Bacteria may be defined as a group of microbes sharing the basic features of the survival means adopted by a prokaryote (a cellular being without nucleus and organelles), single-celled organism. Bacteria are amongst the earliest life forms on the earth.

Bacteria are present in extremely branched out groups that comprise over 2,000 species, each differing from the other relating to their form, structure of their cells, metabolic competencies, nutritional necessities and even favored environments.

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From the point of morphology, on average the bacteria measure about anything between one and two micrometers in diameter. Each bacterium has a cell wall that is present on the cell membrane's outer surface and the organism's typical shape is attributable to it.

The bacterial cytoplasm is located within the cell membrane and it encloses a variety of substances, counting ribosome; but its cytoplasm does not enclose any membrane-bound organelles, for instance, mitochondria or chloroplasts. Being prokaryotic organisms, bacteria also do not possess a distinct nucleus.

However, their cells do enclose a solitary, spherical DNA molecule, which is also known as bacterial chromosome - a substance that encloses all essential genes.

A number of bacteria species are also likely to enclose a relatively smaller portion of DNA known as plasmid, which predetermines or instructs all supplementary functions of the organism. In addition, bacteria may either be inactive or dynamic owing to the existence of an extended appendage called flagella.

Bacteria multiply by means of an uncomplicated method called binary fission, whereas the mother (a solitary cell) increases two-fold both in size as well as quantity of diverse constituents prior to dividing into two approximately identical progeny cells.

It has been found that different types of bacterium have a very wide range of enzymes that facilitate metabolism and also make it possible for them to survive in an extensive variety of ecological conditions, in addition to helping them in obtaining nourishments as well as vigour from an assortment of sources, counting the sunlight as well as organic and inert chemicals.

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Bacteria acquire their nourishments by assimilation of a variety of forms of food molecules, but do not take in large particles of food or any other living being. Irrespective of their aptitude to undertake photosynthesis, all bacteria have one common aspect - they are all unable to make oxygen.

Majority types of bacteria exist freely geographically and owing to their dissimilar metabolic competences, they inhabit nearly in all places of the environment.

It is worth mentioning here that bacteria possess the aptitude to serve several important jobs that are vital for the survival of humans, for instance, bacteria have the ability to ferment milk to form cheese and yogurt as well as manufacture vitamins in the stomach.

In spite of these beneficial actions, bacteria may also cause a number of most awful risks for our health as well as comfort. They are also responsible for most of the main diseases that affect humans.

As aforementioned, bacteria are present in diverse shapes. Those that usually have a circular shape are known as cocci (singularly called coccus). The term 'coccus' has been derived from the Latin form of a Greek word 'kokkos' denoting a berry.

It may be noted that the bacteria cocci do not have a precisely spherical shape and, often they may be somewhat noticeably deformed. Generally, Streptococcus pneumonia cells exist in pairs and are called diplococci, which have a compressed appearance similar to that of tiny lancets.

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Owing to their particular shape and occurrence in pairs, this bacterium is often called lanceolate diplococcic. The bacterium Neisseria gonorrhoeae is one more type of diplococcus having cells that seem to be compressed against each other. Streptococcus pneumonia cells are arranged in a line along their extended axis having tiny axes occurring correspondingly.

On the other hand, Neisseria gonorrhoeae cells make a parallel line the length of their tiny axis, having extended axes separately. In addition, cocci may possibly also occur in bunches of in excess of two cells.

On the other hand, the bacteria Streptococci occur in the shape of chains owing to cell division on a singular surface, whereas the bacteria Staphylococci appear in clusters akin to that of grapes owing to cell division all along irregular surfaces.

The term streptos meaning twisted is taken from Greek; while it is worth mentioning here that the term 'Staphylococci' has been derived from the Greek word 'staphule' denoting grapes. In effect, bacteria also have the aptitude to grow in the cuboidal packets because of normal division of cells the length of three planes.

The word 'bacillus' ('bacilli' in plural) has been drawn from the Latin term 'bacillus' denoting a stick. Therefore, as the name suggests, the shape of the bacteria Bacilli is akin to that of rods and they are found in various forms.

Although they belong to one genus of bacteria, they are found to exist in extensively varied shapes. For instance, the cells of the bacterium Clostridium peifringens are comparatively small and stout and the shape of the vegetative (asexual) cell is not distorted despite the occurrence of spore (reproductive structure), while the cells of the bacterium Clostridium tetani are extended as well as thin.

A lot of times, the cells are distended at their end with a view to provide accommodation to their spore. This makes the Clostridium tetani cells appear like drumsticks.

In the same manner in which several cocci position themselves in particularly twisted clusters, a number of bacilli are also related to specific arrangements. For instance, the bacterium Bacillus anthracis, the corroding agent present in anthrax, develops in the form of an extended string of cells arranged the length of an elongated axis.

The cells belonging to the bacterial genus Beggiatoa, as well as cells of several cyanobacteria take the shape of chains wherein the cells are connected intimately. These cells arrange themselves in elongated strings wherein every cell is partitioned by means of cross-walls. This type of strain is known as trichomes, as they appear like hairs.

It may be noted that the term 'trichome' has been derived from the Greek word 'trikhos' denoting hair. Bacteria belonging to the genus Corynebacterium possess cells that arrange themselves the length of an elongated axis in a corresponding manner, quite similar to the posts of fencing. Such an arrangement is called a palisade, a word that has been derived from the Latin term denoting a stake or prop becoming palus.

The bacteria Streptomycetes create elongated, multiple nuclei hyphae, which turn out to be branched. A group of hyphae is known as a mycelium. In effect, the expressions mycelia and hyphae are also used to describe or denote similar structures in fungi.

It may be noted that bent rods are often called vibrios. This term point to the vibrating motion witnessed in this type of bacteria. The cells of bacteria may possibly develop in helical, twisted form. While the cells of spirilla are firm, those of spirochaetes possess an extremely supple structure. In Greek, a coil is called 'speira', while it is called 'spira' in Latin. The Greek term for 'long hair' is khaite.

As the name suggests, pleomorphic bacteria exist in changeable forms (the term 'pleomorphic' has been derived from two Greek words 'pleon' denoting more and 'morphe' meaning shape).

Several bacteria are pleomorphic to certain extent. In cultures, sometimes the cells of Proteus mirabilis develop in the form of elongated strings, while most of the cells develop in the form of small rods. In the same way, the cells of the bacterial species Haemophilus inftuenzae may possibly develop in the form of strands, rods and also like coccobacilli.

While Corynebacteria generally have the appearance of bacilli, they have a very inconsistent form. In a number of instances, pleomorphism is attributable to the environs where the bacteria are cultivated. In the case of artificial cultures, bacteria belonging to the genus Rhizobium develop as the normal forms of bacilli having somewhat consistent shapes.

However, when Rhizobium bacteria are observed in minuscule cultivation of the root swellings of plant that are able to fix nitrogen, their cells appear to be extremely degenerated as well as asymmetrical and are known as bacteroids.

Diversity of bacterial nutrition

It is worth mentioning here that bacteria are very widely distributed and are present everywhere from the North Pole to the South Pole. In fact, scientists have even isolated bacteria from the frozen wastes found in the Arctic as well as the Antarctic regions, discovered them close to volcanoes present in the bed of oceans, and even the water emerging from hot springs.

Precisely speaking, the assortment of habitats were bacteria can exist is really astonishing. This manifests the fact that bacteria are able to make use of energy supplies from wide range of sources. In addition, it also reflects how successfully bacteria have been able to change to take advantage of the environment in which they are present.

While light is the source of energy for some varieties of bacteria, there are some others that need chemical energy supply for sustenance. Then again, there are bacteria that make use of organic amalgams, while some others need supplies from an inorganic matter.

In effect, bacteria demonstrate a wide assortment of nourishment and this is necessary for the complete processing of elements passing across their biological systems. In addition, it is also a precondition for unrelenting sustenance of existence on earth.

We are familiar with the fact that all life forms need nutrients for growth as well as multiplication (reproduction). Basically, nutriments are the basic elements that make up all life forms. To facilitate the assemblage of these elements, living beings need an energy source.

Bacteria whose source of energy is light are called phototrophs. When translated into English, this literally denotes being nourished by light (the term has been derived from two Greek words, 'photos' meaning light, and 'trophe' meaning nutrients).

Similarly, the bacteria that depend on supply of chemical energy are called chemotrophs. However, classifying bacteria in these two groups is not essentially absolute, and the prevailing ecological situations may often be responsible for the source of energy which bacteria make use of.

When the bacterial species Rhodospirillum rubrum is cultivated in an anaerobic situation (absence of oxygen) it is known to be a phototroph and needs light as the source of energy. On the other hand, if it is cultivated in the presence of oxygen, the same bacterium is capable of growing in darkness in the form of a chemotroph.

Autotrophic bacteria

Every form of life on this planet is built on carbon and, therefore, it is essential to have adequate supplies of compounds containing carbon for the organisms to grow as well as multiply. In fact, autotrophs as self-nourishing microbes possessing the aptitude to develop on organic compounds by means of fixing carbon dioxide from the environment.

It is possible to cultivate autotrophic bacteria in ordinary salt solutions that are needed to supply the maturing cells with trace elements. However, they do not require any source of carbon, except carbon dioxide.

Autotrophs form an important constituent of food networks, because they generate a renewed organic compound supply required by heterotrophs - which are basically organisms that are unable to develop their entire individual organic compounds and, therefore, need others to nourish them.

Autotrophic bacteria that make use of light as their energy source are called photoautotrophs and sometimes they may also obtain their required energy from oxidized organic (inanimate) chemical compound and, in such instances, are known as chemoautotrophs.

Photoautotrophs bacteria are generally characterized by the purple-colored sulphur bacteria, for instance, Chromatium spp.; the cyanobacteria; and the green-colored sulphur bacteria.

Bacteria classified as chemoautotrophic comprise iron bacteria, hydrogen bacteria, sulphur-oxidizing bacteria and nitrifying bacteria. Iron bacteria like Gallionella supp. produce ferric ions from ferrous ions, while hydrogen bacteria like Hydrogenobacter spp. make use of hydrogen in the form of an electron contributor and oxygen in the form of an electron receiver to make water through the process of reducing molecular oxygen and hydrogen.

On the other hand, nitrifying bacteria facilitate the reaction of oxygen with ammonia and the bacteria that make oxygen react with ammonia to produce nitrite are known as nitrosifying bacteria. Bacteria that are genuinely nitrifying have the ability to further make oxygen react with nitrite to produce nitrate.

Bacteria such as Nitrobacter spp. are a perfect example of truly nitrifying bacteria. Then again, Thiothrix spp. are truly aerobic and acquire their requisite energy by making oxygen react with hydrogen sulphide.

All bacteria classified as autotrophic acquire their requisite energy by means of inorganic compound uptake. Owing to this property of autotrophic bacteria, they are referred to as lithotrophs that acquire nourishments from stones.

It may be noted that the term 'lithotroph' has been derived from the Greek word 'lithos' meaning stone. Accordingly, one may mention photoautotrophs as photolithotrophs, while chemoautotrophs may be referred to as chemolithotrophs.

In addition, autotrophs bacteria posses the aptitude to develop on the entire organic compounds needed for their development as well as multiplication using carbon dioxide. In fact, bacteria belonging to this group are able to fix carbon dioxide.

Majority of the bacteria are basically heterotrophic and are not able to accomplish this act. As an alternative, heterotrophic bacteria are dependent on a supply of natural carbon amalgams from outside.

Heterotrophic bacteria

It is interesting to note that the first bacteria that were referred to as chemolithotrophs were those belonging to the genus Beggiatoa. However, several strains of bacteria of this genus do not have the aptitude to grow till they are provided with compounds that are organic.

Therefore, while they are able to acquire their requisite energy from sulphur oxidation and may possibly be regarded as lithotrophic, they are also referred to as heterotrophic, as they need a carbon source that is fixed, for instance, acetate, for their growth. This type of bacteria is known as mixotrophs (also called mixed feeders).

A number of photosynthetic bacteria make use of uncomplicated organic compounds like acetate, methanol and formate as their only source of carbon instead of binding to carbon dioxide. This type of bacteria are known as photoorganotrophs and represented by bacteria known as purple non-sulphur like Rhodospirillaceae when they are cultivated in anaerobic conditions, but in the presence of light.

Unless there is light, Rhodospirillaceae are not able to grow in the absence of oxygen. Nevertheless, in anaerobic conditions, they may possibly grow in darkness in the form of chemoorganotrophs, which make use of organic compounds for their nourishment. Besides, they also need a supply of adequate chemical energy. It may be noted that the largest part of chemoorganotrophs comprise bacteria that are non-photosynthetic in nature.

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