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Bacteria

From Molecular Biology Wiki

At the bottom of the scale of living things there exists a group of organisms to which the name of bacteria is usually applied. These are apparently of very simple structure and may be subdivided into two sub-groups, a lower and simpler and a higher and better developed.


The wide distribution of the Protozoa is exceeded by the Bacteria. Representatives are literally found everywhere: floating with dust particles in the air; in salt and fresh water; in the water of hot springs; frozen in ice; in the upper layers of the soil; and in the bodies of plants and animals. Bacteria have received a considerable notoriety under the names of 'microbes' and 'germs,' owing to the fact that certain types get a living within the human body as parasites and bring about disturbances, chiefly chemical, which we interpret as disease. But aside from these forms, which are relatively few in number, human life and life in general on the Earth could not long continue without their services. It is this aspect of the Bacteria which concerns us at present.

Among the Bacteria are the smallest organisms known!

Some species are less than one fifty-thousandth of an inch in length and much less in breadth. None of the typical forms comes within the range of unaided vision, indeed there is room and to spare for thousands of millions of Bacteria to live in a thimble-full of sour milk.

The small size and similarity of structure of many of the Bacteria render their study particularly difficult, and accordingly. Chief types of Bacteria. A, cocci; B, bacilli; C, spirilla; D, branched filamentous form. (From Buchanan.)

They are grouped and classified largely on the basis of chemical changes which they produce, rather than on structural characteristics. However, there are three chief morphological types: the rod -like forms or Bacilli; the spherical forms or cocci; and the spiral forms or SPIRILLA. Bacilli or cocci may be associated in linear, branching, or plate-like series, or grouped together in colonies.

The individual bacterium is generally regarded as a single cell though in most species there is no definite nuclear body; the chromatin material being distributed in the form of granules throughout the cytoplasm. A cell wall chemically similar to protein is usually present. Some forms show active movements by means of prolongations of the cytoplasm, or flagella, as in the case of the common Spirillum of decaying vegetable infusions.

Reproduction is by a process of cell division which, under favorable conditions, may occur as often as every half hour.

The vast multitude of cells thus produced before long exhaust the food supply and contaminate with excretion products the medium in which they are living, so that further growth is inhibited. Under these circumstances the protoplasm within the cell wall ordinarily assumes a spherical form and secretes.


Types of flagellation in Bacteria. 1, without flagella (atrichous forma) ; 2, with flagella (trichous forms). (From Buchanan.)

A protecting coat about itself, and thus enters upon a resting state. In this spore form the Bacteria can withstand drying and variations in temperature to which in the active state they would readily succumb, and thereby the organisms tide over periods of unfavorable conditions and are ready to start active life again when the opportunity presents itself.



Contents

Forms of Bacteria

The lower forms are the more numerous, and consist of minute unicellular masses of protoplasm devoid of chlorophyll, which multiply by simple fission. Some are motile, others non motile. Their minuteness may be judged of by the fact that in one direction at least they usually do not measure more than (25 microns)These forms can be classified according to their shapes into three main groups:


(1) A group in which the shape is globular. The members of this are called cocci.

(2) A group in which the shape is that of a straight rod the proportion of the length to the breadth of the rod varying greatly among the different members. These are called bacilli.

(3) A group in which the shape is that of a curved or spiral rod. These are called spirilla. The full description of the characters of these groups will be more conveniently taken later. In some cases, especially among the bacilli, there may occur under certain circumstances changes in the protoplasm whereby a resting stage or spore is formed.

The higher forms show advance on the lower along two lines, (i) On the one hand they consist of filaments made up of simple elements such as occur in the lower forms. These filaments may be more or less septate, may be provided with a sheath, and may show branching either true or false.


Video of Bacterial Forms

There are many different species of bacteria. Scientists often classify bacteria on the basis of shape, size, and other morphological characteristics, as well as the genetic material that each type of bacteria contains. The Gram stain is one important tool that scientists use to identify and characterize bacteria.

History of Bacteriology

see Bacteriology


Morphology and Structure of Bacteria

The minute structure of the elements comprising these filaments is analogous to that of the lower forms. Their size, however, is often somewhat greater. The lower forms sometimes occur in filaments, but here every member of the filament is independent, while in the higher forms there seems to be a certain inter-dependence among the individual elements. For instance, growth may occur only at one end of a filament, the other forming an attachment to some fixed object. (2) The higher forms, more over, present this further development that in certain cases some of the elements may be set apart for the reproduction of new individuals.

While no "living" organisms lower than the bacteria are known (though the occurrence of such is now suspected such as viruses however they may not be classified as living organisms), the upper limits of the group are difficult to define, and it is further impossible in the present state of our knowledge to give other than a provisional classification of the forms which all recognise to be bacteria. The division into lower and higher forms, however, is fairly well marked, and we shall therefore refer to the former as the lower bacteria, and to the latter as the higher bacteria.

For more information see: Bacterial morphology

Morphological Relations of Bacteria

The relations of the bacteria to the animal kingdom on the one hand and to the vegetable on the other constitute a some what difficult question. It is best to think of there being a group of small,, unicellular organisms, which may represent the most primitive forms of life before differentiation into animal and vegetable types had occurred. This would include the flagellata and infusoria, the myxomycetes, the lower algae, and the bacteria. To the lower algae the bacteria possess many similarities. These algae are unicellular masses of protoplasm, having generally the same shapes as the bacteria, and largely multiply by fission. Endogenous sporulation however, does not occur, nor is motility associated with the possession of flagella. Also their protoplasm differs from that of the bacteria in contain ing chlorophyll and another blue-green pigment called phycocyan. From the morphological resemblances, however, between these algae and the bacteria, and from the fact that fission plays a predominant part in the multiplication of both, they have been grouped together in. One class as the Schizophyta or splitting plants (German, Spaltpflanzen) . And of the two divisions forming these Schizophyta the splitting algae are denominated the schizophyceae (German, Spaltalgen), while the bacteria or splitting fungi are called the schizo mycetes (German, Spaltpilzen). The bacteria are, therefore, often spoken* of as the schizomycetes. Certain bacteria which have been described as con taining chlorophyll ought probably to be grouped among the schizophyceae.


Reproduction among the Lower Bacteria

When a bacterial cell is placed in favourable surroundings it multiplies ; as has been said, this, in the great majority of cases, takes place by simple fission. In the process a constriction appears in the middle and a transverse unstained line develops across the protoplasm at that point. The process goes on till two individuals can be recognised, which may remain for a time attached to one another, or become separate, according to the character of the envelope, as already explained. In most bacteria growth and multiplication go on with great rapidity. A bacterium may reach maturity and divide in from twenty minutes to half an hour. If division takes place only every hour, from one individual after twenty-four hours 17,000,000 similar individuals will be produced. As shown by the results of artificial cultivation, others, such as the tubercle bacillus, multiply much more slowly. Sometimes division proceeds so rapidly that the young individuals do not reach the adult size before multiplication again occurs. This may give rise to anomalous appearances. When bacteria are placed in unfavourable conditions as regards food, etc., growth and multiplication take place with difficulty. In the great majority of cases this is evidenced by changes in the appearance of the protoplasm. Instead of its maintaining the regularity of shape seen in healthy bacteria, various aberrant appearances are presented. This occurs especially in the rod shaped varieties, where flask-shaped or dumb-bell-shaped individuals may be seen. The regularity in structure and size is quite lost. The appearance of the protoplasm also is often altered. Instead of, as formerly, staining well, it does not stain readily, and may have a uniformly pale, homogeneous appear ance, while in an old culture only a small proportion of the bacteria may stain at all. Sometimes, on the other hand, a degenerated bacterium contains intensely stained granules or globules which may be of large size. Such aberrant and degenerate appearances are referred to as involution forms. That these forms really betoken degenerative changes is shown by the fact that, on their being again transferred to favourable conditions, only slight growth at first takes place. Many indi viduals have undoubtedly died, and the remainder which live and develop into typical forms may sometimes have lost some of their properties.


Reproduction of Higher Bacteria

Most of the higher bacteria consist of thread-like structures more or less septate, and often surrounded by a sheath. The organism is frequently attached at one end to some object or to another individual. It grows to a certain length, and then at the free end certain cells called gonidia are cast off, from which new individuals are formed. These gonidia may be formed by a division taking place in the terminal ele ment of the filament, such as has occurred in the growth of the latter. In some cases, however, division takes place in three dimensions of space. The gonidia have a free existence for a certain time before becoming attached, and in this stage are sometimes motile. They are usually rod-like in shape, sometimes pyriform. They do not possess any special powers of resistance.


Bacteria Sporulation

See Sporulation.


Bacterial Motility

Many bacteria are motile.

Motility can be studied by means of hanging-drop preparations. The movements are of a darting, rolling, or vibratile character.

The degree of motility depends on the temperature, on the age of the growth, and on the medium in which the bacteria are. Sometimes the movements are most active just after the cell has multiplied, sometimes it goes on all through the life of the bacterium, sometimes it ceases when sporulation is about to occur. Motility is associated with the possession of fine wavy thread-like appendages called flagella or cilia, which for their demonstration require the application of special staining methods. They have been shown to occur in many bacilli and spirilla, but only in a few species of cocci. They vary in length, but may be several times the length of the bacterium, and may be at one or both extremities or all round. When terminal they may occur singly or there may be several. The nature of these flagella has been much disputed. Some have held that, unlike what occurs in many algae, they are not actual prolongations of the bacterial protoplasm, but merely appendages of the envelope, and have doubted whether they are really organs of locomotion. There is now, however, little doubt that they be long to the protoplasm. By appropriate means the central parts of the latter can be made to shrink away from the peripheral (vide infra, " plasmolysis "). In such a case movement goes on as before, and in stained preparations the flagella can be seen to be attached to the peripheral zone. It is to be noted that flagella have never been demonstrated in non-motile bacteria, while, on the other hand, they have been observed in nearly all motile forms. There is little doubt, however, that all cases of motility among the bacteria are not dependent on the possession of flagella, for in some of the special spiral forms, and in most of the higher bacteria, motility is probably due to contractility of the protoplasm itself.

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