Bacteriology is the subdivision of microbiology that deals with the identification, classification, and characterization of bacteria. It also finds ways to make efficient use of bacteria in industry and medicine, to fight bacteria that are dangerous to human life or property, and to gather pure knowledge about the world in microcosm.
The classification of bacteria is the identification of groups of bacteria with common properties. This is bacterial systematics, or taxonomy. One way they can be classed is by gram reaction. This is done by staining an entire sample with a violet-iodine stain in a process invented by Christian Gram in 1884. The sample is then treated with alcohol or acetone, which will not remove the stain from gram-positive bacteria. In gram-negative bacteria the stain will disappear, so that these microbes must be re-stained red to be visually examined.
Bacteria are also classified according to their shape. They may be bacilli, rod-shaped; cocci, spheres; or spirochetes, spirals. Another group of bacteria, the mycobacteria, are amorphous in form. An additional way to group bacteria is into anaerobic and aerobic specimens, depending upon whether they need oxygen to live, or need to shun it. And bacteria are classified by the conditions that suit them, such as a certain temperature range or pH reading.
Today, bacteria are classed into clades by similarities in their genome (DNA or RNA) and also in their proteome (proteins in their cells). Clades are groups of bacteria that share common characteristics because they are descended from the same ancestor. The classification is complicated by Lateral Gene Transfer, in which two unrelated bacteria can actually exchange genetic information. Bacteriologists have developed techniques to get around this problem, such as comparing rRNA, which is less prone to lateral transfer. Most existing bacteria have not been classified as yet, and most cannot be cultured in the laboratory, so far.
There are about 9,000 known species of bacteria. It is estimated that there are 10,000,000 to 1,000,000,000 existing species. The vast majority of the known bacteria are benign, helping animals digest food, breaking down organic matter into a form useful to plant life, and producing substances useful to science, medicine, and agriculture.
Identification of bacteria is done by placing an unknown specimen within the proper group. This can be crucial in a medical situation, when doctors seek the right cure for an unknown disease. Bacteria may be identified using the gram stain, and by visual examination and cultural preferences, but increasingly they are identified using the tools of genetic analysis, such as PCR. Polymerase Chain Reaction multiplies the quantity of DNA or RNA in a sample to the point that it can be analyzed.
Bacteriology has many practical applications. Bacteria can now produce insulin, growth factors, and antibodies. Additional useful medicines should soon be available. Bacteria are currently used to digest some oil spills, and there is room for improvement in this field as well. PCR depends upon an enzyme made by thermophilic bacteria to do its work. In the food industry, bacteriologists improve the creation of bacteria-produced foods like cheese and yogurt, as well as safeguarding against bacterial pathogens.
Bacteriology is a great and growing field, with much to offer the student and the world.