Classification of Bacteria

There are thousands of species of bacteria on earth, many of which have not yet been identified. When attempting to classify a bacterium, a variety of characteristics are used, including visual characteristics and laboratory tests. Bacteria are simple, unicellular organisms. Most are free-living organisms, but a few require animal or plant hosts for survival. Bacteria absorb nutrients from their environments, excrete waste products, and secrete various toxins that help them invade tissues. Bacteria have no enclosed nucleus. Their chromosomal material is in the form of a large loop, packed into the cytoplasm of the cell. Some bacteria can be identified through a simple visual perusal. First, the operator considers the appearance of the bacterial colony (a group of the same kind of bacteria growing together, often on a petri dish.) The operator also views individual bacteria under a microscope, considering their shape, groupings, and features such as the number and location of flagella. A variety of laboratory techniques can be used to narrow down the identity of a bacterial species if a visual survey is not sufficient. The operator can stain the bacteria using a gram stain or an acid-fast stain. The bacteria can be cultured on a specific medium which promotes the growth of certain species, as in the membrane filter method of testing for coliform bacteria. Other tests can detect bacterial by-products, while yet more advanced tests actually analyze the DNA of the bacteria.

Bacterial Shapes The most basic method used for identifying bacteria is based on the bacterium's shape and cell arrangement. This section will explain the three morphological categories which all bacteria fall into - cocci, bacilli, and spirilla. You should keep in mind that these categories are merely a way of describing the bacteria and do not necessarily refer to a taxonomic relationship. The most common shapes of bacteria include rod, cocci (round), and spiral forms. Cellular arrangements occur singularly, in chains, and in clusters. Some species have one to numerous projections called flagella enabling the bacteria to swim, making them motile organisms.

Bacilli singular: Bacillus), rod-shaped bacteria; the most numerous of all types. They include coccobacilli and streptobacilli.

Cocci (singular: Coccus), spherical bacteria. This group is divided into bacteria that occur in pairs, such as the diplococci; in clusters, such as the staphylococci; and in chains, such as the streptococci.

Spirilla (singular: Spirillum), spiral-shaped bacteria; the least numerous type. Some scientists add a fourth category, the vibrios (S-shaped or comma-shaped bacteria) to this list

Gram Stain Procedure The most fundamental technique for classifying bacteria is the gram stain, developed in 1884 by Danish scientist Cristian Gram. It is called a differential stain because it differentiates among bacteria and can be used to distinguish among them, based on differences in their cell wall. In this procedure, bacteria are first stained with crystal violet, then treated with a mordant - a solution that fixes the stain inside the cell. The bacteria are then washed with a decolorizing agent, such as alcohol, and counterstained with safranin, a light red dye. The walls of gram-positive bacteria (ie. Staphylococcus aureus) have more peptidoglycans (the large molecular network of repeating disaccharides attached to chains of four or five amino acids) than do gram-negative bacteria. Thus, gram-positive bacteria retain the original violet dye and cannot be counterstained. Gram-negative bacteria (ie. Escherichia coli) have thinner walls, containing an outer layer of lipopolysaccharide, which is disrupted by the alcohol wash. This permits the orignial dye to escape, allowing the cell to take up the second dye, or counterstain. Thus, gram-positive bacteria stain violet, and gram-negative bacteria stain pink. The gram stain works best on young, growing populations of bacteria, and can be inconsistent in older populations maintained in the laboratory.

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