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Bacteria are classified based on shared characteristics such as RNA sequences, shape, biochemistry, and outer membrane. They are divided into three domains and exhibit extreme diversity. Classification can be difficult due to the absence of complex structures and DNA transfer. Techniques such as shape, size, Gram stain test, metabolism, and molecular analysis are used for classification. Bacteriologists group similar species together based on observable characteristics.
Classification of bacteria is the process of distinguishing types of bacteria from one another and grouping them based on shared characteristics. Such classification is done within the internationally accepted system of biological taxonomy, or the science of classifying organisms. During the classification of bacteria, a number of things are taken into account, in particular the RNA sequences, but also the shape, biochemistry and characteristics of the outer membrane, among others.
While there are differing views on how organisms should be classified, the current prevailing system divides all life into three domains. Bacteria form one of these domains. They exhibit an extreme degree of diversity and, by number, make up the great majority of known species and, by mass, outnumber all multicellular organisms on earth. Classification of bacteria can be difficult due to a number of factors, most notably the absence of complex structures found in more advanced organisms, as well as the tendency of bacteria to transfer segments of DNA.
Bacteriologists use a number of techniques in classifying bacteria. Shape is the simplest way to distinguish bacteria, and these organisms can exhibit a variety of shapes including rods, spirals, and spheres, among others. Shape is not necessarily a unique characteristic, however, and bacteria grouped into very different classifications can have similar shapes.
Other morphological characteristics, such as size, can vary significantly from species to species, and typical groupings can also aid in classification. Many species tend to form clusters of single cells, the number of which can vary greatly. The presence of external structures, such as tiny tentacle-like constructions called flagella, can also help distinguish species of bacteria.
Another key tool in classifying bacteria is a test known as the Gram stain test, named after a 19th-century microbiologist. This test quantifies bacteria based on the thickness of their outer membrane. Gram-negative bacteria have a very thin membrane and Gram-positive bacteria have a thicker membrane.
Factors such as metabolism and other biochemical distinctions are another tool for classifying bacteria. Bacteria metabolize a wide variety of different compounds and the particular compound or compounds used or converted by a specific bacterium can aid in its identification and classification. Analysis of other biochemical data can also aid this process.
Advances in molecular analysis techniques have allowed bacteriologists to differentiate bacteria based on differences in RNA, as well as specific gene sequences. Further analysis of the total amount of individual RNA and DNA proteins and the ratios in which they occur provide another means for classification. Using some or all of these techniques and observable characteristics, bacteriologists are able to classify bacteria according to species and to group similar species together.
