Bacteria – Discovery, Occurrence, Types and Structure

Bacteria – A little Intro

Bacteria are microscopic, unicellular organisms that exist in their millions, which have cell walls but do not have organelles and an arranged nucleus, including some which can trigger the disease.

Microbiologists place bacteria in two major categories: eubacteria: Greek for “true bacteria” and a much smaller division, the archaeobacteria: Greek for “ancient bacteria”.

Discovery of Bacteria

It had actually long been believed that little animals exist which are too little to be seen with the naked eye. However, their discovery was linked to the creation of a microscopic lens. A Dutch Researcher “AntonieVan Leeuwenhoek” (1673) was the first to report the microorganisms such as bacteria and protozoa.

He used a simple microscope to describe bacteria and protozoa with accurate illustrations and descriptions and called these small animals as “animalcules”. He firstly observed small animals in the water, then verified these in saliva, vinegar, infusions, and other compounds.

The progress in understanding the nature and value of these tiny organisms has been sluggish. The existence of microorganisms was additionally confirmed by Louis Pasteur’s work. Pasteur led many discoveries in the field of microbiology and medicine.

His main achievements are the development of vaccines for disease anthrax, fowl cholera, and rabies. He also made significant contributions to the development of the pasteurization process and the advancement of fermentation industries. He proved that microbes might cause illness.

Germ Theory of Disease

Robert Koch created the ‘germ theory of disease’. He isolated common rod-shaped bacteria with squarish ends (bacilli) from the blood of sheep that had actually died of anthrax. Then he found germs that triggered tuberculosis and cholera.

He developed 4 postulates, which are the main pillars of the germ theory of disease’. These are utilized to find out whether the organism found in disease lesions is the causal agent of the disease or not.

1. A specific organism can always be found in association with a provided disease.
2. The organism can be isolated and grown in pure culture in the laboratory.
3. The pure culture will produce the disease when inoculated into a vulnerable animal.
4. It is possible to recuperate the organism in pure culture from the experimentally contaminated animal.

Koch and his associates created lots of strategies concerning shot, isolation, media preparation, maintenance of pure cultures, and preparation of specimens for microscopic examinations.

Occurrence of Bacteria

Bacteria are broad spread in their occurrence. They are present almost all over, in air, land, water, oil deposits, food, decaying organic matter, plants, humans, and animals. Their kind and number differ according to the area and ecological conditions. Some bacteria are constantly present and contribute towards the natural flora. Others exist in specific environments such as warm springs, alkaline/ acidic soil, highly saline environments, in extremely polluted soils and waters.

Mesophiles, or mesophilic bacteria, are the bacteria responsible for a lot of human infections. They grow in moderate temperatures, around 37 ° C. This is the temperature level of the human body.

Extremophiles, or extremophilic bacteria, can endure conditions considered too extreme for most life types.

Thermophiles can reside in high temperatures, up to 75 to 80 ° C, and hyperthermophiles can survive in temperature levels as much as 113 ° C.

Deep in the ocean, bacteria live in total darkness by thermal vents, where both temperature and pressure are high. They make their own food by oxidizing sulfur that originates from deep inside the earth.

Some other extremophiles are:

• halophiles, present in a salty environment.
• acidophiles, some of which live in environments as acidic as pH 0.
• alkaliphiles, residing in alkaline environments approximately pH 10.5.
• Extremophiles can make it through where no other organism can.

Types of Bacteria

The bacteria are divided into 3 categories.

1. Cocci

The cocci are spherical or oval bacteria having one of a number of unique arrangements based upon their planes of division. If division remains in one plane it will produce either a diplococcus or streptococcus arrangement. When cocci occur in sets then the plan is diplococcus, whereas when cocci form a long chain of cells then the arrangement is called streptococci. When the division of cells is in two planes it will produce a tetrad arrangement. A tetrad is a square of 4 cocci.

Thirdly, when the division is in 3 planes, it will produce a sarcina arrangement. Sarcina is a cube of 8 cocci. When division occurs in random planes, it will produce a staphylococcus arrangement in which cocci are set up in irregular, often grape-like clusters.

Examples: Diplococcus pneumoniae and Staphylococcus aureus are some examples of cocci.

2. Bacilli

Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or diplobacillus. Bacillus is a single cell of bacteria. Streptobacillus is a chain of bacilli. When rod-shaped bacteria occur in pairs then the arrangement of cells is called diplobacilli.

Examples: Examples of rod-shaped bacteria are Escherichia coli, Bacillussubtilis, Pseudomonas.

3. Spiral

The spiral-shaped bacteria are spirally coiled. Spirals can be found in one of 3 forms, a vibrio, a spirillum, or a spirochete. Vibrio is a curved or comma-shaped rod. Spirillum is a thick, stiff spiral. The spirochete is a thin, flexible spiral.

Examples: Examples of spiral-shaped bacteria are Vibrio, Hyphomicrobium.

Structure of Bacteria

All bacterial cells inevitably have a cell membrane, cytoplasm, ribosome, and chromatin bodies. The bulk have a cell wall, which offers shape to the bacterial cell. Particular structures like capsule, slime, flagella, pili, fimbriae, and granules are not present in all bacteria.

Size

Bacteria vary in size from about 0.1 to 600 µm over a single dimension. Bacteria vary in size as much as fit. The smallest e.g., some members of the genus Mycoplasma are about 100 to 200 nm in diameter, around the size of the largest viruses (poxviruses). Escherichia coli, a bacillus of about average size, is 1.1 to 1.5 µm large by 2.0 to 6.0 µm long. Some spirochetes sometimes reach 500 µm in length whereas Staphylococci and Streptococci are 0.75 – 1.25 µm in diameter.

Flagella and their functions

These are very thin, hair-like appendages. They come out through cell wall and stem from the basal body, structure simply beneath the cell membrane in the cytoplasm. They are comprised of the protein flagellin. On the basis of the presence of flagella, pattern of the accessory of flagella, and the number of flagella present, bacteria are categorized into various taxonomic groups.

Atrichous means bacteria are without any flagella.

When a single polar flagellum is present then the condition is called monotrichous.

If tuft of flagella exists only at one pole of bacteria then these are lophotrichous flagella.

Amphitrichous is a condition when a tuft of flagella at each of two poles exists.

In peritrichous form, flagella surround the entire cell.

Most of the bacilli and spiral-shaped bacteria have flagella. Cocci really hardly ever have flagella.

The main function of flagella is to help in motility. With the assistance of flagella, flagellate bacteria can also detect and relocate the action to chemical signals which is a type of behavior called Chemotaxis.

Pili and their Functions

These are hollow, nonhelical, filamentous appendages. Pili are smaller than flagella and are not associated with motility. True pili are only present on gram-negative bacteria. They are made up of a unique protein called pilin. They are primarily associated with a mating process between cells called the conjugation process. Some pili work as a means of attachment of bacteria to various surface areas.

The cell wall: The external wrapping of bacteria

Bacterial surface and walls are extremely varied. Collectively complexes of layer external to the cell protoplasm are called as cell envelope and include capsule, slime, and cell wall.

Capsule: Bacteria produce pill, which is comprised of duplicating polysaccharide units, and of protein, or of both, the pill is firmly bound to the cell. It has a thicker, gummy nature that provides sticky characters to colonies of encapsulated bacteria. It provides pathogenicity to bacteria.

Slime: Some bacteria are covered with a loose, soluble shield of macromolecules which is called slime and slime supplies greater pathogenicity to germs and protects them against phagocytosis.

Cell Wall: Below the extracellular compounds and external to the cytoplasmic membrane cell wall exists. It is a rigid structure. It identifies the shape of the bacterium. The cell wall also secures the cells from osmotic lysis. The cell wall is only missing in mycoplasmas.

The cell walls of most bacteria have a special macromolecule called peptidoglycan. Its quantity differs in different types of bacteria. It is composed of a framework of long glycan chains cross-linked with peptide fragments. The intact cell wall also consists of chemical constituents such as sugar molecules, teichoic acid, lipoproteins, and lipopolysaccharides, which are connected to peptidoglycan.

Cell walls of archaeobacteria are different from eubacteria. They do not contain peptidoglycan. Their cell walls are composed of proteins, glycoproteins, and polysaccharides.

Cell Membrane

Simply beneath the cell wall is the cell membrane or plasma membrane. It is very thin, flexible, and totally surrounds the cytoplasm. The plasma membrane is extremely fragile in nature any damage to it leads to the death of the organism. Bacterial membranes differ from eukaryotic membranes in not having sterols such as cholesterol.

The cell membrane regulates the transportation of proteins, nutrients, sugar, and electrons or other metabolites. The plasma membranes of bacteria also contain enzymes for the respiratory metabolic process.

Cytoplasmic matrix

The cytoplasm of the prokaryotic cell does not have membrane-bound organelles and cytoskeleton (microtubules). The cytoplasmic matrix is the substance present in between the plasma membrane and the nucleoid. It has a gel-like consistency. Small molecules can move through it quickly. The plasma membrane and everything present within it are called protoplasts. Thus, the cytoplasmic matrix is a huge part of the protoplast. Other big discrete structures such as chromatin/ nuclear body, ribosomes, mesosomes, and granules and nucleoids are present in this matrix.

Nucleoid

A bacterial cell unlike the cells of eukaryotic organisms lacks discrete chromosomes and a nuclear membrane. The nuclear material or DNA in bacterial cells occupies a position close to the center of the cell. This product is a single, circular, and double-stranded DNA molecule. It aggregates as an irregular shaped thick area called the nucleoid. This chromatin body is actually a very long molecule of DNA that is tightly folded so regarding it inside the cell element. Since bacteria have a single chromosome, they are haploid.

Plasmid

Many bacteria include plasmids in addition to chromosomes. These are the circular, double-stranded DNA particles. They are self-replicating and are not vital for bacterial growth and metabolism. They frequently consist of drug resistant, heavy metals, disease, and insect resistant genes on them.

Ribosomes

Ribosomes are composed of RNA and proteins. Some might be loosely attached to plasma membranes. They are protein factories. There are countless ribosomes in each healthy growing cell. They are smaller than eukaryotic ribosomes.

Mesosomes

The cell membrane invaginates into the cytoplasm forming structure called as mesosome. Mesosomes are in the kind of vesicles, tubules, or lamellae. Mesosomes are involved in DNA replication and cell division whereas some mesosomes are also involved in the export of exocellular enzyme. Respiratory enzymes are also present on the mesosomes.

Granules and storage bodies

Since bacteria exist in an extremely competitive environment where nutrients are usually in short supply. They tend to store extra nutrients when possible. These may be glycogen, sulfur, fat, and phosphate. In addition, cells consist of waste materials that are subsequently excreted. For example, common waste products are alcohol, lactic acid, and acetic acid.

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