Bacteria are the simplest life form and are the most numerous organisms with respect to number of species and total biomass. They are small, unicellular pro-caryotic organisms (Figure 3.1). Bacteria are classified by structure (morphology), response to chemical stains, nutrition, and metabolism.

Except for filamentous forms, cyanobacteria, and spirochetes (Figure 3.2), the range in sizes of most bacteria is from 0.3 to 3 |m. Filamentous bacteria such as Sphaerotilus natans usually are >100 |im in length.The unicellular cyanobacteria have photosynthetic pigments in cell membranes and range in size from 5 to 50 |im. Free-living spirochetes commonly are found in wastewater treatment plants and may be up to 50 |im in length.Although the size range for bacteria is described as a diameter or largest side of the bacterial cell, there are many bacteria that are not spherical. The size of these organisms may be described according to their length and width. For example, Escherichia coli, a common bacterium found in human feces and wastewater treatment plants, is approximately 2 |im in length and 0.5 |im in width.

Young bacterial cells of the same species are smaller than old bacterial cells of the same species and have a higher growth rate. Young bacterial cells increase in size only to reproduce. The higher growth rate of young bacterial cells as compared to old bacterial cells is due to the larger surface-to-volume ratio of young bacterial cells. The larger surface-to-volume ratio provides more surface area for the absorption of substrates and nutrients and, consequently, a higher metabolic rate of activity including growth and reproduction.

Most bacteria can be grouped into three basic shapes: bacillus (rod), coccus (sphere), and spirillum (spiral) (Figure 3.3). In some species of bacteria the offspring or daughter cells do not separate after division. The lack of separation results in the formation of several arrangements of bacterial growth including colonies, tetrads, and chains or filaments (Figure 3.3).

FIGURE 3.2 Spircohetes. Spirochetes are highly motile, rigid, and spiral-shaped bacteria.

Coccus-shaped bacteria can divide in one or two planes. Division in one plane produces cells in pairs (diplococcus) or chains (streptococcus). Division in two planes produces cells in grape-like clusters or tetrads. Bacillus-shaped bacteria divide in only one plane and produce cells in chains that are placed end-to-end or side-by-side. Spirillum-shaped bacterial cells usually are not found in multicellular arrangements. Bacteria typically reproduce by binary fission in which a mother cell divides into two equal daughter cells.

FIGURE 3.3 Bacterial shapes and growth patterns. Basic bacterial shapes consist of bacillus (a), coccus (b), and spriillum (c). Due to the lack of separation of bacterial cells after reproduction, several basic patterns of growth result. These patterns include colonies (d), tetrads (e), and chains or filaments (f).

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