Bacteria

• Categories: Bacteria; medicine and health; microorganisms; taxonomic groups

Although only a relatively few thousand species of bacteria have been discovered and cataloged to date, the total number of bacterial species on Earth is estimated to be in the millions, if not more. Bacteria are extremely abundant; a handful of soil or a spoonful of the organic muck at the bottom of a pond may hold tens of millions of bacteria. So tiny are these smallest of living organisms that a quarter of a million of them can be squeezed into the period at the end of this sentence. Bacteria are undoubtedly the most widely distributed of all organisms in nature. They occur everywhere: in soil, water, and air, as well as within the bodies of virtually all plants and animals.

Bacteria represent Earth’s earliest, and in many ways its simplest, living organisms. Fossils of bacteria have been found in rocks estimated at 3.5 billion to 3.7 billion years old. Thus, bacteria predate eukaryotic cells by at least a billion years.

Although small and simple, bacteria are ecologically and economically among the most important of all living organisms. They are the major organisms of decay and decomposition, fermentation, and nitrogen fixation. Phenomena such as spoiling of food, rotting of flesh, decomposition of plants, and certain diseases are all evidence of bacterial activity. While the vast majority of bacteria are harmless or helpful, a small percentage cause serious and often fatal diseases in humans, other animals, and plants.

Characteristics and Classification

All bacteria are prokaryotic, or single-celled, organisms. A prokaryote cell is a cell that lacks a nucleus. In modern biological classification systems, prokaryotes are divided into two groups, Archaea (previously Archaebacteria, "ancient bacteria") and Bacteria (previously Eubacteria, "true bacteria"). Both groups comprise minute organisms that typically vary in size from about 0.1 to 10 micrometers in diameter (save for outliers such as the bacteria Thiomargarita namibiensis, which has been observed to reach as much as 750 micrometers in diameter). In comparison, eukaryotic cells typically range from about 10 to 100 micrometers in diameter. Archaea were previously thought to be a subtype of bacteria, but while the two groups have many morphological similarities, archaea possess some genetic and biochemical characteristics that are more similar to those of eukaryotes than of bacteria, as well as some that are found in neither. Among the differences between archaea and bacteria are that archaea have a unique RNA structure, lack muramic acid in their cell walls, and produce distinctive lipids.

Known bacteria species are classified on the basis of shape, deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) arrangements, locomotion, pigments, and staining properties. Taxonomists increasingly rely on DNA and RNA comparisons to identify new species and determine relationships among bacteria. One common method of identifying and classifying bacteria is via the 16S ribosomal RNA (rRNA) gene because this gene is ubiquitous in both bacteria and archaea, it is distinct from its eukaryotic homologue (18S rRNA), and it is relatively short compared to other genes that are unique to prokaryotes.

In the three-domain system introduced by Carl Woese and colleagues in 1977, two of the three domains are Bacteria and Archaea; the third, Eukarya (or Eucarya), includes all eukaryotic organisms, including protists, fungi, plants, and animals. In the traditional kingdom classification scheme, the 1969 modification by American ecologist Robert Whittaker introduced a fifth kingdom, Monera, which included both bacteria and archaea. Later revisions to the kingdom system by British zoologist Thomas Cavalier-Smith separated Monera into two prokaryotic kingdoms, Archaea (Archaebacteria) and Bacteria (Eubacteria).

Structure

Bacteria are single-celled organisms encased within a cell wall and lacking a central nucleus. They have a relatively homogenous cytoplasm devoid of membrane-bound organelles, such as Golgi bodies, endoplasmic reticulum, mitochondria, or plastids, but sites along the cell membrane perform some of these organelle functions. Bacteria do have ribosomes for protein synthesis, but these are about half the size of ribosomes that are found in the eukaryotic cells of higher plants and animals.

The nucleic acid of bacterial DNA occurs as a single naked, ringlike form called a nucleoid, which is attached to the cell wall. Bacteria also have small rings of DNA called plasmids, which are dispersed within the cytoplasm. The plasmids replicate separately from the nucleoid, suggesting that they were originally separate organisms that were somehow incorporated into the bacterial system.

The cell walls of bacteria help maintain shape and provide rigid protection. Unlike cell walls of plants, which are composed mostly of cellulose, bacterial cell walls contains peptidoglycans, which consist of large amino acid molecules cross-linked to molecules of polysaccharides. Antibiotics, such as penicillin, work by inhibiting cell wall formation and repair, thereby destroying the bacterial cell.

Reproduction

Most bacterial reproduction is asexual, with individual cells undergoing a form of mitosis called binary fission, during which the nucleoid and plasmids replicate themselves and then migrate to opposite ends of the dividing cell. The cell wall pinches inward to form an interior wall that separates the dividing cell into the two new daughter cells. Under ideal conditions, binary fission can take place every ten to twenty minutes.

Some species can also exchange genetic material by a process called conjugation. Conjugation occurs when two compatible bacteria come into close contact with each other. The cell wall of one of the two bacteria evaginates (grows outward) toward its partner and merges with it to form a hollow, connecting tube called a pilus. When the tube is complete, the DNA segment migrates through the tube to the recipient cell, where it merges with and becomes part of the recipient cell’s nucleoid.

Another process of genetic transfer occurs when a bacterium picks up fragments of DNA released by fragmented dead bacterial cells and incorporates the fragments into its own nucleoid. The absorbed DNA increases the genetic variability of the bacterial cell, and new characteristics may result from the interactions of the original genetic material with the newly acquired genetic material.

Shapes

Most bacteria occur in one of three basic body forms, which also serve as a simple method for recognizing and classifying them. Cocci (singular: coccus) are round, elliptical, or spheroid in shape. Rod-shaped bacteria are called bacilli (singular: bacillus). The third bacterial form is a spiral or corkscrew shape, which includes spirilla (singular: spirillum), elongated coils that are often corkscrew-shaped; spirochetes, which are longer, thinner, and more flexible; and vibrios, which are shaped like a comma. While other shapes occur, these three are the most common.

Cocci bacterial cells may occur in several different forms. In irregular clusters, they are called staphylococci, which cause the well-known staph infections. Those in filament-like or bead-like chains are called streptococci, which cause strep throat, while cocci that occur in pairs are called diplococci. In all of these forms, each cell is completely independent.

Sheaths and Surfaces

Bacteria can develop slimy or gummy capsule-like sheaths around their cells, which, in addition to body shapes and pigments, serve as an aid to classification. Outside the cell walls of some bacteria are sticky capsules, or slime layers, that are made up of polysaccharide or protein. These capsules help certain bacteria that cause disease avoid being detected by an animal’s immune system.

Some bacteria have a mass of hairlike projections called pili covering their surface. They are made of protein, and they generally function to attach to the bacteria of other cells. The tubelike pili found in some bacteria serve as attachment structures that enable the bacteria to remain fastened in place to a suitable substrate. Infectious bacteria, such as the bacteria involved in the sexually transmitted disease gonorrhea, use the pili to attach to the cell membranes of the host, causing infection.

Motility

Although many bacteria are nonmotile, some filamentous bacteria have slender flagella that slowly rotate, propelling them through the medium in a spiraling glide. Flagella help bacteria move into new habitats, follow nutrients, and leave environments that are nonbeneficial. Flagellated bacteria may move toward or away from stimuli, a behavior which is known as a taxis. Bacteria that move in response to chemicals in their environment are called chemotactic, those that move toward or away from light are phototactic, and magnetotatic bacteria respond to the earth’s magnetic field. Occurring in aquatic habitats, the magnetotatic bacteria are able to detect the earth’s magnetic field using iron crystals within their cytoplasm that act as tiny magnets.

If environmental conditions become unfavorable, some bacteria form structures called endospores, which consist of genetic material along with a few enzymes enclosed inside a thick protective layer. Endospores can survive for long periods of time in extremely unfavorable conditions. They are important dispersal mechanisms because they can travel for long distances in the air or water, then produce new bacteria quickly as soon as they find conditions that are favorable.

Nutrition

A major reason for the success of the bacteria is in their various forms of nutrition. All bacteria are either autotrophic (able to manufacture their own food) or heterotrophic (obtaining food by feeding on plants or animals or their remains). The majority of heterotrophic bacteria obtain food directly from the environment by absorbing it across the cell wall, but some are important parasites that cause disease. Other types of bacteria are chemosynthetic, gaining energy through reactions that combine oxygen with inorganic molecules, such as sulfur, ammonia, or nitrite. During the process, they release sulfates and nitrates, crucial plant nutrients, into the soil.

Certain types of bacteria have the ability to break down cellulose, the primary component of plant cell walls. Some of these types of bacteria have formed a symbiotic relationship with mammals called ruminants, which include deer, sheep, and cows. The cellulytic bacteria manufacture and release cellulose-digesting enzymes. They are considered symbiotic because they provide enzymes that enable the animal to digest food it would otherwise be unable to process. In turn, the bacteria inhabit an optimum environment deep within the animal’s stomach and have nutrients supplied directly to them. Other symbiotic bacteria live in the intestines of humans and other animals. They feed on undigested food passing through the gut and synthesize vitamins K and B₁₂, which are absorbed into the human body.

Another significant group of symbiotic bacteria are the nitrogen-fixing bacteria, which are one of the very few groups of organisms able to extract molecular nitrogen from the atmosphere and incorporate it in organic compounds. This ecologically and economically important group grows within root nodules—small, rounded clumps that cluster along the roots of certain plants, such as alfalfa, soybeans, lupines, and clover.

Cyanobacteria

Bacteria have muramic acid in their cell walls. The majority are heterotrophic or parasitic, but some very important groups are autotrophic. One of these groups, the phylum Cyanobacteria, is unique, in that cyanobacteria are the only prokaryotes that undergo oxygen-producing photosynthesis.

Cyanobacteria are often erroneously called blue-green algae, but true algae are eukaryotes, while cyanobacteria, like other bacteria, are prokaryotes. These bacteria possess chlorophyll a, which is also found in green plants. Many cyanobacteria also have a blue pigment called phycobilin, a red pigment called phycoerythrin, and carotenoids, which help gather light energy for photosynthesis. Their distinctive blue-green color is caused by the combination of chlorophyll and phycocyanin pigments. Cyanobacteria are the only organisms that can fix nitrogen and produce oxygen at the same time, producing a nitrogenous food reserve called cyanophycin. They can also produce and store carbohydrates and lipids.

Cyanobacteria occur in chains or hairlike filaments. Some form irregular, spherical, or platelike colonies held together by gelatinous sheaths. These sheaths may be colorless or pigmented with shades of yellow, red, brown, green, blue, violet, or blue-black. Cyanobacteria lack flagella and move by rotating on their longitudinal axis, which gives them a forward gliding motion.

Cyanobacteria occur in soil, in water, on moist surfaces, and in root nodules of plants. They are common in temporary pools and ditches, and often very abundant in freshwater habitats. Cyanobacteria are among the first invaders of newly formed habitats, such as the ash fields around volcanoes and newly opened fissures along deep-sea volcanic ridges and mounts. They are even found in tiny fissures in desert rocks. Some species occur in jungle soils and on the shells of turtles and snails. Others live as symbionts within amoebae, protozoa, diatoms, sea anemones, fungi, and the roots of tropical cycads.

Over billions of years, the photosynthetic activity of cyanobacteria transformed the oxygen-free early atmosphere into the modern atmosphere, in which oxygen plays such an important role for all higher plants and animals. The accumulation of oxygen in the upper atmosphere produced the high-altitude ozone layer, which shields animals and terrestrial plants from the damaging effects of ultraviolet radiation.

Ecologically, cyanobacteria form the base of many food chains, especially in freshwater and marine habitats. During the warm months of the year, cyanobacteria can temporarily become abundant and form floating mats of pond scum that often cover quiet waters of ponds and wetlands in late summer. The sudden increase in populations produces algal blooms (eutrophication) that cause massive die-offs of plant and animal populations because the bacteria populations consume all the available oxygen in the water, thereby asphyxiating other organisms. In reservoirs and other human water supplies, large populations of cyanobacteria clog filters, corrode steel and concrete structures, and cause a natural softening of water. They produce odors and discoloration that sometimes make water unpalatable.

Beneficial Bacteria

The number of bacteria that are beneficial organisms for humans is much greater than the number of disease-causing bacteria. The use of bacteria as biological control agents is typified by Bacillus thuringiensis (B.t.), bacteria available in garden shops as a spray or powder. When placed on plants, it kills caterpillars and worms. It is sold as a mass-produced, stable, moist dust containing millions of spores of the bacteria. It is harmless to humans, birds, earthworms, and other creatures, except moth or butterfly larvae. When the caterpillar ingests the bacterial spores, the spores develop into bacilli, which multiply in the digestive tract and paralyze the gut of the worm. The crop pest usually dies as a result of ingestion in two to four days. Other varieties, such as Bacillus thuringiensis variety israelensis (B.t.i.), is used to control mosquitoes, while Japanese beetles can be controlled by application of a bacterial power containing Bacillus popilliae, which is specific to this beetle.

Other bacteria have important uses in the service of humans. Some are used in bioremediation projects, such as the cleanup of oil spills, sewage treatment plants, and toxic waste dumps. Pseudomonas capacia, for example, is used to decompose oil spills. More benefits from bacteria are being developed using genetic engineering techniques.

Bacteria also play a major role in the dairy industry as cultures in the production of buttermilk, acidophilus milk, yogurt, sour cream, kefir, and cheese. Whey, the watery part of milk left in cheese production, is used in the manufacture of lactic acid. Lactic acid from lactate bacteria is also used in the textile industry, in the preparation of laundry products, in leather tanning, and in the treatment of calcium and iron deficiencies.

Bacteria are cultured in vats and used to manufacture chemicals, such as acetone, butyl alcohol, dextran, sorbose, citric acid, some vitamins, and medicinal preparations. Bacteria are also used to cure vanilla pods, cocoa beans, coffee, and black tea. They are used in the production of vinegar, sauerkraut, and dill pickles. Fibers from linen cloth are separated from flax stems by bacterial action. Green plant material is fermented in silos, providing food for livestock through the action of bacteria. Lastly, bacteria producing the amino acid glutamic acid are used to produce monosodium glutamate (MSG), a common food ingredient.

Plant-Pathogenic Bacteria

Almost all plants are susceptible to a host of bacterial diseases which cause infections, rotting, and death. Economically, the plant-pathogenic bacteria result in enormous losses of crops and other plant products. It has been estimated that about one-eighth of crops are annual losses to plant diseases.

Plant diseases caused by bacteria include galls, rots, wilts, spots, fascination, blights, and soft rots. Blights are caused when bacteria invade plant tissues of stems, leaves, and flowers, producing dead and discolored areas of infection. Fruits and vegetables not completely destroyed are sufficiently discolored to be unmarketable. Soft rots typically occur in fleshy storage organs, such as potatoes, eggplants, squashes, and tomatoes. Bacterial infections that cause drooping or wilting of plant tissues are called wilts. Wilts occur when bacteria colonize xylem vessels, where they block or interfere with water transport, eventually leading to dysfunction and destruction of the plant. Galls are plant swellings produced when bacteria and other organisms invade leaves and stems and lodge in parenchyma tissue. The infected tissue swells to produce a gall that encases the bacterial colony.

Human-Pathogenic Bacteria

Although bacteria do provide some benefits with their feeding habits, certain bacteria have feeding habits that threaten the health of humans. The largest threats stem from bacterial infection. These disease-producing bacteria, which are scientifically referred to as pathogens, synthesize toxic substances that cause disease symptoms. For example, the bacteria Clostridium tetani and Clostridium botulinium cause tetanus and botulism, respectively, the latter of which is a deadly form of food poisoning. These bacteria produce toxins that attack the nervous system. They are anaerobes, meaning they do not require oxygen, and they thrive as spores until they are introduced into a desirable environment. Tetanus enters the body through a deep puncture wound, protecting the bacteria from having contact with oxygen. As the bacteria multiply, they release their poison into the bloodstream.

Bacteria can enter the human body from the air. Every time someone coughs, sneezes, or speaks loudly, they produce an invisible spray of saliva droplets containing bacteria. The fluid around these bacteria quickly evaporates, but the bacteria cling to protein flakes that were also expectorated and enter the lungs during breathing. Once inside the lungs, bacteria access the circulatory system, via which they are transported to tissues and organ systems. Legionnaire’s disease is caused by a bacterium that lives in small amounts of water in air conditioning systems. It can be transmitted throughout an entire building by airborne particles that are blown through the air conditioning ducts. In a particularly deadly form of infection, anthrax bacilli can be inhaled, lodge in the lungs, and quickly kill the host.

Bacteria also gain access to the body through the ingestion of contaminated food and water. Bacterial infections caused by eating contaminated food are widespread, especially in the less developed countries of the world. Open sewers and unsanitary toilet conditions increase the risk of waterborne bacterial diseases, such as cholera, dysentery, and salmonella. Bacteria may also be ingested via improperly stored foods, such as raw chicken, shellfish, and eggs. Once ingested, the bacteria multiply in the intestinal tract and can be passed with urine.

Some bacteria gain access to the body through direct contact. Most of the sexually transmitted diseases are caused by bacteria. Examples include syphilis and gonorrhea. Other diseases, such as anthrax and brucellosis, enter the body through the skin or mucous membranes. Contact anthrax (less deadly than inhalational anthrax, if treated quickly) is a disease of cattle and other farm animals that occasionally infects humans. It is transmitted to workers in the tanning industry who handle hides and wool. Brucellosis is also a disease from farm animals and is transmitted by contaminated milk. It is sometimes called undulant fever and is characterized by a daily rise and fall of temperature associated with the cyclic release of the toxins by the bacteria.

Other bacteria that live in the soil enter the body through wounds. The tetanus or “lockjaw” bacterium is a common soil organism. Puncture wounds caused by stepping on dirty nails and other sharp devices introduce the tetanus bacteria deep into the body. Once inside, it produces toxins that are extremely powerful. Tetanus is easily controlled by immunizations that are developed from an attenuated horse serum. Some soil bacteria can cause a disease called gas gangrene. These bacteria respire anaerobically in the body and basically destroy tissues while emitting damaging gases (hence their name). If untreated, a serious infection by gas gangrene bacteria can result in the loss of a limb.

Many bacteria gain access to the human body through the bites of insects and other organisms. If an organism transmits a disease-causing organism to a different host, the first organism is called a vector. Many of the most feared and deadly of human diseases are caused by bacteria that are transmitted by vectors. Bubonic plague and tularemia are transmitted by fleas, deer flies, ticks, or lice. Of these, the most famous—and historically the most deadly—is bubonic plague, which is transmitted by fleas of rats and other rodents. Throughout recorded history, periodic visitation of plagues resulted in catastrophic die-off of thousands and even millions of people. The populations of Thebes, Athens, Rome, Vienna, and other early cities suffered and survived many plague years. The most potent plague years were in the late Middle Ages, when nearly a third of the European population succumbed to the bacterial disease known as the Black Death. Bubonic plague is still found today in the United States in ground squirrel populations and other rodents as well.

Mycoplasmas and Other “Small” Bacteria

Bacteria of the genera Mycoplasma and Rickettsia and of the phylum Chlamydiae are small and simple prokaryotes that are sometimes collectively labeled “small bacteria.” Of these, the mycoplasmas are the smallest of all bacteria and therefore the smallest of all living organisms, since viruses do not qualify as life. Mycoplasmas are distinctive in that, unlike other bacteria, they lack a cell wall. The relationships and diversity of mycoplasmas are still poorly understood, but the mycoplasmas have the distinction of being the smallest known organisms to cause human disease. One form causes a sexually transmitted disease and another, Mycoplasma pneumonia, causes a form of pneumonia commonly called walking pneumonia.

The rickettsias are tiny bacteria that were first described by Howard Ricketts in 1909. Rickettsiae species are mostly transmitted by animal vectors, such as ticks and lice, and cause typhus and Rocky Mountain spotted fever.

Some Chlamydiae cause airborne diseases, among them Chlamydia psittaci, which causes parrot fever, or psittacosis. Humans are exposed to parrot fever by inhaling the dried droppings or dust from infected birds. Another airborne chlamydial disease of humans is transmitted by respiratory droplets containing Chlamydia pneumoniae. This causes a mild form of walking pneumonia in humans. Other Chlamydiae cause sexual diseases that resemble gonorrhea and can result in serious infections of the urogenital tract if undetected or left untreated.

Genetic Engineering with Bacteria

Since they are relatively simple organisms that can easily be cultivated in large numbers, bacteria are one of the most important of all groups of organisms used in scientific experiments. As a result, they have been the subject of much genetic engineering, which is the artificial introduction of DNA into bacteria to change their characteristics. Genetic engineering, which is also called gene splicing, begins with the isolation of plasmid DNA from a bacterium. The DNA molecule is broken up into separate genes or groups of genes by special enzymes called restriction enzymes. The resulting DNA fragments are then mixed with repair enzymes and injected into another bacterium, which absorbs the new genes and functions in a new way under the influence of the new genes or combination of genes.

BacteriumDiseasesArgobacteriumCane gall, crown gall, hairy root, twig gallClavibacter, RhodococcusFasciation, spots, ring rots, tomato cankers, wiltsErwiniaBlights, wilts, soft rotsPseudomonasBanana wilts, bud blasts, cankers, leaf spots, lilac blights, olive gallsXanthomonasBlack venation, bulb rots, citrus cankers, cutting rots, walnut blightsRhizobiumRoot nodules ()StreptomycesPotato scabsSource: Data are from Peter H. Raven et al., Biology of Plants, 6th ed. (New York: W. H. Freeman/Worth, 1999).

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