Friday, January 3, 2020

Structure of Bacterial Cell

Bacteria are microscopic, single-celled organisms that thrive in diverse environments. These organisms can live in soil, the ocean and inside the human gut.Upon observation under microscope several structural components outside and inside the cell wall are revealed. Some of these structures are found only in certain species of bacteria/fungi. Out of all these only cell wall is common to all microbial cells. However, the cell wall structure differs in different microbial groups; for example the prokaryotic cell wall differs from the eukaryotic cell wall, and Gram-positive bacteria from the Gram-negative bacteria. Fungal and algal cells also differ. The animal cells lack a rigid cell wall, many have flagella and fimbriae or pili. Inside the cell wall there is cell membrane which encloses cell inclusions.
 Structure of Bacterial Cell

Some of the bacterial cells are surrounded by the extracellular polymeric substances (EPS)which are commonly called capsule or glycocalyx (Costerton et al.,1981). It forms an envelope around the cell wall and can be observed under light microscope after special staining technique.The presence of capsule may be detected by negative staining also such as India ink method.
It is an outer covering of thin jelly-like material (0.2 μm in width) that surrounds the cell wall. Only some bacterial species possess capsule. Capsule is usually made of polysaccharide (e.g. pneumococcus), occasionally polypeptide (e.g. anthrax bacilli) and hyaluronic acid (e.g. streptococcus).

The capsule is gelatinous polymer made up of either polysaccharide (Klebsiella pneumoniae or polypeptide (B. anthracis) or both. The polysaccharides may be of a single type of sugars (heteropolysaccharides). The heteropolysaccharide is synthesized by sugar precursors within the cell. Homopolysaccharide constitutes the cansule Acetobacter xylinum, and heteropolysaccharide (consisting of D-glucose, D-galactose, D-mannos D-gluconic acid and D-rhamnose) is secreted by Pseudomonas aeruginosa. The capsule opneumococci is made up of hexoses, uronic acids and amino sugars and that of streptococci consist of L-amino acids. The bacterial capsule is species specific and, therefore, can be used for immunological differentiation of related species (Ferris and Beveridge, 1985). Amount of these polymers vary with bacterial species. It is sticky in nature and secreted from the inner side of cell which gets firmly attached to the surface of cell wall. If the substances are unorganised and loosely attached to cell wall, the capsule is called slime layer. The fresh water and marine bacteria form trichomes which are enclosed inside the gelatinous matrix called sheath. Sheath is also found in cyanobacteria and other algae.

Function of Capsule

Capsule may have a number of functions according to bacterial species. Costerton et al. (1981) have reviewed the function of bacterial glycocalyx, Some of the functions are discussed below.
(i) The capsule may prevent the attachment of bacteriophages.
(ii) It protects the bacterial cells against desiccation as it is hygroscopic and contains water molecules.
(iii) It may survive in natural environment due to its sticky property. After attachment they can grow on diverse surfaces e.g. plant root surfaces, human teeth and tissues (dental carries, respiratory tract), rocks in fast flowing streams, etc.
(iv) They may inhibit the engulfment by WBCS (antiphagocytic feature) and, therefore, contribute to virulence. Capsule protects from phagocytosis for example the capsulated strains of Sreptococcus pneumoniae causes pneumonia and uncapsulated strain is phagocytized.
(v) S. mutans uses its capsules as a source of energy. It breaks down the sugars of capsule when stored energy is in low amount.

(iv) Capsule protects the cell from desiccation, maintains the viscosity and inhibits themovement of nutrients from the bacterial cell (Ferris and Beveridge, 1985).

The motile bacterium may possess a flagellum (plural flagella). The flagellum is hair like, helical and surface appendeges emerging from the cell wall. It is of 20-30 nm in diameter and 15 pm long. It provides various types of motility to the bacterial cell. The flagella of prokaryotes are several time thinner than that of eukaryotes. In addition, the number and position of flagella vary. The arrangement may be monotrichous (a single polar flagellum e.g. V. cholerae), lophotri-chous (a clusture of polar flagella e.g. Spirillum), amphitrichous (flagella at both the ends either singly or in clusture), cephalotrichous (two or more flagella at one end of bacterial cell e.g. Pseudomonas), peritrichous (cell surface evenly surrounded by seveverl lateral flagella.

 structure of Flagella

 (i) Structure of Flagella: The structure and function of bacterial flagella have been described by Simon et al. (1978), Doestsch and Sjoblad (1980) and Ferris and Beveridge (1985). A flagellum consists of three basic parts, the basal body, hook and filament (Fig. 4.3).
(a) Basal body: M. L. De Pamphilis and J. Alder (1971) isolated the basal body of a flagellum of E. coli and B. subtilis and studied its fine structure and arrangement of rings. The basal body attaches the flagellum to the cell wall and plasma membrane. It is composed of a small central rod inserted into a series of rings.

b) Hook : The hook is present outside the cell wall and connects filament to the basal body. It consists of different proteins. The hook in Gram-positive bacteria is slightly longer than the Gram-negative bacteria.

(c) Filament or shaft : The outermost long of the flagellum is called filament or shaft region (Fig. 4.3). It has a constant diameter and is made up of globular proteins, the flagellin. The flagellins are arranged in several chains that intertwine and form a helix around a hollow core . The proteins of flagella act to identify certain pathogenic bacteria unlike eukaryotes, the filaments are not covered by a membrane or sheath.

Pili and Fimbriae

Pili and fimbriae are hair like appendages found on surface of cell wall in Gram negative bacteria (e.g. Enterobacteriaceae, Pseudomondaceae and Caulobacter). Eukaryotic cells lack pili. The term fimbriae is used for all hair like structure covering the surface of the cell. Pili are genetically governed by plasmids, the number of which varies from 3 to 5. The number of fimbriae is around 1,000. However, a similar structure has also been observed only in Corynebacterium renale, a Gram-positive bacterium. Pili differ from flagella in being shorter and thinner, straight and less rigid. But they are in large number. They occur either at the poles of bacterial cell or evenly distributed over the entric surface of the cell. The pili are 0.2-20 um long with a diameter of about 250 Å.

Function of Pili:
There are several functions of fimbriae and pili as given below:
(a) Bacteria containing fimbriae are called fimbriate bacteria. Fimbriae have the adhesive properties which attach the organism to the natural substrate or to the other organ Fimbriae agglutinate the blood cells such as erythrocytes, leucocytes, eptithelial cells, etc.
(6) Fimbriae are equipped with antigenic properties as they act as thermolabile nonspecme agglutinogen.
(c) Fimbriae affect the metabolic activity, The Fim" cells (cells containing fimbriae) possess higher rate of metabolic activity than the Fim cells (cells devoid of fimbriae). Moreover, they function as aggregation organelles i.e. they can form stellate aggregation on a static liquid medium.
(d) The sex pili make contact between two cells. Since they posses hollow core, they act as conjugation tube. The tip of pilus recognises the female (F) cell through which the genetie material of donor (F*) cell passes to the recipient (female) cell. Only F-pili (not I-pili) contain axial hole (Simon et al., 1978).

The Cell Wall (Outer Membrane)

The cell wall of bacteria is a semirigid and complex structure present beneath capsule and external to the plasma membrane. protects the plasma membrane and the other cytoplasmic inclusions from adverse environment. It also protects the bacterial cell from bursting when the osmotic pressure of cytoplasm is higher than that of outside of cell wall. It provides support for attachment to the flagella. It rescues the cells from antibodies and harmful chemicals. is responsible for characteristic shape of the cell. The cell wall The cell wall of Gram-negative eubacteria is comparatively thinner than the cell wall of Gram- positive bacteria. Similar is the situation of Gram-negative archaeobacteria. In addition, chemical composition of cell wall of archaeobacteria differs from eubacteria. Also cell walls of eukaryotic microorganisms (e.g. algae, fungi) differ chemically from those of prokaryotes. The cell envelope of Gram-negative bacteria consists of two unit membranes of 75 Å wide, separated by 100Å wide periplasmic space. Peptidoglycan is present in periplasmic space in Gram-negative bacteria.

The plasma membrane:
The plasma membrane also called cell membrane or cytoplasmic membrane is a structure internal to the cell wall. The term cell membrane was coined by C. Nugeli and C. Cramer in 1855. However, no cell could be alive without a plasma membrane. It is situated just beneath the cell wall and is 7.5 nm thick. It consists of proteins (20-70%), lipids (28-80%), oligosaccharides (1- 5%), and water (20%). The plasma membrane consists of a continuous bilayer of phospholipid molecules in which globular proteins are embedded.

Mesosomes are the invaginated structures formed by the localized infoldings of the plasma membrane. The invaginated structures comprise of vesicles, tubules of lamellar whorls. Generally mesosomes are found in association with nuclear area or near the site of cell division. They are absent in eukaryotes. The lamellae are formed by flat vesicles when arranged parallely. Some of the lamellae are connected to the cell membrane. The Jamellar whorl can be observed in Nitrobacter, Nitromonas and Nitrococcus. The vesicles are formed probably by invagination and tubular accretion of the plasma membrane. The structure of vesicle becomes interrupted due to constriction at equal distance. The constriction does not cause the complete separation of tubules. Closely packed spherical vesicles are seen in Chromatium and Rhodospirillum rubrum. In some purple bacteria the vesicular bodies are flattened and stacked into the regular plates like thylakoids. Salton and Owen (1976) have suggested that the mesosomes are formed due to vesicularization of outer half of the lipid bilayer. However, they are the special cell membrane components, the proteins of which differ from the cell membrane.   

Cytoplasm - The cytoplasm, or protoplasm, of bacterial cells is where the functions for cell growth, metabolism, and replication are carried out. It is a gel-like matrix composed of water, enzymes, nutrients, wastes, and gases and contains cell structures such as ribosomes, a chromosome, and plasmids. The cell envelope encases the cytoplasm and all its components. Unlike the eukaryotic (true) cells, bacteria do not have a membrane enclosed nucleus. The chromosome, a single, continuous strand of DNA, is localized, but not contained, in a region of the cell called the nucleoid. All the other cellular components are scattered throughout the cytoplasm.
One of those components, plasmids, are small, extrachromosomal genetic structures carried by many strains of bacteria. Like the chromosome, plasmids are made of a circular piece of DNA. Unlike the chromosome, they are not involved in reproduction. Only the chromosome has the genetic instructions for initiating and carrying out cell division, or binary fission, the primary means of reproduction in bacteria. Plasmids replicate independently of the chromosome and, while not essential for survival, appear to give bacteria a selective advantage.
Plasmids are passed on to other bacteria through two means. For most plasmid types, copies in the cytoplasm are passed on to daughter cells during binary fission. Other types of plasmids, however, form a tubelike structure at the surface called a pilus that passes copies of the plasmid to other bacteria during conjugation, a process by which bacteria exchange genetic information. Plasmids have been shown to be instrumental in the transmission of special properties, such as antibiotic drug resistance, resistance to heavy metals, and virulence factors necessary for infection of animal or plant hosts. The ability to insert specific genes into plasmids have made them extremely useful tools in the fields of molecular biology and genetics, specifically in the area of genetic engineering.

Cytoplasmic Membrane - A layer of phospholipids and proteins, called the cytoplasmic membrane, encloses the interior of the bacterium, regulating the flow of materials in and out of the cell. This is a structural trait bacteria share with all other living cells; a barrier that allows them to selectively interact with their environment. Membranes are highly organized and asymmetric having two sides, each side with a different surface and different functions. Membranes are also dynamic, constantly adapting to different conditions.

Nucleoid - The nucleoid is a region of cytoplasm where the chromosomal DNA is located. It is not a membrane bound nucleus, but simply an area of the cytoplasm where the strands of DNA are found. Most bacteria have a single, circular chromosome that is responsible for replication, although a few species do have two or more. Smaller circular auxiliary DNA strands, called plasmids, are also found in the cytoplasm.

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