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Bacterial size
Bacterial size
Bacteria are very small, most being approximately 0.5 to 1.0 micrometer diameter. Smallest bacteria are Mycoplasmas, as small as 0.2 micrometers (almost as small as largest poxviruses!) Accepted wisdom is that bacteria are smaller than eukaryotes. But certain cyanobacteria are quite large; Oscillatoria cells are 7 micrometers diameter, size of red blood cells. And certain eukaryotes (e.g. Nanochlorum eukaryotum) are very small, only 1 to 2 micrometers, but true eukaryotes (nucleus, chloroplast, mitochondrion are present). So size difference is, like many generalizations, only a useful yardstick, not an absolute truth.
Epulopiscium fishelsoni, discovered in 1985 in intestinal tract of sturgeon fish, is an enormous, cigar-shaped cell, as large as 80 x 600 micrometers (that's 0.6 mm, large enough to be seen by the naked eye). Amazingly, this cell is prokaryotic! Initial evidence by EM was hard to believe, but confirmed rRNA comparisons with other organisms, a cousin of Gram-positive Clostridium genus.
Bacterial shapes
Bacterial shapes
The shape of the bacterium is governed by its rigid cell wall. Typical bacterial cells are spherical (Cocci and Coccus) or straight rod shaped (Bacilli and Bacillus) or helically curved rods (Spirilli and Spirillum). Few bacteria can exhibit variety of shapes that are referred as Pleomorphic. (Ex. Arthrobacter).
Cocci appear in several characteristic arrangements, depending on the plane of cellular division and whether the daughter cells stay together or not. Spherical is called coccus. Division along the same plane forms chains; 2 cocci together - Diplococcus; 4 - 20 in chains - Streptococcus. Division along 2 different planes – Tetrads or Tetracoccus; Division along 3 planes regularly - Sarcinae; Division along 3 planes irregularly – Staphylococcus.
Bacilli are not arranged in patterns as complex as those of cocci and mostly occur in single or in pairs. The following are some arrangements of rod shaped bacteria. Rod shape is called Bacillus; two bacilli together - Diplobacilli; Chains of bacilli are called Streptobacilli; Palisades - Rods side by side arrangement like match sticks or in X, V or Y figures. Ex. Corynebacterium diphtheria.
Curved bacteria (spirilla) vary with the number of twists they had. Spiral shape that is rigid is called Spirillium. Spirals with less than one complete twist are called as Vibrioid shapes (Ex. Vibrio). If the organism is flexible and undulating, it is called Spirochete.
Other shapes:
Filamentous shaped – Streptomyces sp. form long, multinucleated, branched filaments called as hyphae
Pear shaped – Ex. Pasteuria
Lobed spheres - Ex. Sulfolobus
Rod with square end – Ex. Bacillus anthracis
Bacterial shapes: A video
Bacteria – Morphology
Bacteria – Morphology
Bacteria are unicellular organisms of relatively simple construction, especially if compared to eukaryotes. Whereas eukaryotic cells have a preponderance of organelles with separate cellular functions. Prokaryotes carry out all cellular functions as individual units.
A prokaryotic cell has five essential structural components: a genome (DNA), ribosomes, cell membrane, cell wall and a surface layer.
Other than enzymatic reactions, all the cellular reactions incidental to life can be traced back to the activities of these macromolecular structural components. Thus, functional aspects of prokaryotic cells are related directly to the structure and organization of the macromolecules in their cell make-up, i.e., DNA, RNA, phospholipids, proteins and polysaccharides. Diversity within the primary structure of these molecules accounts for the diversity that exists among bacteria.
This video will describe the size, shape and components of bacterial cell
1. Flagella
1. Flagella
Flagella (singular flagellum) are hair like, filamentous helical appendages that protrude through cell wall which is responsible for movement of the bacterium. They are very thinner than the eukaryotic flagellum. The diameter is about 20 nm, well below the resolving power of compound microscope. The flagellum is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in the fluid environment.
Flagellum is composed of three parts namely, basal body (associated with cytoplasmic membrane and cell wall), hook (next to basal body) and a filament (a hair like, longer than the cell). The flagellum is made up of flagellin protein.
The flagellum is powered by the proton motive force of the cell membrane. About half of the rod shaped, all the spiral and curved and very few spherical bacteria are motile by flagella.
The distribution/ arrangement of flagellum in the bacteria vary with different types.
Monotrichous flagellum – single flagellum present in the pole of the bacterium (Ex. Pseudomonas aeruginosa)
Lophotrichous flagella – cluster of flagella present in the pole of the bacterium (Ex. Pseudomonas fluorescens)
Amphitrichous flagella – single or cluster of flagella present in the both pole of bacterium (Ex . Aquaspirillum serpens)
Peritrichous flagella – flagella surrounded through out the body of the bacterium (Ex. Salmonella typhi)
Endoflagella
The flagella or filaments, present between the outer membrane and inner membrane of the cell wall of the bacterium are called as endoflagella or periplasmic flagella or axial filaments. They are attached at one end of the cell. Ex. Spirochetes- group of bacteria; Bacterium : Spirochete, Leptospira
The flagella are responsible for motility of the bacterium. It rotates like a propeller. Rings in the basal body rotate relative to each other causing the flagella to turn. The energy to drive the basal body is obtained from the proton motive force. The average speed of bacterial movement is 50 mm/sec which equivalent to its 10 body length.
How flagellum works? Youtube video
2. Pili
2. Pili
Pili (singular pilus) are hollow, non-helical, filamentous appendages thinner, shorter and more numerous than flagella. They also present in the non-motile bacteria too. Like flagella, they composed of protein, called pilin. They also called as Fimbriae. They are very common in gram –ve bacteria, but occur in some archaea and gram +ve bacteria too. Pili are mainly involved in the adherence or attachment of bacteria to surfaces, substrates and other cells in nature.
Some pili (F or sex pili) are involved in the transfer of DNA from one cell to other, called conjugation. In E. coli, both sex pili and common pili are present).
They also involve the virulence determination character of a bacterium. Ex. Neisseria gonorrhoeae causes disease if pili are present.
The pili also give resistance to bacterium from phagocytosis (Ex. Streptococcus pyrogenes)
The number of pili varies from few 1-4 to 100 – 200 per bacterium.
3. Capsule
3. Capsule
Some bacterial cells are surrounded by viscous substance forming a covering layer or envelope around the cell wall, are called as capsules. If it is visible under light microscope using special staining, it is called as capsule and if it is so thin, not able to see under light microscope, called as microcapsules. If it is so abundant that many cells are embedded in a common matrix, are called as slime.
Note : The capsule or slime or microcapsule often referred as Glycocalyx
Most of the capsules are polysaccharide nature, specifically homopolysaccharides. Ex. Streptococcus mutans capsule is made up of glucan.
Some bacterial capsule made up of polypeptides. Ex. Bacillus anthracis capsule composed of polymers of glutamic acid.
Functions :
Provide protection against temporary drying
Block the attachment of bacteriophages
Antiphagocytic (Ex. Streptococcus pneumoniae)
Provide virulence to the bacteria (Ex. S. pnemoniae causes disease only if capsulated)
Promote attachment of bacteria (ex. Streptococcus mutans adheres smooth surface of teeth and causes dental caries)
Prevent cell aggregation in the suspension by electrical charges
Important role in biofilm application
Other appendages / structures outside the cell wall
Sheath : Some species of bacteria, particularly those from fresh water and marine environment form chains or trichomes that are enclosed by hollow tube are called as Sheath. Ex. Sphaerotilus
Prosthecae : Some species of bacteria had semi-rigid extension of cell wall and cell membrane are called as prosthecae. This is to increase the surface area of the cell for nutrient absorption. Ex. Caulobacter.
Stalk : The non-living ribbon or tubular appendage that are excreted by the bacterial cell are referred as stalk. Ex. Caulobacter and Gallionella. They aided for attachment of the bacteria to surface.
4. Cell wall
4. Cell wall
All the bacteria have rigid cell wall. The cell wall is the essential structure that protect the cell from mechanical damage and from osmotic rupture of lysis. Prokaryotes live relatively diluted atmosphere (has lower osmotic pressure than the osmotic pressure of inside the cell), which accumulate high salt concentration inside. The osmotic pressure against the inside of the plasma membrane may be the equivalent to 10—25 atm. Since the plasma membrane is delicate, plastic structure, it must be restrained by an outside wall made of porous, rigid material that has high tensile strength. The cell wall of bacteria deserves special attention for several reasons.
They are essential for viability
They are composed of unique compounds no where else in nature
They are one of the important site for attack by antibiotics
They provide adherence and receptor sites for drug or virus
They cause symptoms of disease in animals
They provide immunological distinction and variation among strains of bacteria
The bacteria can be divided in to two major groups based on their cell wall chemistry, called as gram positive and gram negative bacteria. The gram negative cell wall is a multilayered and quite complex, where as the gram +ve cell wall consists of a single type of molecule and often much thicker.
Peptidoglycan layer: The rigid layer of both gram +ve and –ve bacteria is similar in chemical composition, called as peptidoglycan (or murein). It consists of N-acetyl glucosamine and N-acetyl muramic acid and small group of amino acids such as L-alanine, D-alanine, D-glutamic acid, and lysine or diamino pimelic acid. N-acetyl glucosamine and N-acetyl muramic acid were linked by beta 1,4 glucoside linkage and the aminoacids were cross linked. In G+ve bacteria, 90% of the cell wall consists of peptidoglycan layer and few quantities of teicholic acid usually present. Many bacteria have several layers (about 25) of peptidoglycon. In g-ve bacteria, only 10 per cent of wall is peptidoglycon and the major of the wall is consists of outer membrane.
The peptidoglycan is signature of the bacteria can be destroyed by certain agents. Ex. Lysozyme, a protein that break the b-1,4 glucoside linkage of peptidoglycan and weaken the cell wall. Water then enters to the cell, and cell swells and eventually bust. Lysozyme is present in the tears, saliva and body fluids.
If proper concentration of solute that does not allow the water to penetrate into the cell, such as sucrose is added to the medium, the solute concentration will be balanced between in side and out side the cell. Under those condition, the lysozyme still digest the peptidoglycan, but lysis does not occur, instead, protoplast is formed. (Protoplast referred as the cell with out cell wall, but somatically protected). If these sucrose stabilized cells are placed in water, lysis occurs immediately. Spheroplast often referred as the cell with some remaining of cell wall. If peptidoglycan layer is remove in G+ve cell, protoplast will be formed and in g-ve bacteria will form spheroplast.
Outer membrane: Besides peptidoglycan layer, g-ve bacteria contain additional wall layer made of lipo-polysaccharide. The layer is called as lipopolysaccharide layer or LPS layer or outer membrane. The lipids and polysaccharides are intimately linked and formed this layer. The polysaccharide has two portions namely, o- polysaccharide and core polysaccharide. O-polysaccharide contains galactose, glucose, rhamnose, mannose, and one or more dideoxy sugar like abequose, colitose, paratose or trylose. These sugars were connected four or five membered sequence which often branched. The core polysaccharide consists of keto deoxyoctonate, glucose, galactose and N-acetyl glucosamine. The lipid portion of the LPS referred as lipid A, is not a glycerol lipid but instead, the fatty acids are connected by ester amine linkage to disaccharides composed of N-acetyl glucosamine. The fatty acids are caproic, lauric, mysteric, palmitic, stearic acids. A lipid protein is also found on the inner side of the outer membrane of gram negative bacteria. It acts like an anchor between outer membrane and peptidoglycan layer.
Endotoxins : In Salmonella, Shigella, Escherichia, the outer membrane causes toxic and pathogenic symptom to human and mammals referred as endotoxins.
Porins : Protein substance present in the outer membrane of g-ve bacteria. They act like channel for entrance and exit of hydrophobic low molecular substances.
Periplasm : The space in between peptidoglycan and outer membrane is referred as periplasm or periplasmic space. In g+ve bacteria, it refers the space between peptidoglycan and cell membrane. This space harbours hydrolytic enzymes, binding proteins, detoxifying enzymes etc.
Gram Positive vs Gram Negative cell wall: Animation
5. Cell membrane :
5. Cell membrane :
The cell membrane is the most dynamic structure in prokaryotes. They act as selective permeability barrier that regulates the passage of substance in and out of the cell. Bacterial membrane consists of 40 per cent phospholipids and 60 per cent protein. The phospholipids forms bilayer and the proteins are present scatterly (called integral protein) and some protein present on the surface of the layer (called peripheral protein). The phospholipids are phosphoglycerides in which straight chain fatty acids are ester linked to glycerol.
Functions of cell membrane:
Osmotic or permeability barrier
Location of transport systems for specific solutes (nutrients and ions)
Energy generating functions, involving respiratory and photosynthetic electron transport systems, establishment of proton motive force, and transmembranous ATP-synthesizing ATPase
Synthesis of membrane lipids (including lipopolysaccharide in Gram-negative cells)
Synthesis of murein (cell wall peptidoglycan)
Assembly and secretion of extra cytoplasmic proteins
Coordination of DNA replication and segregation with septum formation and cell division
Chemotaxis (both motility and sensing functions)
Cell membrane structure and chemical difference among the organisms
Cell membrane structure and chemical difference among the organisms
6. Mesosomes
6. Mesosomes
The cytoplamic membrane invagination in the form of tubular or vesicle shaped are referred as mesosomes. If the mesosome is present in the centre of the cell referred as central mesosomes and they are involved in the cell division.
The mesosome which is present in the peripheral region (aside) of the cell are referred as peripheral mesosomes and they involved in the transport of extracellular enzymes from cytoplasm to external regions.
7. Cytoplasm
7. Cytoplasm
The cytoplasm of bacterial cell is the place where the functions for cell growth, metabolism, replication carried out. The gel like mat composed of water, protein, enzymes, nutrients, wastes, gases, like non-cellular materials and chromosomal DNA, ribosomes, plasmids like cellular components are present.
8. Chromosomal DNA
8. Chromosomal DNA
The bacteria do not contain true membrane enclosed nucleus and chromosomes. They have a long coiled double strand single circular structure often called as chr. DNA or nucleoid or bacterial chromosome or chromatin body.
9. Plasmids
9. Plasmids
A small circular, covalently closed, self replicating extra chromosomal DNA is also present in many bacteria. They are referred as plasmids. They are mostly controlling the special characters like pathogenicity, nodulation etc.
(The chromosomal DNA and plasmids are collectively called as genomic DNA)
10. Ribosomes
10. Ribosomes
The ribosomes are the granular appearance in the cytoplasm involved in the protein synthesis. The size of prokaryotic ribosomes is of 70 S (sedimentation coefficient, Svedberg unit) and are of two sub units of 50S and 30S. The bacterial ribosomes never bound to any organelles in the cytoplasm.
11. Cytoplasmic inclusions
11. Cytoplasmic inclusions
The cytoplasm of bacteria often contains inclusion granules referred as cytoplasmic inclusions. Inclusion bodies are usually storage materials of energy, carbon, phosphorus etc. Some aquatic bacteria possess large sized gas vacuoles which will help to float them.
12. Spores and modifications
12. Spores and modifications
Endospore
Certain species of bacteria produce spores either within the cells are referred as endospores. They are unique to bacteria and Bacillus, Clostridium are common spore forming bacteria.
The endospores are extremely resistance to desiccation, staining, disinfecting chemicals, radiation and heat. The endospore can able to survive in boiling water, uv light and many harmful chemicals. They can survive even 100 years. First, the DNA fragmented and followed by invagination of cytoplasmic membrane. Then, the portion of the invagination is separated by complete formation of septum (this stage is called forespore). Then formation of cell wall around the forespore and followed by cortex, spore coat and exosporium synthesis occurred. Finally, cell lysed and the spore is released to the external environment.
The outer most layer is called as exosporium, a thin delicate covering made of protein. The next layer is spore coat, composed of spore specific proteins. Below the spore coat is cortex, composed of cross linked peptidoglycan. Inside the cortex is spore protoplast which has cell wall (core wall), cell membrane, DNA, ribosome and little cytoplasm. One chemical substance that is characteristic of endospores not in vegetative cells is dipicolinic acid in the core region. They occupy 10% of total dry weight of the spores and responsible for heat resistance.
How endospores are formed?
Cyst : Cyst is dormant, thick walled desiccation (drying) resistant form of cell. It can be differentiated from vegetative cells and can germinate under suitable condition. The cyst don’t have temperature resistance. Ex. Cyst of Azotobacter
Sporangiospores and Conidiospores : the filamentous bacteria, actinomycetes produce these kind of spores. The spores were produced at the tip of the hyphae and if the spores are formed in a sac like structure (Sporangium), called as sporangiospores, if not called as conidiospores. These spores do not have heat resistance but can survive long period of drying.
Youtube video describing bacterial morphology
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