Cytology of typical bacterial Cell
Bacterial Morphology And Ultra Structure
Bacteria are unicellular, free-living, microscopic organisms. Morphology of bacteria includes the size, shape and arrangement of microbial cell. However, these features vary with species of microorganisms.
The structure of microbial cells reveals the composition and topography of chemical constituents building the cell wall, the components outside and internal to the cell walls.
Morphology of bacterial cell:
Size, shape and arrangement of bacterial cell: The size, shape and arrangement of microbial cells vary with species to species. Size: Bacterial cell have size about 0.1um – 20um in diameter e.g. size of disease causing bacteria Streptococcus pneumoniae – 1.25µm, Clostridium tetanae – 2.5µm Vibrio cholerae – 1-5µm, Salmonella typhi – 0.5-4µm.(fig.1.1) Fig. 1.1 Shape and arrangement bacterial cell
Ultra structure of bacterial cell:
The structure of bacterial cell consists of outer layer or cell envelope it contains two components such as cell wall and a cytoplasmic membrane or plasma membrane. Inside the plasma membrane, there is protoplasm comprising the cytoplasm, cytoplasmic inclusions such as ribosome’s, mesosome, granules, vacuoles and nuclear body.
The cell may be enclosed in a viscous layer, which may be a loose slime layer or organized as a capsule. Many bacteria have filamentous appendages called fimbriae or pili.Many bacteria also posses flagella, which are organs of locomotion (fig.1.2) Fig.1.2 Bacterial cell Ultra Structure
Many prokaryotic microorganisms synthesize amorphous organic exopolymers which are deposited outside the cell wall called capsules or slime layer or sugar coat. The term capsule refers to the layer tightly attached to the cell wall while the slime layer is the loose structure that often diffuses into the growth medium. Capsule layer may be thin in size less than 0.2µm called microcapsule and thick layer of size more than 0.2µm to 10µm called macro capsule.
Capsulated bacteria produce smooth colonies on surface of agar media. These bacteria are usually non-motile as flagella remains unfunctional in the presence of capsule. Development of capsule is dependent on the existence of favorable environmental conditions such as sugar concentration, blood serum or growth in a living host. Capsule may be composed of a complex polysaccharide or polypeptide or hyaluronic acid. Water (98%) is the main component of bacterial capsule. Capsule or slime layer has less affinity for basic dyes and is not visible in gram staining. Special capsule staining technique is used by using copper salts as mordant. Bącterial Morphology And Ultra Structure
- It gives protection against dry condition.
- It store food of bacteria.
- It also store waste material.
- It protect cell from unfavorable conditions.
- Capsule gives protection from phagocytosis.
- Capsule play important role in disease causing ability in some bacteria for example Diplococcus pneumoniae.
2) Cell wall :
The cell wall of bacteria is a semi rigid complex structure present between the capsule and cytoplasmic membrane. It gives protection to inner cytoplasm and nucleic acid present in cytoplasm. Cell wall gives size, shape, rigidity to bacterial cell. It is about 10-20nm in thickness and constitutes 20-30% of the dry weight of the cell. Cell wall cannot be stain easily by different staining reagents.
It can be demonstrated by differential staining, electron microscopy and by specific antibody. The cell wall of bacteria contains diaminopimelic acid (DPA), muramic acid and teichoic acid. These substances are joined together to give rise to a complex polymeric structure known as Peptidoglycan or murein or mucopeptide. Peptidoglycan consists of three parts (fig.1.3)
- A backbone composed of alternating N-acetyl muramic acid.
- A set of tetra peptide side chains attached to N-acetylmuramic acid.
- A set of penta peptide cross-bridges.
Fig.1.3 Organization of Peptidoglycan The backbone is same for all bacterial species. Tetra peptide side chains and nort peptide cross-bridges vary from species to species but the most commonly found side chain contains four amino acid as L-alanine, D-alanine, D-glutamic acid and diaminopimelic acid (DPA). Peptidoglycane is the major constituent of the cell wall of Gram-positive bacteria (50-90%). Gram-negative cell wall is more complex and the cell wall contains lipopolysaccharides (LPS). Cell wall synthesis may be inhibited or interfered with by many factors. Lysozyme enzyme present in many tissue fluids causes lysis of bacteria. If the Gram-positive cell is treated with Lysozyme and the cell wall is completely removed, the cell is called protoplast. If the Gram-negative cell is treated with EDTA and the cell wall is partially removed, such a cell is called spheroplast.
Functions of cell wall:
- Cell wall is involved in growth and cell division of bacteria.
- It gives shape to the cell.
- It gives protection to the internal structure and acts as a supporting layer.
- It contains receptor sites for phages and colicin.
- It provides attachment to complement.
- It shows resistance to the harmful effects of environment.
Differences between Gram-positive cell wall and Gram-negative cell wall.
|Sr. No||Components||Gram Positive||Gram Negative|
|1.||Thickness||Thick about (20-25nm)||Thin(10-15m)|
|2.||Peptidoglycan||More (50-90%)||Less (5-10%)|
|5.||Lipids||Less or Absent||Present|
|7.||Outer membrane and periplasmic space||Absent||Present|
|8.||Effect of Lysozyme||Easily destroyed||Resistant|
|9.||Types of amino acids||Few||Several|
|10.||Susceptibility to streptomycin and tetracycline||Slight||Marked|
|11.||Susceptibility to penicillin and sulfonamides||Marked||Much less|
|12||Examples||Staphylococcus aureus||Escherichia coli|
Unit membrane/Cytoplasmic membrane/ plasma membrane :
The cytoplasmic (plasma) membrane is a thin (5 to 10nm) layer lining the inner surface of the cell wall and separating it from the cytoplasm. It is composed of phospholipids (20-70%).The phospholipids form a bilayer in which the proteins are tenaciously held and are called integral proteins. These proteins can be removed only by destruction of the membrane with treatment by detergents. Other proteins are loosely attached and can be removed by mild treatments such as osmotic shock. These proteins are called peripheral proteins. The phospholipids molecules are arranged in two parallel rows, called a phospholipids bilayer. Each phospholipids molecule contains a polar head composed of a phosphate group and glycerol. The non-polar tails are in the interior of the bilayer and the polar heads are on the two surfaces of the phospholipids bilayer (fig. 1.4). Electron microscopy shows the presence of three layers constituting a ‘unit membrane’ structure. Fig.1.4 Cytoplasmic membrane
1) It provides mechanical strength to the bacterial cell.
2) It acts as a semi permeable membrane controlling the inflow and outflow of metabolites in the cell.
3) It helps in DNA replication.
4) It contains the enzyme, permease, which plays an important role in the Passage of selective nutrients through membrane. It serve as an attachment site for chromosomal and plasmid DNA.
5) It contains the enzyme involved in the biosynthesis of membrane lipids and various other macromolecules of the bacterial cell wall.
The bacterial cytoplasm is a suspension of organic, inorganic solutes in a viscous water solution. The cytoplasm of bacteria differs from that of higher eukaryotic organisms in not containing endoplasmic reticulum, Golgi apparatus, mitochondria and lysosome. It contains the nucleus, ribosomes, proteins and other water soluble components and reserve material. In most bacteria, extra chromosomal DNA (plasmid DNA) is also present.
All living cells contain ribosomes which act as site of protein synthesis. High number of ribosomes require for high rate of protein synthesis. Cytoplasm of prokaryotic cell contains about 10,000 ribosomes. The prokaryotic ribosome is free in cytoplasm but eukaryotic ribosomes are attached to plasma membrane. The prokaryotic ribosome is smaller and less dense than eukaryotic ribosomes. Prokaryotic ribosomes are called 70s ribosomes while eukaryotic ribosomes are called 80s ribosomes. The ribosome consists of two subunits. Prokaryotic ribosomes sediment into 70s made up of 50s and 30s subunit (fig. 1.5). While eukaryotic ribosomes sediment into 80s made up of 60s and 40s. The letter ‘S’ refers to Svedberg unit which indicates the relative rate of sedimentation during ultracentrifugation. Sedimentation rate depends on size, shape and weight of particles. Fig.1.5 Ribosome
Functions of ribosome :
The function of ribosome is that it is site for protein synthesis because in ribosome, the enzyme for peptide bond formation between two amino acid is present i.e peptidal transferase.
Nuclear membrane /Nucleus:
Bacterial nucleus can be demonstrated by acid or ribonuclease hydrolysis. They may be seen by a light microscope after staining or by electron microscopy. They appear as oval or elongated bodies, generally one per cell. The genome consists of a single, large double stranded DNA. It forms closed circular strand with extensive folding. It may open under certain conditions to form a long chain about 1000um in length. The bacterial chromosome is haploid and replicates by simple fission instead of mitosis as in a eukaryotic cell. A bacterial cell may posses extra nuclear genetic elements consisting of DNA. These cytoplasmic carriers of genetic information are termed as plasmids or episomes. They may be transmitted to daughter cells during binary fission or by conjugation. Presence of Plasmid confers the characteristics of drug resistance, conjugation ability, pathogenesis and nitrogen fixation ability
Mesosome is complex, localized infolding of the cell membrane. (fig.1.6). They are commonly observed in gram positive bacteria. They are particularly developed in bacilli. The mesosome consists of invaginated cell membrane with many vesicles, tubules or lamellar whorls filling invagination. The lipid components of the mesosome appear to be similar to that cell membrane. The Salton and Owen (1976) suggested that mesosome may believė to equivalent of the mitochondria of higher cells. Fig.1.6 Mesosome
1) The main function of mesosome is the storage of food material.
2) It is the site for DNA replication
3) Almost all major function of the bacterial cells has been attributed in mesosome.
4) They also play role in the respiratory acti vity
Flagella are long thin-hair-like appendages, responsible for the motility of bacteria (fig. 1.7). They are found on both Gram positive and Gram negative bacteria. They are 0.01 to 0.02µm in diameter and 3 to 20 um in length. Flagella can be seen by an ordinary light microscope by special staining technique. These are organs of locomotion. Fig.1.7 Arrangement of flagella Fig. 1.8 Structureof flagella
Structure of flagella:
The Bacterial flagella have main three structure parts.
a) Basal b) Hook and c) Filament
a) Basal body :
The basal body structure is complex and different in Gram positive ane Gram negative bacteria. In Gram negative bacteria four rings are mounted on central rod In Gram negative bacteria two pairs of rings, the proximal ring and the distal ring are connected by a central rod.
- L-(Lipopolysaccharides) ring
- P-(Peptidoglycan) ring
- S- (Super membrane) ring
- M-(Membrane) ring.
The outer pairs of rings, L-ring and P- ring are attached to respective polysaccharide and Peptidoglycan layer of ell wall and the inner pair of ring i.ce. S-ring to pass through it. In Gram positive bacteria only the distal (inner) pair of ring is present. The S-ring is attached to inside thick layer of Peptidoglycan and M-ring is attached to cell membrane.
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 Gram negative bacteria.
c) Filament or shaft:
The outermost long region of the flagellum is called filament or shaft. 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 as identify of certain pathogenic bacteria unlike eukaryotes, the filaments are not covered by a membrane or sheath.
Mechanism of flagella movement :
Motility of bacteria may be observed microscopically using the hanging drop technique or by detccting the spreading growth in semi solid agar medium. Under the microscope, active motility has to be differentiated from the passive movement of the cells which is either due to air currents or due to Brownian movement some helical bacteria like spirochetes exhibit swimming motility in highly viscous media. However, those have flagella like structures located within the cell, just beneath the outer cell envelope these are called periplasmic flagella or endoflagella or axial fibrils e.g. Treponema pallidum. Some bacterial species are motile by gliding motility. The movement of a bacteria towards (positive) or away (negative) from a particular stimulus is called taxis. The bacterial movement in response to chemical stimulus is called chemotaxis. The bacterial movement in response to magnetic stimulus called magnetotaxis or in response to light is called phototaxis. If a suspension of bacteria is placed on a slide under cover slip. the aerobic bacteria accumulate at the edges of cover slip, while the anaerobic bacteria at the center it is called aero-taxis.
Pili are very fine, straight, filamentous appendages much smaller than flagella. They are usually present on Gram negative bacteria except a single Gram positive bacteria Corenebacterium renale. Pili differ from flagella in being shorter and thinner, straight and less rigid but they are large in number. They occur either at the poles of bacterial cell or evenly distributed over the enteric surface of the cell. The pili are 0.2-20um long with a diameter of about 250 A. According to function pili are two types ie common pili which act to adhere the cell surface and sex pili which capable of transferring DNA from conjugation tube during conjugation.They originate from cytoplasm and penetrate through the peptidoglycane layers of the cell wall. The fimbriae of E-coli are nearly 100% protein. The protein in fimbriae has been called as fimbrilin or pilin.
Functions of pili :
- 1) Sex pili present on male cell and it works as a bridge of conjugation.
- 2) They are antigenic in nature.
- 3) They play main role in infection to pathogenic bacteria.
- 4) They have strong tendency to adhere to mammalian cells or other surface.
- 5) Sex pili also serve as phage receptor site.
Reserve food material (cytoplasmic inclusion):
The cytoplasm of bacterial cell contain various different types of reserve food materials like
1) PHB (poly B-hydroxybutyrate granules/lipid granules)
A granular energy reserve compound is found in several bacterial cells. Lipids are found in high amount in several species of Bacillus, Azotobacter, Mycobacterium etc. These are present as a storage materials as a polymer poly B-hydroxybutyrate (PHB). PHB formed by condensation of acetyl coenzyme A forming acetoacetyl CoA. In Bacillus megaterium PHB is about 60% when bacteria are growing on an acetate or butyrate medium. These granules are 0.2 to 0.3um in diameter. These granules are stained by using Sudan black B or Sudan dye. Bacteria use PHB granules as a source of food material and energy.
2) Volutin granules/ Phosphate granules/ Metachromatic granules:
These granules are found in algae, fungi, protozoa and bacteria. These are present in higher number in Corenebacterium diphtheriae. These granules are made up of polyphosphate or inorganic phosphate which is form when cells are growing in a rich phosphate medium. These granules are stain with Methylene blue and look like reddish purple in color; therefore it is called as Metachromatic granule. These granules store phosphate in form of linear chain of inorganic pyrophosphate.
3) Glycogen :
It is storage product found in prokaryotes. It is polymer of glucose molecules are linked by a,1-4 glycosidic linkage and at branched point a,1-6 branching is there. It is also found in the mammalian liver. Some fungi contain glycine which are B, 1-3 linked polymers.
Glycogen is also use as a storage food material or source of carbon and energy in Bacillus and Clostridium and some genera of Cynobacteria and enteric bacteria.
Bacterial cell division :
Bacteria reproduce asexually as well as sexually. Many microorganisms multiply by the asexual process of cell fission which results in division of the cell into two or more vegetative cells. Most bacteria multiply by transverse binary fission,that division into two equal cells.
The circular chromosome divides into two identical circles, which segregate at apostate ends of the cell. Simultaneously, the cell wall is laid down in the middle of the cell, which finally grows to produce two new cells each with its own wall and nucleus (fig. 1.9). Fig.1.9 Binary cell division
Bacterial Morphology And Ultra Structure
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