What is the Size of Bacterial Cell?
Bacteria are among the smallest of all organisms. Most bacteria range from 0.5 to 2.0 μm in diameter. For comparison, a human red blood cell is about 7.5 μm in diameter. Keep in mind, however, that although we often use diameter to specify cell size, many cells are not spherical in shape.
Some spiral bacteria have a much larger diameter, and some cyanobacteria (formerly called blue-green algae) are 60 μm long. Because of their small size, bacteria have a large surface-to-volume ratio.
For example, spherical bacteria with a diameter of 2 μm have a surface area of about 12 μm2 and a volume of about 4 μm3. Their surface-to-volume ratio is 12:4, or 3:1. In contrast, eukaryotic cells with a diameter of 20 mm have a surface area of about 1,200 μm2 and a volume of about 4,000 μm3.
Their surface-to-volume ratio is 1,200:4,000, or 0.3:1—only one-tenth as great. The large surface-to-volume ratio of bacteria means that no internal part of the cell is very far from the surface and that nutrients can easily and quickly reach all parts of the cell.
What are the Shape of Bacterial Cell?
Typically, bacteria come in three basic shapes—spherical, rodlike, and spiral but variations abound. A spherical bacterium is called a coccus (plural: cocci), and a rodlike bacterium is called a bacillus (plural: bacilli).
Some bacteria, called coccobacilli, are short rods intermediate in shape between cocci and bacilli. Spiral bacteria have a variety of curved shapes. A comma-shaped bacterium is called a vibrio; a rigid, wavy-shaped one, a spirillum (plural: spirilla); and a corkscrew-shaped one, a spirochete. Some bacteria do not fit any of the preceding categories but rather have spindle shapes or irregular, lobed shapes.
Square bacteria were discovered on the shores of the Red Sea in 1981. They are 2 to 4 μm on a side and sometimes aggregate in waffle-like sheets.
Triangular bacteria were not discovered until 1986. Even bacteria of the same kind sometimes vary in size and shape. When nutrients are abundant in the environment and cell division is rapid, rods are often twice as large as those in an environment with only a moderate supply of nutrients.
Although variations in shape within a single species of bacteria are generally small, there are exceptions. Some bacteria vary widely in form even within a single culture, a phenomenon known as pleomorphism.
Moreover, in aging cultures where organisms have used up most of the nutrients and have deposited wastes, cells not only are generally smaller, but also often display a great diversity of unusual shapes.
What are the Arrangements of Bacterial Cell?
In addition to characteristic shapes, many bacteria also are found in distinctive arrangements of groups of cells. Such groups form when cells divide without separating.
Cocci can divide into one or more planes, or randomly. Division in one plane produces cells in pairs (indicated by the prefix Diplo-) or in chains (strepto-).
Division in two planes produces cells in tetrads (four cells arranged in a cube). Division in three planes produces sarcinae (singular: sarcina; eight cells arranged in a cube). Random division planes produce grapelike clusters (staphylo-).
Bacilli divide in only one plane, but they can produce cells connected end-to-end (like train cars) or side-by-side. Spiral bacteria are not generally grouped together. Prokaryotes divide by binary fission, rather than by mitosis or meiosis. New cell wall material grows, and the cell pinches in half through this area. Inside, the chromosome has been duplicated, and one is found in each daughter cell.