Eubacteria

Eubacteria (more commonly known as bacteria) are prokaryotic microorganisms that can be found almost everywhere on Earth. They are usually single cells but can also be found in chains, filaments, or multicellular clusters. Most are about 1 micron (1 µm), or one millionth of a meter in length, although some of the largest can be up to half a millimeter (500 µm). They come in a variety of shapes such as rods, filaments, spirals, vibrio (comma-shaped), and cocci (ball-shaped). Some have stalks that can be used for attachment. Many of them can move by gliding or by rotating small, projecting filaments called flagella. They lack the complex intracellular motility and mitosis found in eukaryotic cells.

Cellular Structure

Bacterial cells are fairly simple in structure when compared to the eukaryotic cells of fungi, protists, plants, and animals. As seen with an electron microscope, the majority of bacterial cell volume is filled with ribosomes, the sites of protein synthesis. Some bacteria, such as those that are photosynthetic, contain many internal membranes where metabolic processes take place. They contain no internal organelles, such as mitochondria and chloroplasts. Like archaea, bacteria have a prokaryotic cell organization: their deoxyribonucleic acid (DNA) is loosely gathered into a nucleoid and is not surrounded by a nuclear membrane, like that found in Eukaryotes. The DNA usually occurs as a single long circular strand, but some bacteria have linear chromosomes or divide their genetic material into several DNA molecules. Although they are different from the chromosomes of Eukaryotes, the large circular or linear prokaryotic DNA molecules are often termed chromosomes as well. Bacteria can also have smaller circles of DNA called plasmids, which usually carry a small number of genes used for specific metabolic functions—for example, to allow bacteria to metabolize certain compounds. Plasmids can easily be passed from cell to cell, allowing bacteria to rapidly pick up new metabolic functions, and are the basis for many advances in genetic engineering.

Like all living cells, bacteria are surrounded by lipid membranes. Most bacteria also have cell walls made up of a peptidoglycan called murein. The peptidoglycan layer is made up of a single kind of molecule from covalently linked sugar derivatives and amino acids. This molecule surrounds the bacterial cell like chain mail armor. Together with the osmotic pressure, the wall gives cells rigidity and shape. The cell wall structure and the presence or absence of a second lipid membrane surrounding the murein layer determine how bacteria react in a procedure called the Gram stain. Gram-positive organisms, which take up Gram stain, have a single membrane and a very thick outer peptidoglycan layer. Gram-negative organisms do not take up the stain and have two membranes in between which is a thin layer of peptidoglycan.

Distribution and Ecological Roles

Although bacteria may appear simple, they excel in the diversity and complexity of their metabolic capabilities and they are able to survive in many places. Bacteria are found everywhere on Earth where life is able to exist. They are plentiful in soils, bodies of water, on ice and snow, and are even found deep within Earth's crust. They often take advantage of living in and on other organisms in symbiotic relationships and can be found inhabiting the intestinal tracts and surfaces of animals, including humans. For the most part, the bacteria in and around us bring us more benefit than harm. Sometimes however, bacteria can be pathogenic, or disease causing. This can happen for a number of reasons, such as when the host has a compromised immune system or when a bacterium acquires genes that make it grow more aggressively or secrete toxins into its host environment.

Oxygen-producing photosynthesis, which is so familiar in plants, is actually a bacterial invention. Many bacteria are photosynthetic and use light energy to turn CO₂ from the atmosphere into cell material. Among these only the cyanobacteria produce oxygen during photosynthesis. Plastids, the photosynthetic organelles found in plants and algae, evolved from cyanobacteria through a process called endosymbiosis, in which cyanobacteria lived inside the cells of other organisms that were the ancestors of green algae. Mitochondria, found in most eukaryotic cells, also evolved from nonphoto-synthetic respiring bacteria in this way.

Bacteria are crucial for the cycling of elements necessary for all life. Through various processes, which we generally call decomposition, bacteria break down the cell materials of dead organisms into simpler carbon-, phosphorus-, sulfur-, and nitrogen-containing nutrients that can be used again by other organisms for growth. Without bacteria to recycle these essential nutrients, they would remain within the dead organisms or sediments and would thus be unavailable for use by other organisms.

J. Peter Gogarten

Lorraine Olendzenski

Bibliography

Needham, Cynthia, Mahlon Hoagland, Kenneth McPherson, and Bert Dodson. Intimate Strangers: Unseen Life on Earth. Washington, DC: ASM Press, 2000.

Schlegel, H. G. General Microbiology, 7th ed., tr. M. Kogut. Cambridge: Cambridge University Press, 1993.