Essay on Bacteria!
Size and Shape of Bacteria:
Bacteria are microscopic, unicellular, asexually reproducing prokaryotes. They lead either saprophytic or parasitic life. The saprophytic forms are of great economic importance to man. The parasitic forms cause diseases in plants and animals including man.
Bacteria are very small organisms hardly visible under microscope. The smallest bacterium is Dialisier pneumosintes (0.15 to 0.30 µ) and largest bacterium is Spirillum voluians (13-15 µ) in length.
Bacteria occur in three main shapes, spherical, rod-like and spiral:
These are spherical bacteria. The cells may occur in pairs (Diplococci), in groups of four (tetracocci), in bunches (Staphylococci), in a bead like chain (Streptococci) or in cubical arrangement of eight or more (Sarcinae).
These are rod-like bacteria. The cells may occur singly but occasionally found in pairs (Diplobacilli) or chains (Streptobacilli).
These are spiral shaped bacilli or commashaped.
Structure of Bacteria:
A typical bacterial cell has an outer covering and cytoplasm.
The outer covering is made up of three layers:
1. Plasma membrane:
It is composed of lipoproteins and is rich in oxidative enzymes and respiratory chain enzymes. In some bacteria plasma membrane sinks deep into the cytoplasm to form multilayered structures called desmosomes.
In others, plasma membrane gives out certain invaginations or infoldings of complex whorls of convoluted membranes called mesosomes. These contain enzymes of electron transport system. In addition, mesosome helps in secretion, receive DNA during conjugation and are sites of DNA replication.
2. Cell Wall:
The plasma membrane is followed and is covered by a strong rigid cell wall. Besides containing carbohydrates lipids, phosphorus and certain inorganic salts, cell wall contains an amino acid, diaminopimelic acid (found only in bacteria) and a derivative of glucose called muramic acid.
Outside to the cell wall of most bacteria occurs a slimy capsule which is composed of largely polysaccharides. It serves as an additional protective covering.
The cytoplasm of bacterial cells is dense, colloidal and contains granules, of glycogen, proteins and fats. It also contains poly-B-hydroxy- butyric acid, granulose (a polymer of glucose), volutin (metaphosphate granules) and elemental sulphur as the stored food materials. It lacks the cellular orgenelles like endoplasmic reticulum, Golgi complex, mitochondria, lysosomes, centriolcs etc.
However, the bacterial cells have ribosomes which occur freely in the cytoplasm and do not bounded to membranes. Bacterial ribosomes are of 70S type consisting of two subunits 30S and 50S. During protein synthesis many ribosomes reads the codes of mRNA and form the polyribosomes or polysome.
The genetic information’s of each bacterial cell is carried by DNA molecules which is often called bacterial chromosome or gonophores. The DNA is present in a distinct light coloured region which is often called nucleoid. The nucleoid lacks any delimiting membrane. Cocci usually have one nucleoid per cell, rod-like bacilli usually have two or more nucleoids per cell. The RNA molecules are also found scattered in the cytoplasm.
Certain photosynthetic bacteria have chromophores which have bacteriochlorophyll and other photosynthetic pigments. These differ somewhat from the comparable pigments of eukaryotes. The photosynthetic pigments and enzymes are associated with internal membranes that are arranged as lamellae, tubules or vesicles in different species to form the so called chromatophores.
Many bacteria have whip-like outgrowths called flagella. A bacterial flagellum is formed of a single fibril about 100-200Å thick and several microns in length. It arises from an intracellular basal granule. It is made up of a protein flagellin. Most rod- and spiral-shaped bacteria have flagella, most spherical ones do not.
Pili or Fimbriae:
In addition to flagella, certain bacteria have numerous finer projections called pili (“hair”, Sing., Pilus) or fimbriae with diameters of 0.01 um or less and lengths upto one urn or two. Most pili seems to have a construction somewhat similar to that of flagella, including a basal body anchor within the cytoplasm.
However, male bacteria of certain strains carry one or two special hollow sex pili that form a bridge with the female cell during mating. In few pathogenic strains of bacteria, pili are responsible for recognizing and attaching to the host.
Bacteria are broadly classified according to their response to a staining procedure developed by Danish bacteriologist Christian Gram in 1884. The bacteria that are stained by this method are called gram-positive, the others are gram-negative. The distinction in the staining is related to a fundamental difference in the structure of cell wall and also a corresponding difference in their sensitivity to various chemical agents other than the stain.
Gram stain is a mixture of crystal violet and iodine. The cell walls of Gram-positive bacteria contain polysaccharides and mucopeptides. They also contain teichoic acid and traces of RNA. The macromolecules of polysaccharides and mucopeptides remain extensively linked to one another, producing a complex multimolecular network that provides considerable strength to the wall.
The walls of Gram-negative bacteria contain a similar network but have in addition an outer layer of lipids, complexed with protein and polysaccharides. They lack RNA and teichoic acid. The cell walls of Gram-negative bacteria are so thin that they appear like unit membrane in the electron microscope. Some of the outer layer components in Gram- negative cell walls are toxic to animals and may account for certain effects of bacterial infection.
Eg. Gram positive – Diplococcus pneumoniae
Gram negative – Escherichia coli
It is a gram-negative bacillus. This is nonpathogenic inhabiting the mammalian digestive tract. It is the most convenient experimental organism for cell biologists, geneticists, biochemists and molecular biologists. An E. coli cell is about 2u long and 0.8 to 1 u thick. It has an outer cell wall of 100Å thickness.
The cell wall is composed of molecules of polysaccharides, lipids and proteins and also of certain mineral salts. The cell wall is followed by a true plasma membrane which contains the molecules of lipids and proteins like the ‘unit membrane’ of eukaryotic cells. Due to the absence of mitochondria, the respiratory mechanism of bacterial cell is performed by the plasma membrane.
The cytoplasm of E. coli is colloidal containing roughly 5000 distinguishable components, ranging from water to DNA and other macromolecules (such as RNA, enzymatic proteins, glycogen etc.). It has no endoplasmic reticulum, Golgi complex, mitochondria, centriole, lysosome, flagella and pili, but has about 20,000 to 30,000 particles of 200Å diameter, known as ribosomes. The ribosomes of E. coli is 70S consisting of two subunits 30S and 50S. The ribosomes contain ribosomal RNA and proteins and are the sites for protein synthesis.
All the genetic information’s of an E. coli are carried by a single, double stranded, circular and 1000-1500 u long molecule of DNA. DNA molecule of E. coli does not remain wrapped in histone proteins like the DNA of eukaryotes, but certain polyamines may be found to some of its phosphate groups.
Pettijohn (1973) had made a two dimensional model of the condensed E. coli chromosome. The circular DNA molecule is folded into chromosome containing 12 to 80 domains (in diagram only 7 domains are given for simplicity).
Bacterial Diseases in Human beings:
1. Pneumonia (Lobar)
9. Whooping cough
11. Boils & Carbuncles
13. Scarlet fever
16. Food poisoning, and