Bacteria are prokaryotes (unicellular organisms with no membrane-enclosed nucleus) with simple structures that typically range in size from about 0.5 to 20 micrometers.
Bacteria are named according to the binomial (two-name) system of nomenclature first used by Swedish botanist Carolus Linnaeus in the eighteenth century. The first name, or generic name, indicates the genus of the bacteria (a group of closely related species). The second name, or specific name, indicates the species (a group of bacteria that share a number of characteristics). Examples of generic names include Staphylococcus and Esherichia; aureus and coli are examples of specific names.
The genus and species names of bacteria often reflect their shape; for example, the Bacillus family of bacteria are bacillior rod-shaped. Others are named for their founders (e.g. Yersinia pestis, the causative agent of bubonic plague, is named for Alexandre Yersin) or for their preferred habitat (e.g., Thermoplasma prefer temperatures up to 149°F, or 65°C).
Under a microscope, different families of bacteria have different shapes. Typical cell shapes are straight rods (bacilli), spheres (cocci), bent or curved rods (vibrios), spirals (spirochetes), or thin filaments. Some bacteria exist as single cells, while others form clusters of
various shape and complexity. Acetobacter aceti, for example, excretes a substance called cellulose that surrounds the cells to form a skinlike layer. Staphylococcus aureus forms grapelike clusters of cells.
Many groups of bacteria have a cell wall, a structure surrounding the cell. Peptidoglycan (a chemical composed of carbohydrates and proteins) is a major component of the cell wall, although the exact composition of peptidoglycan varies according to bacteria group. Gram-positive organisms have a relatively thick layer of peptidoglycan and stain violet when applied with certain dyes; gram-negative organisms have a thin layer of peptidoglycan covered by an outer membrane and stain red under the same application of dyes. Gram staining is therefore an important method for identifying bacteria.
Bacteria may be classified by their biochemical composition, and analysis of the protein and lipid content of an organism is often a means of identification. Growth requirements are often used as a means of classification: Mycobacterium tuberculosis, for example, is an obligate aerobe and therefore requires oxygen for growth, while the presence of oxygen is toxic to the anaerobe Clostridium tetani.
The most precise method of classification, however, is genetic analysis. Each species of bacteria has a unique genetic makeup, and therefore a unique sequence of deoxyribonucleic acid (DNA) bases. Some sequences remain constant by genus or species, while others vary considerably. These distinguishing factors are used to trace genetic relatedness and are often used for identification of unknown organisms.
Stéphanie Islane Dionne, The Gale Group Inc., Gale, Detroit,