Bacteria are prokaryotic single-celled or colonial microorganisms Assignment Help
Bacteria are prokaryotic single-celled or colonial microorganisms
Characteristics of Bacteria
Lack Green Pigment Chlorophyll
Reproduce by Transverse Fission
Bacteria display a wide diversity of shapes and sizes.
0.5 µm diameter
Length 0.5 µm – 80 µm
Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometers in length. However, a few species are visible to the unaided eye—for example, Thiomargarita namibiensis is up to half a millimeter long] and Epulopiscium fishelsoni reaches 0.7 mm.] Among the smallest bacteria are members of the genus Mycoplasma, which measure only 0.3 micrometers, as small as the largest viruses.] Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied.
Spherical – coccus
Rod-shaped – bacillus
Vibrio – Comma shaped
Spiral-shaped – spirillum
Spherical bacteria are known as cocci (singular coccus, Rod-shaped bacteria are called bacilli. Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of other unusual shapes have been described, such as star-shaped bacteria.
Arrangements of Bacterial Cells
Bacteria are unicellular or colonial
Colonial – cells remain together after division
Colony type – depends on plane of cleavage and planes of successive cleavage.
Bacillus – can only divide in one plane, at right angles to the long axis of the cell.
Diplobacillus – remain in pairs following division. after 4 chain fragments
Spirillum- (spiral) divides in one plane
Spherical (coccus) can initially divide in any plane. Great variation in colony types.
Cells divide simultaneously
If after 4 unit, chain fragments into chains of 2 organisms each – diplococcus
Cells divide at right angles to the preceding division
Sarcina – 3 planes of division. Successive planes are at right angles
Sarcina colonies are cuboidal. All dimensions are the same.
Staphylococcus – irregular cluster of spherical cells. Cells divide in any plane. No pattern
Coccus organism –the type of colony is a species characteristic. It can be used to identify an organism. The colony type is often indicated by the generic name. This is not true of bacillus or spirillum. The colony type can be varied by environment or temperature.
Many bacterial species exist simply as single cells; others associate in characteristic patterns: Neisseria forms diploids (pairs), streptococci form chains, and staphylococci group together in “bunch of grapes” clusters. Bacteria can also group to form larger multicellular structures, such as the elongated filaments of Actinobacteria species, the aggregates of Myxobacteria species, and the complex hyphae of Streptomyces species. These multicellular structures are often only seen in certain conditions. For example, when starved of amino acids, myxobacteria detect surrounding cells in a process known as quorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometers long and containing approximately 100,000 bacterial cells. In these fruiting bodies, the bacteria perform separate tasks; for example, about one in ten cells migrate to the top of a fruiting body and differentiate into a specialized dormant state called a myxospore, which is more resistant to drying and other adverse environmental conditions.
Structure of the Prokaryotic Cell
Structures External to the Cell Wall
Among the structures external to the cell wall are the:
Capsule (slime layer)
Axial filaments, and
Capsule, or Slime Layer
A capsule is a jelly-like coating that surrounds the cells of certain bacteria.
Chemically, the capsule is composed of a gelatinous polymer of polysaccharide, polypeptide, or both.
- It appears to prevent desiccation (drying) of the organism under adverse conditions.
- Capsules often protect pathogenic bacteria from phagocytosis by cells of the host.
Flagella are long threadlike appendages used for locomotion in certain bacteria.
Bacteria can swim by rotating their flagella.
The flagellum has three basic parts:
- The filament, the outer threadlike part composed of the protein flagellin.
- Hook– a curved portion attached to the proximal end of the filament.
- Basal body– anchors the flagellum to the cell wall and cytoplasmic membrane.
The structure of the flagellum of bacteria is completely different from the cilia and flagella of eukaryotic cells.
Mechanism of Movement
In the basal body there is a helical rotor powered by a proton gradient that pushes the cell by spinning either clockwise or counter clockwise around its axis.
Axial Filaments consist of numerous fibrils that arise from both poles of the cell and are encased within a sheath.
Axial filaments are found only in the spirochetes. These are corkscrew-shaped bacteria. One of the best-known spirochetes is Treponema pallidum, the causative agent of syphilis.
The axial filaments are similar in structure to flagella but instead of being found outside the cell as flagella are, they are found inside the cell. They are attached to both poles of the cell and spiral around the organism between the plasma membrane and the cell wall.
The function of axial filaments is movement. As they rotate or contract, the axial filaments cause the spirochete cell to turn in a corkscrew-like manner.
Pili and Fimbriae
Pili and fimbriae are filamentous projections that extend from the surface of certain bacteria.
Fimbriae are shorter in length than pili and present in high numbers. Fimbriae function in the attachment of a bacterium to a surface. Neisseria gonorrhoeae, the bacterium that causes the disease gonorrhoeae, uses fimbriae to adhere to the cell it infects.
Pili function in the process of bacterial conjugation in which genetic material is exchanged between two bacterial cells. Non-sex pili also function in attachment of bacteria to surfaces.
The Cell Wall
The cell wall is a semi rigid structure that surrounds the bacterial cell.
The cell wall protects the cell when it is in a dilute environment. The high concentration of solute within the bacterial cell creates a high osmotic pressure that leads to the entry of water into the cell. The cell wall resists the pressure created by the inward flow of water preventing the cell from bursting.
Structure of the Cell Wall
The bacterial cell wall is surrounded by the cell wall, which is composed of a material called peptidoglycan (also called murein). Peptidoglycan consists of glycan chains of (poly-N-acetylglucosamine and N-acetylmuramic acid) cross linked by way of peptide side chains.
Bacterial cell walls are distinguished from the cell walls of plants, which are made of cellulose, and fungi, which are made of chitin. The cell wall of bacteria is also distinct from that of Achaea, which do not contain peptidoglycan. The cell wall is composed of S-layers, which can be either protein or glycoprotein.
The antibiotic penicillin (produced by a fungus called Penicillium) kills bacteria by inhibiting a step in the synthesis of peptidoglycan.
The Gram Stain
Bacteria can be divided into two large groups on the basis of a differential staining technique called the gram stain. One large group is called gram-positive and the other, gram-negative. Following the gram-staining procedure, gram-positive organisms will appear purple, gram-negative organisms will appear pink or red. This staining procedure is based upon differences in the structure of the cell wall between the two groups.
Gram-positive bacteria have a thicker peptidoglycan wall. The cell wall contains polyalcohols called teichoic acids, some of which are lipid-linked to form lipoteichoic acids. Lipoteichoic acids link the peptidoglycan to the cytoplasmic membrane.
Gram-negative bacteria contain less peptidoglycan. In the gram-negative bacteria, a thin layer of peptidoglycan is sandwiched between the plasma membranes and a second outer membrane. The outer membrane contains phospholipids and lipopolysaccharide, lipids with polysaccharide chains attached.
Procedure for the Gram Stain
- A bacterial smear is prepared and then stained with the purple stain crystal violet.
- The slide is washed off with distilled water.
- The slide is covered with Gram’s iodine, which is a mordant. The iodine combines with crystal violet to form a compound or precipitate that remains in gram-positive bacteria, but can be removed from gram-negative bacteria by washing with ethyl alcohol.
- The slide is flooded with ethyl alcohol until the purple dye no longer appears in the alcohol flowing from the slide. If the bacteria are gram-positive, they will not be decolorized. The crystal violet dye will remain in the cells. Gram-negative bacteria are decolorized by the alcohol, losing the purple color of the crystal violet.
- The slide is washed using distilled water, stopping the action of the alcohol.
- The bacterial smear is counterstained using the red dye safranin. Gram-positive bacteria will retain the purple color of the crystal violet stain. Decolorized gram-negative bacteria will be stained pink by the safranin.
- The slide is washed, blotted dry, and allowed to dry at room temperature.
This slide is examined microscopically. Gram-positive bacteria will appear purple. Gram-negative bacteria will appear pink.
Structures Internal to the Cell Wall
Plasma (Cell) Membrane
The plasma membrane is a thin membrane internal to the cell wall that encloses the protoplasm of the cell.
It is composed of protein and phospholipid molecules.
Functions of the Cell Membrane
- It controls the transport of most compounds entering and leaving the cell.
- Produces a separation of protons (H+) from hydroxyl ions (OH-) generating a proton motive force. This force is responsible for driving functions such as transport, motility, and synthesis of ATP.
Cytoplasm is the substance contained within the cell membrane.
Cytoplasm consists mostly of water. Dissolved and suspended in the water there are many substances including inorganic ions, nucleic acids, proteins, carbohydrates, lipids, inorganic ions, and a variety of compounds of low molecular weight.
There are no membranous organelles in the cytoplasm of a bacterial cell, but there are ribosomes, internal membranes, a cytoskeleton, and storage granules.
In photosynthetic bacteria, internal membranes within bacterial cells may serve as a location for photosynthetic reactions.
Unlike eukaryotic cells, bacteria usually lack large membrane-bound structures in their cytoplasm such as a nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells.
Among the best studied of the bacterial organelles are the magnetosomes, round structures that build magnetic particles within their lipid bilayer membranes. The organelles allow aquatic “magnetotactic” bacteria to navigate vertically along the Earth’s magnetic fields toward the low-oxygen depths in which they thrive https://www.quantamagazine.org/bacterial-organelles-revise-ideas-about-which-came-first-20190612/
Tantalizing examples appear in a group of oval-shaped aquatic bacteria known as planctomycetes. Some species of planctomycetes contain a membrane-bound organelle called an anammoxosome, which sequesters a chemical reaction that produces nitrogen along with toxic intermediaries. Anammoxosomes act like energy factories for the bacteria, much as mitochondria do in eukaryotes, though anammoxosomes do not seem to be remnants of symbionts as mitochondria are. https://www.quantamagazine.org/bacterial-organelles-revise-ideas-about-which-came-first-20190612/
The bacterial phylum Planctomycetes has revealed a number of compartmentalization features. The Planctomycetes cell plan includes a intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space). Membrane-bound anammoxosomes have been discovered in five Planctomycetes “anammox” genera, which perform anaerobic ammonium oxidation. https://en.wikipedia.org/wiki/Organelle
The prokaryotic cytoskeleton consists of structural filaments within the protoplasm. The Cytoskeleton functions in cell division, or to produce changes in cell shape.
Storage granules contain phosphate or sulfur.
Magnetosomes are particles of the iron mineral magnetite – Fe3O4. They allow bacteria to respond to a magnetic field.
Ribosomes are small granules that are composed of RNA and protein.
Ribosomes are the site of protein synthesis.
Ribosomes are numerous in the cytoplasm of bacterial cells. Observation with the electron microscope shows that the cytoplasm is quite densely packed with ribosomes.
Several antibiotics, such as streptomycin, neomycin, and tetracycline exert their antimicrobial effects by inhibiting protein synthesis.
The nuclear area, or nucleoid, of bacterial cells contains a single, long, circular molecule of DNA, referred to as the bacterial chromosome. This is the cell’s genetic information.
Unlike the chromosomes of eukaryotic cells, bacterial chromosomes are not surrounded by a nuclear envelope. Eukaryotic cells have rod-shaped chromosomes containing linear DNA bound to special proteins known as histones.
Bacteria often contain, in addition to the bacterial chromosome, small cyclic DNA molecules called plasmids.
Plasmids are extrachromosomal genetic elements; that is, they are not connected to the main bacterial chromosome.
Plasmids are used to transfer genetic material from one cell to another. Plasmids can pass from one cell to another cell by passing through the cell wall. When it enters the cell that receives it, it introduces new genetic information into that cell.
Plasmids are now used in Genetic Engineering Research to introduce genetic material into recipient cells.
Endospores are highly durable, dehydrated bodies with a thick wall.
Endospores are formed by bacterial cells in response to harsh conditions such as lack of food, lack of water, high temperatures, freezing temperatures, etc.
They are formed inside the bacterial cell wall.
Since one vegetative cell forms a single endospore, which after germination remains one cell, sporogenesis in bacteria is not a means of reproduction. There is no increase in the number of cells.
Endospore formation is important from a clinical viewpoint, because endospores are quite resistant to processes that normally kill vegetative cells. Such processes include heating, freezing, desiccation, use of chemicals, and radiation. Whereas temperatures above 70º C kill most vegetative cells, endospores may survive in boiling water for an hour or more. Endospore-forming bacteria are a problem in the food industry, since some species produce toxins that result in food spoilage and disease.