Reproduction and growth
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Bacteria reproduce primarily by binary fission, an asexual process whereby a single cell divides into two. Under ideal conditions some bacterial species may divide every 10–15 minutes—a doubling of the population at these time intervals. Eukaryotic microorganisms reproduce by a variety of processes, both asexual and sexual. Some require multiple hosts or carriers (vectors) to complete their life cycles. Viruses, on the other hand, are produced by the host cell that they infect but are not capable of self-reproduction.
The study of the growth and reproduction of microorganisms requires techniques for cultivating them in pure culture in the laboratory. Data collected on the microbial population over a period of time, under controlled laboratory conditions, allow a characteristic growth curve to be constructed for a species.
Metabolism
Collectively, microorganisms show remarkable diversity in their ability to produce complex substances from simple chemicals and to decompose complex materials to simple chemicals. An example of their synthetic ability is nitrogen fixation—the production of amino acids, proteins, and other organic nitrogen compounds from atmospheric nitrogen (N2). Certain bacteria and blue-green algae (cyanobacteria) are the only organisms capable of this ecologically vital process. An example of microbes’ ability to decompose complex materials is shown by the white and brown rot fungi that decompose wood to simple compounds, including CO2.
Laboratory procedures are available that make it possible to determine the biochemical capability of a species qualitatively and quantitatively. Routine techniques can identify which compounds or substances are degraded by a specific microbe and which products are synthesized. Through more elaborate experimentation it is possible to determine step-by-step how the microbe performs these biochemical changes. Studies can be performed in a number of ways using growing cultures, “resting cells” (suspensions of cells), cell-free extracts, or enzyme preparations from cells.
Certain biochemical tests are routinely used to identify microbes—though more in the case of bacteria than algae, fungi, or protozoa. The adoption of routine sets of laboratory tests has allowed automated instrumentation to perform the tests. For instance, technicians often simply inoculate individual units of a “chamber” that is preloaded with a specific chemical substance (the substrate) and then place the chamber into an apparatus that serves as an incubator and analyzer. The apparatus automatically records the results and is frequently capable of calculating the degree of accuracy of the identification.
Pathogenesis
Some microorganisms cause diseases of humans, other animals, and plants. Such microbes are called pathogens. Pathogens are identified by the hosts they infect and the symptoms they cause; it is also important to identify the specific properties of the pathogen that contribute to its infectious capacity—a characteristic known as virulence. The more virulent a pathogen, the fewer the number needed to establish an infection.
Antigenic characteristics
An antigen is a substance that, when introduced into an animal body, stimulates the production of specific substances (antibodies) that react or unite with the antigen. Microbial cells and viruses contain a variety of antigenic substances. A significant feature of antigen-antibody reactions is specificity; the antibodies formed as a result of inoculating an animal with one microbe will not react with the antibodies formed by inoculation with a different microbe. Antibodies appear in the blood serum of animals, and laboratory tests of antigen-antibody reactions are performed by using sera—hence the term serological reactions. Thus, it is possible to characterize a microorganism by its antigenic makeup as well as to identify microorganisms by using one of many different serological tests. Antigens and antibodies are important aspects of immunity, and immunology is included in the science of microbiology.
Genetic characterization
Since the last quarter of the 20th century, researchers have accumulated a vast amount of information elucidating in precise detail the chemical composition, synthesis, and replication of the genetic material of cells. Much of this research has been done by using microorganisms, and techniques have been developed that permit experimentation at the molecular level. For instance, experiments determining the degree of similarity between different organisms’ DNA and RNA have provided new insights for the classification of microorganisms. Test kits are available for the identification of microorganisms, particularly bacteria, by DNA probes.
Since the invention of recombinant DNA technology in 1973, techniques have been developed whereby genes from one cell can be transferred to an entirely different cell, as when a gene is transferred from an animal cell to a bacterium or from a bacterium to a plant cell. Recombinant DNA technology opened the door to many new medical and industrial applications of microbiology, and it plays a central role in genetic engineering.
Applied microbiology
Genetic engineering is an example of how the fields of basic and applied microbiology can overlap. Genetic engineering is primarily considered a field of applied microbiology (that is, the exploitation of microorganisms for a specific product or use). The methods used in genetic engineering were developed in basic research of microbial genetics. Conversely, methods used and perfected for applied microbiology can become tools for basic microbiology. Applied microbiology can, however, be divided under the following headings.
Soil microbiology
However “dead” soil may appear, it is in fact teeming with millions or billions of microbial cells per gram, depending upon soil fertility and the environment. Dead vegetation, human and animal wastes, and dead animals are deposited in or on soil. In time they all decompose into substances that contribute to soil, and microbes are largely responsible for these transformations.
Two great pioneer soil microbiologists were Martinus W. Beijerinck (1851–1931), a Dutchman, and Sergey N. Winogradsky (1856–1953), a Russian. These researchers isolated and identified new types of bacteria from soil, particularly autotrophic bacteria, that use inorganic chemicals as nutrients and as a source of energy. The relationship between legumes and bacteria in the nodules of legume roots was discovered by other scientists in 1888. The nodules contain large numbers of bacteria (Rhizobium) that are capable of fixing atmospheric nitrogen into compounds that can be used by plants.
The ecology of fertile soil consists of plant roots, animals such as rodents, insects, and worms, and a menagerie of microorganisms—viruses, bacteria, algae, fungi, and protozoa. The role of this microbial flora can be conveniently expressed in Earth’s natural cycles. In the nitrogen cycle, for example, microorganisms capture nitrogen gas from the atmosphere and convert it into a combined form of nitrogen that plants can use as a nutrient; the plant synthesizes organic nitrogen compounds that are consumed by humans and animals; the consumed nitrogen compounds eventually reach the soil; microorganisms complete the cycle by decomposing these compounds back to atmospheric nitrogen and simple inorganic molecules that can be used by plants. In similar cycles for other elements such as carbon, sulfur, and phosphorus, microbes play a role; this makes them essential to maintaining life on Earth.
Microbiology of water supplies, wastewater, and other aquatic environments
Long before the establishment of microbiology as a science, water was suspected of being a carrier of disease-producing organisms. But it was not until 1854, when an epidemic of cholera was proved to have had its origin in polluted water, that contaminated water was considered more seriously as a source of disease. Since that time there has been continuous research on the microbiology of public water supplies, including the development of laboratory procedures to determine whether the water is potable, or safe for human consumption. At the same time, purification procedures for these supplies have emerged.
A highly standardized and routine laboratory procedure to determine the potability of water is based upon detecting the presence or absence of the bacterium Escherichia coli. E. coli is a normal inhabitant of the intestinal tract of humans; its presence in water indicates that the water is polluted with intestinal wastes and may contain disease-producing organisms.
The principal operations employed in a municipal water-purification plant are sedimentation, filtration, and chlorination. Each of these operations removes or kills microorganisms, and the microbiological quality of the treated water is monitored at frequent intervals.
The used water supply of a community, commonly referred to as sewage, is microbiologically significant in two ways. First, sewage is a potential carrier of pathogenic microorganisms, so measures such as chlorination must be implemented to prevent these microbes from contaminating drinking-water supplies. Second, sewage-treatment plants purify water by exploiting the biochemical abilities of microbes to metabolize contaminants. Raw sewage is processed through large tanks, first for anaerobic degradation of complex substrates and later for aerobic oxidation of soluble products. This “activated sludge” treatment is dependent upon incubation conditions that favour the growth and metabolic activity of appropriate microorganisms.
Another aspect of the microbiology of water pertains to natural bodies of water such as ponds, lakes, rivers, and oceans. Aquatic microbes perform a host of biochemical transformations and are an essential component of the food chain in these environments. For example, the microbial flora of the sea comprises bacteria, algae, fungi, and protozoa. The microorganisms inhabiting aquatic environments are collectively referred to as plankton; phytoplankton refers to the photosynthetic microbes (primarily algae), whereas protozoa, and other small animals, are zooplankton. Phytoplankton is responsible for converting solar energy into chemical energy—the components of plankton cells that serve as food for higher aquatic life. The magnitude of this process can be appreciated by calculations indicating that it takes 1,000 tons of phytoplankton to support the growth of one ton of fish.
Large populations of archaea live in volcanic ridges 2,600 metres (8,500 feet) below the ocean surface in areas immediately surrounding hydrothermal vents (deep-sea hot springs). The vents spew superheated water (350 °C [662 °F]) that contains hydrogen sulfide (H2S); the water surrounding the vents has a temperature range of 10–20 °C (50–68 °F). Many bacteria concentrate in this region because of the availability of H2S, which they can use for energy. The abundance of animal life that also inhabits this region is completely dependent on the microbes for food.
There is a growing interest in other ecological aspects of aquatic microbiology, such as the role of microbes in global warming and oxygen production. Experimental approaches are being developed to study the complex biology and ecology of biofilms and microbial mats. These assemblages of microbes and their products, while potentially useful in several ways, are complex. In many instances the microbial flora involved must sometimes be studied in its natural environment because the environment cannot be reproduced in the laboratory.
