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.