Arrangements Of Bacteria
The patterns of arrangement of bacteria depend on plane of cell division and ability of cells to remain together after division. The cocci in one, two or three planes. The bacilli in only one place. Division in one planes produce single cells, pairs or chains; division in two planes produce clusters and division in three planes produces packets.
The patterns of arrangement of bacteria are important for identification and classification purpose.
I. The Arrangement Of Cocci :
The cocci shaped bacteria show different patterns of arrangement. The patterns of arrangement. The most common are :
1. Diplococci :
The cells divide in one place and remain attached predominantly in pairs.
2. Streptococci :
The cells divide in one place and remain attached in chains.
3. Tetrads :
The cells divide in two planes and characteristically form tetrads.
4. Staphylococci :
The cell divide in three planes in an irregular pattern to form clusters like bunches of grapes.
5. Sarcinae :
The cells divide in three planes in a regular pattern to form cuboidal packets of cells.
II. The arrangement of bacilli
The bacilli do not show complex patterns of arrangement like cocci because they divide in only one plane (transverse plane ). The bacilli can occur singly, in pairs (diplobacilli) or in chains (streptobacilli). Some species such as Bacillus subtilis to form chains called streptobacilli. Certain bacteria show characteristic arrangement. The examples include:
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| Diplobacilli |
a. Palisade arrangement :The Corynebacterium diphtheriae cells produce group of cells lined side by side like matchsticks in the match box called palisade arrangement.
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| Single cells |
Streptomyces species form long branched multinucleate filaments called hyphae, which collectively form mycelium.
III. The arrangements of spiral bacteria :
The spiral of helical bacteria occur more commonly as unattached single cells.
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| Styoptobacilli |
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| palisade arrangement |
Size of Bacteria
Bacteria show considerable variation in size. The size of bacteria ranges from 0.05 μ to 750 μ. The smallest bacteria known are Nanobacteria that have diameter in the range of 0.05 to 0.2 μ. The biggest bacterium known is Thlomargarita namibiensis. It is 100 to 750 μ in diameter.
The size of bacteria can be measured by a technique known as micrometry. This technique uses an ocular and a stage micrometer. The ocular is calibrated using stage micrometer and size is measured using calibrated ocular. The size can also be measured by photographic micrometry or electron microscope micrometry. A spherical organism is measured in terms of its diameter and rod or spiral form by its length or width. The true length of spiral organism may be calculated by measuring length of each turn of the spiral.
The unit of measurement used in bacteriology is the micron (μ) also called (micrometer (μm)). Other units used are given below:
Micron (μ) = 1/1000th part of millimeter (mm); (i.e., 1000 μ = 1mm)
Nanometer (nm) = 1/1000th part of micron 9i.e., 1000 nm = 1 μ)
Angstrom (A°) = 1/10th part of nanometer (i.e., 10A° = 1 nm)
The cocci have diameter ranging from 0.5 to 3.0 μ. One of the smallest bacteria, Mycoplasma have diameter in the range of 0.14 to 0.3 μ. The size of bacilli, range from 0.15 to 2.0 in diameter and form 0.5 to 20 μ in length. The size of spirochete, range from 0.1 to 0.3 μ in diameter and from 0.5 to 200 μ in length. One of the biggest bacterium, Epulopiscium fishelsoni has size of 80 μ by 600 μ. The average size of E. coli is 1.1 to 1.5 μ wide by 2.0 to 6.0 μ long.
The small size of bacteria gives high surface area to volume ratio. The entry of nutrients and exit of waste is carried out through large surface area. This results in unusually high rate of growth and metabolism. In bacteria, there is no need of circulatory system because the area of cell to be nourisned is very close to surface.