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In this article we will discuss about Bacteria:- 1. Definition of Bacteria 2. Characteristics of Bacteria 3. Practical Work.
Definition of Bacteria:
Bacteria can be defined briefly as minute microorganisms with a prokaryotic form of cellular organisation. They are usually unicellular, they may divide by binary fission and grow attached to one another in clusters, chains, filaments (hyphae) or a mycelium (Actinomycetales—higher bacteria). They usually measure from 0.4-to 1.5 micron, smaller than yeast, fungi and protozoa and are devoid of chlorophyll.
They have rigid cell walls which cover very closely cytoplasmic membrane and are responsible for the maintenance of their characteristic shape; the shape may be:
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1. Spherical (Coccus);
2. Rod shaped (Bacillus);
3. Comma shaped (Vibrio);
4. Spiral (Spirillum and Spirochaete);
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5. Filamentous (Actinomyces)—Fig. 2.1.
Their nuclei can be seen only under electron microscope, but not under light microscope. A capsule which is outside the cell wall is a protective gelatinous layer, if it is too thin, it is called microcapsule.
There are two filamentous appendages protruding outwards from the bacterial cell wall:
(a) Flagella which are organs of locomotion, and
(b) Fimbriae (syn, Pilli) which are organs of adhesion.
The cell wall, capsule, microcapsule, flagella and fimbriae play a special role in the process of infection.
Characteristics of Bacteria:
In addition to these morphological features, bacteria can be identified by the following characteristics after staining which will be performed in the next practical.
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(a) Shape arid manner of grouping of bacteria manner of grouping or characteristic arrangement of bacteria depend upon their mode of division e.g. axis of division and tendency of the daughter cell to adhere to one another, Cocci form clusters (Staphylococci—Gr. Staphyle, cluster of grapes) or chains (Streptococci—Gr. Streptos, chain).
(b) Size of Bacteria Cocci their size ranges from 0.4 to 2 microns in diameter, bacilli vary from 1.2 to 12 microns in length; spirilla have variable length of 114 microns. Size of bacteria can be measured by micrometry or by electron microscopy.
(c) Motility of bacteria it is due to the presence of organ of locomotion i.e. flagella— hair-like appendages. Flagella can be demonstrated by Leifson’s flagellar staining. Hanging drop preparation is routinely adopted method to demonstrate the motility.
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Instead of using a hanging drop preparation, a film of the fluid between a microscopic slide and coverslip may be used, but in this case the edge of the coverslip should be sealed with vaseline to prevent evaporation of the fluid.
Other methods are by using semi-solid agar, by agglutination with specific antisera, by dark illumination, fluorescence or electron microscopy. Spirochaetes move with the help of long filaments applied to their bodies showing rotatory cork screw-like motility. Campylobacter jejuni, Vibrio cholerae, and Spirillum minus show darting motility.
True motility of bacteria is indicated by their change in position in relation to their surroundings. It differs from Brownian movement which is a peculiar motion possessed by all fine or minute particles suspended in fluid (Table 2.1).
(d) Spore of Bacteria—Bacteria can be spore (Fig. 2.1) or non-spore formers.
Practical Work for Detection of Bacteria:
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A. Hanging Drop Preparation:
1. A glass slide with a circular concavity in its centre is used;
2. A thin ring of vaseline is outlined around the concavity by means of a matchstick dipped in vaseline (petroleum jelly);
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3. With the help of a cooled sterile platinum wire loop, a loopful of fluid containing organisms was taken from bacterial suspension (or, alternatively, a small amount of culture from the surface of a solid medium is thinly emulsified in a drop of normal saline) and placed on the coverslip laid on the bench;
4. The slide is inverted over the coverslip so that it will adhere to vaseline;
5. It is quickly turned around the slide so that the coverslip is uppermost and the drop should then be “hanging” from the coverslip in the centre of concavity;
6. The slide is placed on the stage of the microscope;
7. The condenser is slightly racked down and the diaphragm is partially closed (excess of illumination renders the organisms invisible);
8. The edge of the drop of fluid is focused with the low power objective so that it appears across the centre of the field;
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9. The high power lens is turned in position and the edge of the drop is focused; it is advisable to use the high power dry lens, NOT the oil immersion objective (Fig. 2.2).
B. Simple Staining Method:
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This method shows the presence of organisms and the nature of cellular content in the exudate. The fixed smear is covered with dilute oxalated gentian violet for 2 minutes, then it is washed with running tap water for half a minute.
The smear is dried between the two folds of a filter paper. Shape and arrangement of microorganisms can be studied under oil immersion objective, then the observations should be entered in the practical notebook.
