Microbiologists place bacteria in two major categories: eubacteria(Greek for true bacteria) and much smaller division, the archaebacteria (Greek for ancient bacteria).
Occurrence: Bacteria are wide spread in their occurrence. They are found almost everywhere in air, land, water, oil deposits, food, decaying organic matter, plants, humans and animals. Their kind and number vary according to locality and environmental conditions. Some of them are always present and contribute towards natural flora. Others are present in specific environments such as hot springs, alkaline or acidic soil, highly saline environments and in highly polluted soils and waters.
Size: Bacteria range in size from about 0.1 to 600 micrometer over a single diameter. They vary in size as much as in shape. Smallest(some members of genus Mycoplasma) are about 100 to 200 nanometer in diameter. Escherichia coli a bacillus of about average size is 1.1 to 1.5 micrometer wide by 2.0 to 6.0 micrometer long. Some spirochetes occasionally reach 500 micrometer in length whereas staphylococci and streptococci are 0.75 to 1.25 micrometer in diameter.
Shape: On the basis of general shape bacteria are classified into three categories cocci, bacilli and spiral. Although most of them have fairly constant characteristic cell shape, yet some cells are pleomorphic and they can exist in variety of shapes. Cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division. If division is in one plane it will produce either diplococcus or streptococcus arrangement. When cocci occur in pairs then arrangement is diplococcus whereas when cocci form long chain of cells then arrangement is called as streptococci. When division of cells is in two planes it will produce tetrad arrangement. Tetrad is square of 4 cocci. When division is in three planes it will produce sarcina arrangement. It is cube of 8 cocci. When division occur in random planes it will produce staphylococcus arrangement in which cocci are arranged in irregular often grape like clusters. Diplococcus pneumonia and staphylococcus aureus are some examples of cocci. Rod shaped bacteria are called bacilli. All bacilli divide in one plane producing bacillus, streptobacillus or diplobacillus. Bacillus is single cell. Streptobacillus is chain of bacilli. Diplobacillus occur in pairs. Examples are Escherichia coli, Bacillus subtilis and pseudomonas. Spiral shaped bacteria are spirally coiled. Spirals come in one of three forms vibrio, spirillum and spirochete. Vibrio is curved or comma shaped. Spirillum is thick rigid spiral. Spirochete is thin flexible spiral. Examples are Vibrio and Hyphomicrobium.
Structure: Bacterial cell structure.
Flagella: These are extremely thin hair like appendages. They come out through cell wall and originate from basal body. They are made up of protein flagellin. On the basis of presence of flagella, pattern of attachment of flagella and number of flagella present bacteria are classified into different taxonomic groups. Atrichous means bacteria are without any flagella. When single polar flagellum is present then condition is known as monotrichous. If tuft of flagella is present only at one pole then these are lophotrichous. Amphitrichous is condition when tuft of flagella at each of two poles is present. In peritrichous form flagella surround whole cell. Most of bacilli and spiral have flagella. Cocci very rarely have flagella. Primary function of flagella is to help in motility. With help of flagella, flagellate bacteria can also detect and move in response to chemical signals which is type of behavior called chemotaxis.
Pili: These are hollow, nonhelical, filamentous appendages. Pili are smaller than flagella and are not involved in motility. True pili are only present on gram negative bacteria. They are made up of special protein called pilin. They are primarily involved in mating process between cells called conjugation process. Some pili function as means of attachment to various surfaces.
Capsule: Bacteria produce capsule which is made up of repeating polysaccheride units and protein or of both. Capsule is tightly bound to cell. It protects cells from engulfment by eukaryotic cells such as macrophages.
Slime: Some bacteria are covered with loose soluble shield of macromolecules which is called slime layer. It protects from environmental dangers such as antibiotics and desiccation.
Cell wall: Beneath extracellular substances and external to cytoplasmic membrane cell wall is present. It is rigid structure. It determines shape of bacterium. Cell wall also protect cells from osmotic lysis. It is only absent in mycoplasmas. Christian Gram developed technique of gram stain. Bacteria could be divided into two groups based on their response to gram staining procedure. By this staining technique gram positive bacteria are stained purple( retain primary dye due to formation of CV-1 complex) and gram negative bacteria are stained pink ( retain secondary dye) in color. Cell wall have unique macromolecule called peptidoglycan. Its amount varies different. It is composed of framework of long glycan chains cross linked with peptide fragments. It also contains chemical constituents such as sugar molecules, teichoic acid, lipoproteins and lipopolysaccharides which are linked to peptidoglycan. Cell walls of archaebacteria are different from eubacteria. They do not contain peptidoglycan. Their cell wall composed of proteins, glycoproteins and polysaccharides.
Cell membrane: Just beneath cell wall is cell membrane or plasma membrane. It is very thin, flexible and completely surrounds cytoplasm. Plasma membrane is very delicate in nature any damage to it results in death of organism. Bacterial cell membranes differ from eukaryotic membranes in lacking sterols such as cholesterol. It regulates transport of proteins, nutrients, sugar and electrons or other metabolites. It also contains enzymes for respiratory metabolism.
Cytoplasmic matrix: Cytoplasm of prokaryotic cell lacks membrane bound organelles and cytoskeleton. Cytoplasmic matrix is substance present between plasma membrane and nucleoid. It has gel like consistency. Small molecules can move through it rapidly. Plasma membrane and everything present within it is known as protoplast. Thus cytoplasmic matrix is major part of protoplast. Other large discrete structures such as chromatin body, ribosomes, mesosomes, granules and nucleoid are present in this matrix.
Nucleoid: Bacterial cell unlike the cells of eukaryotic organisms lacks discrete chromosomes and nuclear membrane. Nuclear material or DNA occupies position near to center of cell. This material is single, circular and double stranded DNA molecule. It aggregates as an irregular shaped dense area called nucleoid. This chromatin body is actually extremely long molecule of DNA that is tightly folded so as to fit inside cell component. Since bacteria have single chromosome, they are haploid.
Plasmid: Many bacteria contain plasmids in addition to chromosomes. These are circular double stranded DNA molecules. They are self replicating and not essential for bacterial growth and metabolism. They often contain drug resistant, heavy metals, disease and insect resistant genes on them.
Ribosomes: Ribosomes are composed of RNA and proteins. Some may also be loosely attached to plasma membranes. They are protein factories. There are thousands of ribosomes in each healthy growing cell. They are smaller than eukaryotic ribosomes.
Mesosomes: Cell membrane invaginates into cytoplasm forming structure called as mesosomes. It involved in DNA replication and cell division where as some of them are also involved in export of exocellular enzyme. Respiratory enzymes are also present on them.
Granules and storage bodies: Bacteria exist in very competitive environment where nutrients are in short supply. They tend to store extra nutrients when possible. These may be glycogen, sulfur, fat and phosphate. In addition cells contain waste materials that are subsequently excreted. Common waste materials are alcohol, lactic acid and acetic acid.
Spores: Certain species of bacteria produce spores either external to vegetative cells called exospores or within vegetative cells called endospores. They are metabolically dormant bodies and are produce at late stage of cell growth. Spores are resistant to adverse physical environmental conditions such as light, high temperature, desiccation, PH and chemical agents. Under favorable conditions they germinate and form vegetative cells.
Cysts: Cysts are dormant thick walled desiccation resistant forms and develop during differentiation of vegetative cells which can germinate under suitable condition. They are not heat resistant.
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