Bacteria are small, single cells and are the only ones characterized by prokaryotic organization. Too tiny to see with the naked eye, a bacterial cell is usually one of three basic shapes; bacillus, a rod shaped cell; coccus, a spherical cell; or spirillum, a spiral cell. A few kinds of bacteria aggregate into stalked structures or filaments.
A bacterium's plasma membrane is encased within a cell wall. Members of the kingdom Eubacteria have a cell wall made of peptidoglycan, a network of polysaccharide molecules linked together with chains of amino acids. Some eubacteria have a cell wall covered with an outer membrane layer made of large molecules called lipopolysaccharides. A lipopolysaccharide is a chain of sugar molecules with a lipid attached to one end. Outside of the cell wall and membrane, many bacteria have a gelatinous layer called a capsule.
Eubacteria are classified by differences in their cell walls.
A bacterium with a cell wall containing a large amount of peptidoglycan is classified as
gram-positive. A bacterium with a cell wall containing a thin layer of peptidoglycan covered by an outer membrane is classified as gram-negative.
These terms refer to a bacterium's reaction to a staining procedure developed by the Danish microbiologist Hans Gram.
Some bacteria form thick-walled endospores around their chromosomes and a small bit of cytoplasm when they are exposed to harsh conditions such as drought or high temperatures. The formation of endospores in the bacterium Clostridium botulinum is responsible for botulism, a disease often considered to be a form of food poisoning.
Bacteria, which outnumber all eukaryotes combined, differ from eukaryotes in at least seven important respects.
1. Internal compartmentalizationBacteria are prokaryotes and lack a cell nucleus, unlike eukaryotes. The cytoplasm of bacteria has very little internal organization. It contains no internal compartments or membrane systems.