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Pathology => Microbiology => Bacteria
Bacteria
INTRODUCTION
Bacteria (bacterium, singular), microorganisms that lack a nucleus and have a cell wall composed of peptidoglycan, a protein-sugar molecule. Bacteria are the most common organisms on earth and are intimately connected to the lives of all organisms.
Most bacteria are less than 1 micron (0.001 mm/0.00004 in) in length. Hundreds of thousands of bacteria can fit into a space the size of the period at the end of this sentence. However, colonies of bacteria, such as those found on a laboratory culture plate or on the surface of salt marsh muds, can be viewed easily without a microscope.
CLASSIFICATION
A variety of classification systems are used to order bacteria. Bacteria are described as prokaryotes, organisms whose cells lack nuclei, to distinguish them from eukaryotes, organisms such as fungi, plants, and animals, whose cells contain nuclei. Because of this fundamental difference, bacteria are placed in their own kingdom, Kingdom Monera, also called Kingdom Prokaryotae.
Recent molecular studies have found that a small group of organisms now known as the archaebacteria, formerly classified with bacteria because they lack a nucleus, actually have significant differences in the composition of their cell wall, plasma membrane, and other key molecular features. These structural differences have led some scientist to favor a classification scheme that groups all organisms into three domains. In this scheme, the bacteria are placed in the domain Bacteria, the archaebacteria in Archaea, and eukaryotes in Eukarya.
Bacteria are often classified on the basis of their physical shapes. Bacteria can be spherical (cocci), rod-shaped (bacilli), or corkscrew-shaped (spirochetes). Another classification system divides bacteria into gram-negative or gram-positive according to the composition of their cell walls, a distinction identified by a staining technique called the Gram stain. Scientists also classify bacteria according to whether or not they require oxygen to survive. Bacteria that require oxygen are called aerobic bacteria, or aerobes. Bacteria that live without oxygen are called anaerobic bacteria, or anaerobes.
Bacteria are also classified by their methods of obtaining carbon and energy. Carbon is the element needed to build the complex molecules required for life, and energy is required to carry out all life activities. Some bacteria are autotrophs, organisms that obtain carbon from carbon dioxide. Autotrophs derive energy from different sources. Photoautotrophs, represented by the cyanobacteria (formerly known as the blue-green algae), derive their energy from light and use it to carry out photosynthesis, the process in which light energy is converted to the chemical energy of glucose. Chemoautotrophs, including the soil bacteria Nitrobacter, derive their energy from inorganic compounds such as hydrogen sulfide and use the energy to power the cell's activities. The remaining bacteria are heterotrophs, organisms that obtain carbon by ingesting organic molecules released from decaying organisms or by preying on other bacteria. Photoheterotrophs gain energy from light, and chemoheterotrophs obtain energy from organic molecules.
To derive energy from organic molecules such as sugars, bacteria carry out either fermentation or cellular respiration, processes that utilize the potential energy in organic molecules for synthesis of adenosine triphosphate (ATP). ATP is used for growth, movement, temperature regulation, and a host of other survival processes that require energy. Fermentation, the process used by most bacteria, typically takes place in an anaerobic environment. A variety of fermentation pathways are utilized by different species of bacteria, and each produces unique byproducts, including alcohol, carbon dioxide, lactic acid, formic acid, or acetic acid. Some species of bacteria rely on cellular respiration, a more complex process that can be carried out with or without oxygen, in which the potential energy of organic molecules is more efficiently utilized to synthesize ATP. Carbon dioxide and water are typical byproducts of cellular respiration.
STRUCTURE
Like all cells, bacteria contain genetic material known as deoxyribonucleic acid (DNA). However, the DNA of bacteria is arranged in a single circular chromosome, unlike the DNA of eukaryote cells, which is arranged in several to many rod-shaped chromosomes. Bacteria also often have additional DNA in the form of smaller rings called plasmids. Bacterial DNA is not enclosed in a nucleus, as is the DNA of eukaryotic cells. Like eukaryotic cells, bacteria have ribosomes, structures active in protein synthesis, but they are smaller and have a slightly different molecular structure.
Many bacteria feature small protrusions from their outside cell surface known as pili (singular, pilus). These hairlike outgrowths assist the bacteria in attaching to teeth, intestines, rocks, and other surfaces. Other hairlike extensions called flagella (singular, flagellum) are much longer than pili and can be found at either or both ends of a bacterium or all over its surface. Flagella beat in a propeller-like motion to help the bacterium move toward nutrients, away from toxic chemicals, or, in the case of the photosynthetic cyanobacteria, toward the light.
Some bacteria form thick-walled structures known as endospores in response to lack of nutrients or water. These thick-walled bodies are extremely resistant to environmental stresses and enable bacteria to survive harsh conditions for decades or even centuries.
REPRODUCTION
A bacterium reproduces by means of a process called binary fission. In binary fission, the single chromosome is replicated, the bacteria divides into two cells, and each cell receives one chromosome. The two cells are thus genetically identical.
Binary fission does not provide bacteria with a way to acquire genetic diversity. Such diversity is necessary to enable a species to withstand changing environments. Many higher organisms gain genetic diversity through the union of reproductive cells from two parents. Lacking this capability, bacteria shuffle DNA between cells by several processes, including transformation, conjugation, and transduction.
In transformation, bacteria take up fragments of DNA released into the soil or water as dead bacteria are decomposed. In conjugation, a donor bacterium attaches itself to a recipient bacterium, generates a tube called a pilus, and transfers fragments of plasmid DNA to the recipient. Transduction involves the transfer of DNA fragments between bacteria cells by a bacteriophage, a virus that infects bacteria. Through mixing genetic material, bacteria develop new traits, including the ability to withstand changes in acidity and temperature, and resistance to antibiotics.
DISEASE
Of the thousands of bacterial species on the earth, only a small fraction cause disease. Nevertheless, the history of human cultures around the world has been greatly shaped by bacterial infections. Outbreaks of plague, the deadly disease caused by the bacterium Yersinia pestis, have killed hundreds of millions of people throughout recorded history. Cholera is caused by Vibrio cholerae, a bacterium found in drinking water that has been contaminated with human feces that harbor the bacteria. Cholera epidemics erupted repeatedly in the 1800s in Europe and Asia, claiming thousands of lives, and the epidemic in South America in the early 1990s caused over 6000 deaths. The bacterium Mycobacterium tuberculosis has caused tuberculosis in millions of people throughout human history. Although the incidence of tuberculosis greatly declined during the mid-20th century, the disease has recently become a major worldwide problem again. One of the primary drugs originally used to cure it, streptomycin, is ineffective today because the overuse of antibiotics has enabled resistant strains of bacteria to evolve. Other bacterial diseases such as certain forms of pneumonia and strep throat are also proving resistant to antibiotics, a cause of grave concern among physicians and other health care professionals.
HISTORY
In the late 17th century, the Dutch microscope maker Antoni van Leeuwenhoek became the first person to systematically study bacteria. Leeuwenhoek spent hundreds of hours to make the finest ground glass for his simple microscopes. Considered the founder of microbiology, he was the first to discover and describe a variety of very minute organisms, many of which we now know were bacteria. Leeuwenhoek's work set the stage for later researchers such as French biologist Louis Pasteur, who showed that microbes do not arise from nonliving matter, as scientists of his day believed, and German scientist Robert Koch, who showed that bacteria could cause disease.
Other notable landmarks in the study of bacteria include the work of the Russian soil scientist Sergei Winogradsky, who in the late 19th century described important energy-yielding metabolic reactions in bacteria and discovered many anaerobic microorganisms. He is considered a founder of microbial ecology. Also in the late 19th century, the Dutch microbiologist Martinus Beijerinck discovered the role of microorganisms in the cycling of nutrients, especially nitrogen. In the 1940s, American microbiologist Selman Waksman discovered a wide range of soil bacteria that produce antibiotics. His discovery paved the way for development of antibiotics, which revolutionized the practice of medicine, since diseases that once were crippling or fatal could be controlled or even cured.
In the 1960s, American microbiologist Lynn Margulis resurrected the forgotten early-20th-century studies by Russian biologists Konstantin S. Mereschkovsky and B. M. Kozo-Polyansky and American biologist Ivan Wallin, and demonstrated the prokaryotic nature of eukaryotes such as plants and animals. Margulis's studies led to the hypothesis of endosymbiosis, the idea that key eukaryotic features such as the energy-generating centers called mitochondria in all animals, plants, and fungi and the photosynthesizing centers called chloroplasts in all algae and plants were derived from ancient bacteria.
Studies of bacteria led to the revolutionary view, put forward in the 1970s by American biologist Carl Woese, that the separation of organisms into prokaryotes and eukaryotes does not represent the fundamental distinction between all organisms. Woese suggested that the archaebacteria, which resemble both prokaryotes and eukaryotes, represent one ancestral lineage, bacteria another, and eukaryotes a third, and proposed that each be placed in their own domain
BACTERIA IN OUR DAILY LIVES
Bacteria are like living paint, covering nearly every surface imaginable and living within a variety of living and nonliving things. Many exist in a symbiotic condition in which they function as partners with other organisms. This symbiosis has profound consequences on people's lives. For example, the agricultural industry depends on the symbiosis between the roots of certain plants and nitrogen fixing bacteria, which transform the nitrogen gas from the atmosphere into ammonia in the soil that plants can use.
Cyanobacteria play an extremely important role in aquatic ecosystems. They are a significant component of the phytoplankton, floating microscopic organisms that carry out photosynthesis. In the process of photosynthesis, cyanobacteria produce the sugar glucose, which is sometimes stored as starch in their cells. Cyanobacteria therefore are a rich food source for zooplankton, floating animal-like microorganisms, which in turn are food for larger aquatic organisms. During photosynthesis, cyanobacteria also release oxygen, which dissolves in the water. A great variety of aquatic organisms rely entirely on this oxygen for their survival. Many scientists are concerned that breakdown of the ozone layer may damage cyanobacteria and other phytoplankton, threatening the survival of the organisms that depend on them for food and oxygen.
Bacteria are also important recyclers. Like fungi, many bacteria feed on dead and dying organisms, breaking down their tissues and cells into nutrient-rich molecules, some of which remain in the soil or water. This process, known as decomposition, is as significant as photosynthesis, for it provides germinating seeds, algae, and other aquatic life forms with the nutrients needed for growth. Further, without decomposition, fallen trees, leaves, and other refuse would simply pile up. Bacteria also strongly influence the movement of key elements, such as sulfur, iron, phosphorus, and carbon, around the globe. The weathering of rocks, which releases elements into the soil and atmosphere, is substantially enhanced by the metabolic activities of certain bacteria.
There is a bacterial species involved with the production of many familiar products. Foods such as cheese and yogurt are developed through the metabolic processes of bacteria. Vinegar, which is used as both a flavor enhancer and an important food preservative, results from the conversion of ethyl alcohol to acetic acid by acetic-acid bacteria. Specific enzymes extracted from bacteria are used in spot removers, meat tenderizers, laundry starches, and household detergents. Bacteria that can digest petroleum are even used in oil-spill cleanups (see Bioremediation). Bacteria have gained enormous importance throughout the biotechnology industry, where they are used in genetic engineering to develop new medications.
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