The bacterial cells are surrounded by a wall made up of mucopeptide, which is peculiar to bacteria and not found elsewhere. Its amount varies in the cell walls of the two main divisions of bacteria; Gram +ve and Gram -ve. In Gram +ve bacteria, it is the major cell wall component (80 per cent). However, in Gram -ve bacteria, it is present in small quantity; the major portion being formed by lipoprotein and lipopolysaccharide.
The cell wall is surrounded, in some bacteria, by a polysaccharide in the form of a definite layer called capsule. When the polysaccharide is more fluid in consistency, it forms a loose mass of slime surrounding the wall. Motile bacteria have flagella whose number and position differ. Bacterial flagella lack the ‘9 + 2’ structure (characteristic of eukaryote flagella) and are made up of flagellin molecules.
The pili, which are minute, hair-like, superficial appendages, covering the surface of most of the Gram -ve bacteria, are helpful in holding the cells to water surface, and also during conjugation. Inside, the cell shows very little structure, and lacks an organized nucleus and other organelles-chloroplast, mitochondria, golgi body, endoplasmic reticulum and lysosomes.
The cytoplasm contains- (a) particles, (b) reserve materials as granules, and (c) soluble substances. The genome is not organized into chromosomes but represented by a circular, long DNA molecule attached to the cell membrane. It is concentrated in the cytoplasm, which can be stained and seen in light microscope. The photosynthetic bacteria have lamellae (thylakoids) and vesicles, which constitute the photosynthetic apparatus.
Mesosomes are particles concerned with bacterial genome-replication and septum formation during the cell division. Ribosomes, having the sedimentation coefficient of 70S, lie free in the cytoplasm. There are inclusions- granules (reserve materials) and vesicles. The cell sap contains high molecular weight (e.g., various soluble enzymes and t-RNAs) and low molecular weight substances (amino acids, nucleotides), which occur in different zones of the cytoplasm a ‘pools’.
We shall now study the various parts in some detail.
Structurally, the bacterial cell can be divided into 5 regions:
1. Surface appendages-flagella, fimbriae and pili
2. Surface adherents-capsules and slime layers
3. Cell wall
4. Cytoplasm and organelles
5. Special structures-endospores, stalks.
1. Surface Appendages:
Flagella (= sing, flagellum) are the organs of locomotion in motile forms and many times longer (4-5 µm long) than the bacterial cell. They are fundamentally different from the flagella or cilia of eukaryotes in lacking the ‘9+2’ structure. Flagella and cilia of eukaryotic cells are morphologically and physiologically similar. Cilia are shorter, present in greater number and have a coordinated beat.
The bacterial flagellum (120-150 Å in diameter) is a cylindrical, hollow strand made up of protein molecules, called flagellin which is structurally similar to the proteins of the hair and muscles. Each flagellin molecule is 40 Å in diameter. Several (usually 3-8) longitudinal chains of flagellin molecules run longitudinally, twining around each other to form a wavy helical or rope-like structure. A cross section of the flagellum reveals 8 flagellin molecules around a central space.
The flagellum consists of three morphological parts- a basal body, the hook, and the filament. The basal body is anchored in the plasma membrane; the hook penetrates the wall and the filament is the part that appears in the stained preparations. The cell wall is necessary for the flagellar movement. If the wall is removed, the flagellar movement ceases. The antigenic property of some of the bacteria (e.g., Salmonella, H antigen) resides in the flagellin molecules.
The flagellation is described as monotrichous (single flagellum on one end), lophotrichous (a tuft of flagella on one end), amphitrichous (tuft on both the ends), or peritrichous (flagella all over the cell).
Fimbriae and Pili (sing, fimbria, pilus) are terms which have customarily been used interchangeably, but these are distinctly different.
The fimbriae are shorter and numerous while the pili are longer and sparse. The fimbriae have a tendency to stick to each other and to surface, resulting in aggregation of cells on the surfaces of liquids and in adhesion to substrates during colonization. When they bring about adhesion of bacterial cells with red blood cells, the phenomenon is called hemagglutination.
The pili (also called sex-pili) are elongate, rigid, tubular appendages made of a special protein called pilin. These have so far been reported only from Gram negative bacteria where they serve to connect two cells during conjugation (conjugate means linked together), and allow the DNA to pass from donor cell into the recipient.
2. Surface Adherents:
Some bacteria have a gelatinous covering around them. When it is narrow and well-defined, it is called a capsule. When it is a loose mass, it is called slime. Chemically, capsule and slime are the same-a polysaccharide of glucose. In Bacillus anthracis, it is a polypeptide. Depending on the thickness, the capsule is designated as a macrocapsule (more than 0.2 µm thick) or a microcapsule (less than 0.2 µm).
The production of capsule or slime is a hereditary, mutable character. The capsules protect the pathogenic bacteria from phagocytosis and also serve as a storage product, which may be consumed when needed. Certain pathogenic bacteria (e.g., Pseudomonas solanacearum, which causes wilt disease in plants) owe their virulence to the capsule — polysaccharide or slime.
3. The Cell Wall:
The cell walls of bacteria and cyanobacteria differ from plants in being made up of mucopeptide (= peptidoglycan) and not cellulose. This difference provides a site where bacterial pathogens can be attacked by antibiotics without damaging the diseased eukaryotic plant or animal cells.
Mucopeptide is a polymer made up of alternating units of NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid) joined by β, 1-4 linkages. NAG and NAM are amino sugars. The mucopeptide chains are laterally linked by short chains of amino acids, which originate at the carboxyl group of the muramic acid molecules. A diamino acid like lysine is essential for cross-linking of the animo acid chains. Some of the amino acids are D-isomers, which is quite peculiar, as in normal metabolism only L-amino acids are used.
The cell walls of Gram +ve and Gram -ve bacteria differ in their chemical composition. The wall of the Gram +ve bacteria is homogeneous containing 85 per cent or more of mucopeptide and simple polysaccharide, like teichoic acids (teichos = wall) which are polymers of ribitol and glycerol phosphates. Teichoic acids serve as antigens and also regulate entry of ions.
The cell wall of Gram -ve bacteria contains only 3 to 12 per cent mucopeptide, the rest being lipoprotien and lipopolysaccharide. The wall of Gram -ve bacteria, in electron microphotographs appears tripartite, i.e., three-layered. It is formed by an inner cytoplasmic membrane, middle periplasmatic space containing mucopeptide and an outer membrane, which contains lipopolysaccharide (also called endotoxin) in the outer lipid layer, and lipoprotein in its inner lipid layer.
The wall of Gram -ve bacteria functions like a sieve. The outer membrane contains proteins, called porins, which form aqueous channels through which small molecules can pass.
4. Cytoplasm and Organelles:
The Nucleoid:
In bacteria, the genome (DNA) is not bound with proteins. The ‘chromosome’ consists of a single closed ring 1,000 µm long. Thus, in a bacterial cell the DNA molecule is over a thousand times longer than the cell itself. The DNA helix must untwist itself every half an hour at every replication during cell division.
The expression ‘the tangled skein of life’ for the DNA is most appropriate. DNA molecule is attached to the cell membrane possibly at the mesosome. The DNA, in fulgen-stained preparations, is seen concentrated, forming a gel-like structure less dense than the cytoplasm. This concentrated structure is called nucleoid or nucleoplasm.
The Plasmids:
In addition to the ‘chromosome’, some bacteria have one or more, small circular DNA molecules, called plasmids. These provide additional genetic information which, though not essential for basic life processes, helps the bacteria in various ways. The plasmids can exist either free in the cytoplasm or integrated with the chromosome, and accordingly, replicate independently or along with the chromosome.
The types of plasmids and their functions are as follows:
1. F-plasmids (fertility factors), code for proteins of sex-pili.
2. Resistance (R) plasmids carry genes that provide resistance to antibiotics like chloramphenicol and tetracycline, and to heavy metals such as arsenic, mercury, etc.
3. Bacteriocinogens or Col-factors, carry genes for bacteriocins- proteins killing closely-related bacteria.
4. Virulence Plasmids produce toxins in disease, as in Salmonella.
5. Tumor (Ti) plasmids, responsible for tumor formation in plants (now used in genetic engineering of plants).
6. Catabolicplasmids, contain genes for catabolic enzymes.
Although plasmids usually contain only 1-5 per cent of the DNA of the bacterial chromosome, the effect of this small genetic information can decide survival, as happens with the antibiotic-resistance gene (R plasmids).
Lamellae and Chromatophores:
Photosynthetic bacteria and cyanobacteria have lamellae (thylakoid) or chromatophores, instead of chloroplasts. Lamellae consist of two parallel unit membranes, which may be small or long, extending throughout the cytoplasm.
Chromatophores (or vesicles) are hollow spherical structures about 300Å in diameter. Much of the cytoplasm appears occupied by them. The bacterial photosynthetic apparatus (lamellae and chromatophores) contain pigments together with enzymes and the electron transport system for the photosynthetic phosphorylation of the light reaction. They are devoid of the enzymes associated with the dark reaction.
Ribosomes:
These are small particles (100 Å in diameter), which, along with the reserve materials, give a granular appearance to the otherwise homogeneous cytoplasm. In photosynthetic bacteria, however, the ribosomes are overshadowed by the chromatophores. Ribosomes of bacteria differ from eukaryotic ribosomes in that they lie free in the cytoplasm (in eukaryotic cells they lie attached to the endoplasmic reticulum), and that they have a sedimentation coefficient of 70S against 80S of eukaryote ribosomes.
Mesosomes:
These are extensions of the plasmamembrane, which simultaneously initiate DNA replication and septum formation during cell division. The available evidence suggests that mesosomes, most likely, have diverse functions which vary from cell to cell and even vary from one growth phase to another. In recent years, the status of mesosomes has been controversial and there is doubt whether these are actual structures or artifacts of fixation.
Inclusions:
The cytoplasm shows a variety of small bodies, collectively called inclusions. These include granules and vesicles.
Granules:
The cytoplasm itself is a homogeneous aqueous solution of soluble proteins, enzymes, cell solutes, inorganic ions, and metabolites of small molecular weights. Under electron microscope, the cytoplasm appears granular due to the reserve materials. Some reserve materials lie in a state of fine dispersion in the cytoplasm.
The reserve materials can be classified into three categories:
1. Organic polymers-polysaccharides (including glycogen and PBHB (poly beta hydroxy butyrate) formed within special single-layered membrane), lipids, etc.
2. Inorganic metaphosphate granules-volutin or metachromatic granules which are polymers of phosphate insoluble reserves.
3. Elemental sulfur – present in sulfur-oxidizing bacteria; serves as energy reserve.
Vesicles:
Some aquatic bacteria and cyanobacteria have membrane-bound gas vesicles (or Vacuoles) that provide buoyancy which is helpful in floating.
Magnetosomes:
These are membrane-bound vesicles containing magnetite (Fe3O4) found in magnetotactic bacteria. The presence of these magnetic inclusions helps these bacteria, which are anaerobic or microaerophilic, to swim towards the magnetic field i.e., downwards where microaerophilic conditions exist.
The magnetosomes are arranged in parallel chains. Experiments are underway to use these bacteria in the manufacture of magnets, audio and videotapes. Magnetosomes have been found in meteorites from Mars, associated with bacillus shaped structures and it is believed to represent bacterial life on Mars.
5. Special Structures:
Endospores:
The genera Bacillus and Clostridium, as well as a few cocci and spirilla, form heat-resistant structures known as endospores which are difficult to stain. Usually, one endospore is formed in each sporangium. The endospore may be central, lateral or terminal in position.
Sometimes, it is larger than the diameter of the cell. About 15 per cent of the dry weight of the endospores is due to dipicolinic acid and calcium, which provide extreme resistance to heat. The endospores can survive boiling temperature for one and half hours. As soon as the environment becomes congenial, the endospore germinates and produces the bacterium.
Stalks:
The stalked-bacteria have stalks which differ in their mode of origin and nature. In Caulobacter group, the stalk is a narrow extension of the cell; the wall of the stalk being continuous with the wall of the cell. The cell division is by a transverse fission and not by binary fission as the two daughter cells are not identical. Apical cell carries the polar flagella and later develops the stalk.
The lower daughter cell develops the flagella. The other type of stalk is found in Gallionella. The stalk is not a part of the cell but a secreted product. It is heavily impregnated with ferric hydroxide and for this reason these bacteria are referred to as Iron bacteria. When a cell divides, the stalk bifurcates, each branch bearing one daughter cell.
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