A phospholipid bilayer controlling what enters and leaves. In bacteria it also hosts energy-making machinery.
- Cell wall. Most bacteria have a wall of peptidoglycan, a mesh of sugar chains cross-linked by short peptides, which resists bursting from osmotic pressure. Archaea do not make peptidoglycan.
Gram-positive and Gram-negative
The Gram stain divides most bacteria in two. A Gram-positive cell has a thick peptidoglycan layer and keeps the purple crystal violet-iodine complex after the alcohol rinse. A Gram-negative cell has a thin peptidoglycan layer under an outer membrane that contains lipopolysaccharide (LPS). The alcohol removes the purple dye, and the cell is coloured pink by the counterstain.
Extras outside the wall
A capsule is a sticky polysaccharide layer that helps cells attach and resist engulfment by immune cells. Flagella drive swimming. Pili (fimbriae) help attachment, and a special type takes part in DNA transfer. Endospores, made by genera such as Bacillus and Clostridium, are dormant, tough structures that survive heat, drying and chemicals. One cell makes one spore, so spores are for survival, not reproduction.
Reproduction
Bacteria reproduce by binary fission: the chromosome is copied, the cell elongates, and a new wall divides it into two similar cells.
Worked example
A technician sees purple rods under the microscope after a Gram stain, and the cells have no nucleus. Reasoning step by step: purple means the dye was retained, so the wall is thick peptidoglycan, which is Gram-positive. No nucleus means prokaryote, and a peptidoglycan wall points to Bacteria rather than Archaea. Had the cells been pink, we would infer a thin peptidoglycan layer plus an outer membrane with LPS. The stain tells us about wall structure, not about whether the organism causes disease.