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The below mentioned article provides a study note on bacterial motility.
Some bacteria are self-motile. They are able to swim through the liquid in which they live. They cannot crawl over dry surfaces or fly through the air. Motility is universal among the spirilla, common among the bacilli but lacking or rare in coccal forms.
The organs of locomotion are small whips or hair-like appendages called the flagella. These fine filaments are of uniform thickness. They are about 120A°-150A° thick and 4 or 5µ long.
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They are cytoplasmic nature. Each flagellum arises within the cytoplasmic membrane but very in nature. Each flagellum arises within the cytoplasmic membrane but very close to it from a granule called a blepharoplast and passes out through the cell wall.
1. Flagellation (Distribution of Flagella):
The flagella are distributed over the surface of the bacterial cell in a characteristic manner. Their number, position and arrangement varies with the species.
They may be restricted to one or both the ends of the bacterium cell (polar flagellation) or may be distributed uniformly all over the body surface (non-polar flagellation). Bipolar flagellation is of rare occurrence.
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On the basis of number and arrangement of flagella, bacteria are classified into the following categories:
A. Polar Flagellation (Fig. 18.5 A-D):
This type of flagellation is restricted to a rather homogenous group of bacilli and spirilla. They are all gram-negative.
Polar flagellation is of the following four types:
(a) Monotrichous:
In this case the bacterium cell bears only single flagellum which is inserted at or near one pole of the cell (A). The flagellum has more than two curves. All vibrios (Vibrio cholerae and V. metschnikovii) are monotrichous.
(b) Amphitrichous:
There is one flagellum at each pole of the bacterium cell (B). This type of flagellation is of are occurrence.
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(c) Cephalotrichous (Thimman, 1959):
There are two or more flagella in a bunch at one pole of the bacterium cell (C). Rod-shaped bacterium, Pseudomonas fluoresces is an example of this category. The flagellum shows more than two curves.
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(d) Lophotrichous (Thimman, 1959):
There are two or more flagella at both the poles of the bacterium cell (D). The flagella show one or two curves. The spirilla are Pseudomonas fluoresces the common examples (Spirillum volutans).
B. Non-polar Flagellation (Fig. 18.5 E):
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In this case the flagella are found all over the cell surface. It is of one type only.
(e) Peritrichous (E):
The bacterial cell bears a large number of flagella which are distributed evenly all over the cell surface. Proteus vulgaris and Bacillus typhosus are the common examples. In fact, peritrichous flagellation is more commonly found among the bacteria than polar flagellation.
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(f) Atrichous (Fig. 18.5 F):
All bacteria which lack flagella are called atrichous. Diptheria bacilli and Lactobacillus—a milk souring group of bacteria—are entirely without flagella and thus non- motile. The passive type of movement is exhibited by most of the bacteria.
It is purely physical and is also known as Brownian movement. This type of movement is shown by all small particles suspended in a liquid. The movement of bacterial flagella is screw-type motion in three dimensions.
2. Structure of Bacterial Flagellum (Fig. 18.6 A-C):
The bacterial flagella are the most primitive of all motile organs. Structurally they are unique. Each is composed of a single thin fibril as against the 9+2 pattern of fibrillar structure characteristic of eucaryotic motile cell types.
The bacterial flagellum is thus equivalent to one of the peripheral half fibrils (microfibrils) of the flagellae of all motile eukaryotic organisms. Electron microcopy reveals that the bacterial flagellum is helical in shape.
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Because of its spiral patterned surface it apparently seems to consist of a few fine fibrils twised tightly into a rope-like structure coiled in an open wave form (A). The flagellum is composed entirely of protein which is Flageliin.
It gives a typical helical diffraction pattern in the electron microscope. Its molecular weight is about 40,000. According to Lowy and Hanson (1965), the bacterial flagellum, under high resolution of electron microscope, is seen to be composed of globular subunits arranged in helices of various kinds.
The diameter of each subunit is approximately 40-50A (Angstrom units). The flagellum is composed of several chains of subunits (flagellin molecules) forming a hollow cylindrical thread.
There is no axoneme. Whether the hollow core is empty or filled with a similar or different protein is not known. The number and orientation of chains in the flagellum varies with the species. It is not a uniform feature of all bacterial flagella.
In Salmonella, for example, the flagellum consists of four helices or spiral chains of subunits as shown in figure 18.6 B. One of the chains is shown by black globules. Ringo (1967) investigated the molecular structure of the flagellum of a species of Chlamydomonas.
He reported that in a cross section each of the peripheral half fibrils (microfibril) consists of a ring of 13 subunits surrounding a homogeneous core. The diameter of each subunit is 40A which corresponds to a protein with a molecular weight of approximately 40,000.
Pili (Fig. 18.7):
These are minute, straight, hair-like, non-flagellar appendages found extending out from the cell wall of many gram- negative bacteria. Like flagella they can only be seen in electron micrographs. They are somewhat rigid and composed entirely of a protein known as pilin.
They may be several microns long but are smaller in diameter which is around 30-50A. There may be one hundred to four hundred pili or fimbriae distributed over the surface of the bacterium cell. Pili are considered as the organelles of attachment.
