Bacteria Bacteria are all prokaryotes. The main structures of a bacterial cell are:

Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles

Ribosomes. The

smaller (70 S) type, all free in the cytoplasm, not attached to membranes (like RER). For protein synthesis.

Nucleoid. The region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane.

DNA. Always circular (i.e. a closed parts shown here loop), and not associated with any proteins to form chromatin. Sometimes confusingly referred to as the bacterial chromosome.

Plasmid. Small circles of DNA, used to exchange DNA between bacterial cells, and very useful for genetic engineering.

Flagellum. A rigid rotating helical-shaped tail used for propulsion. The motor is embedded in the cell membrane and is driven by a H+ gradient across the membrane. Clockwise rotation drives the cell forwards, while anticlockwise rotation causes a chaotic spin. This is the only known example of a rotating motor in nature.

Cell Membrane. made of phospholipids and proteins, like eukaryotic membranes.

Mesosome. This is now thought to be an artefact of the electron microscope and not a real structure. A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis. Can also be associated with the nucleoid.

Capsule (or Slime Layer). A thick polysaccharide layer outside of the cell wall, like the glycocalyx of eukaryotes. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against

phagocytosis. In some species the capsules of many cells in a colony fuse together forming a mass of sticky cells called a biofilm. Dental plaque is an example of a biofilm.

Cell Wall. Made of murein (not cellulose), which is a glycoprotein or peptidoglycan (i.e. a protein/carbohydrate complex). There are two kinds of bacterial cell wall, which are identified by the Gram Stain technique when observed under the microscope. Gram positive bacteria stain purple, while Gram negative bacteria stain pink. The technique was discovered by Christian Gram in 1884 and is still used today to identify and classify bacteria. We now know that the dif- ferent staining is due to two types of cell wall:

Bacteria have a variety of distinctive shapes when seen under a microscope:

Coccus (spheres)

Bacillus (rods)

Spirillum (helical)

Vibrio (crescent)