BACTERIA –Morphology and ultra structure



Morphology and Fine Structure of Bacteria

Bacteria are very small. Most being approximately 0.5 -1μm. Surface area and volume ratio of bacteria is exceedingly high compared to the same ratio for larger organism of similar shape. Relatively large surface area and a small volume of cell substance to be nourished accounts for unusual high rate of metabolism and growth. The cell substance very close to the surface and therefore no circulatory mechanism is needed to cytoplasmic movements within a bacterial cell is seen. In spite of a high surface area and volume ratio bacteria are microscopic in size.

Shape and Arrangement:

The shape of bacterial cell is due to a rigid cell wall.

The different shapes of bacteria may be

1. Spherical shaped: Coccus ( Cocci- plural )

2. A rod shaped: Bacillus (Bacilli -Plural)

3. Helically twisted (Rigid Helix): Spirillum (Spirilla -Plural), Spirochetes-highly flexible helical bacteria

4. Coma shaped: Vibrio ( Vibria-Plural )

5. Pleomorphic: No definite shape.

Bacteria are arranged in different arrangements.

Arranged: in cluster- Staphylococcus in chains -Streptococcus in pairs -Diplococcus

Cylindrical cells with much larger area of contact between adjacent cells are called Trichome.

Arranged side by side like match sticks -Palisade arrangement.

Arranged like stacks of coins- Ex: Caryophanon

Arranged in lobed spheres- Ex: Sulfolobus.

Flagellum:

These are hair like helical appendages that protrude through cell wall and are responsible for swimming motility of bacteria. Flagella may be arranged in various ways.

1. Monotrichous: If only one flagellum protrudes from one side of the cell. Ex: Pseudomonas aeroginosa

2. Lopholotrichous: If several and numerous flagella protrude from one pole. Ex: Pseudomonas fluorescens.

3. Amphitrichous: At least one flagellum on both sides. Ex: Aqauspirillum serpens.

4. Peritrichous: Flagella present all over surface of the bacterium. Ex: Escherichia coli

Structure of Flagellum: The entire flagellum apparatus is made up of three distinct parts.1.Filament, 2.Hook, and 3.Basal body.

1. Filament: The outer most regions of flagella is called filament. It is helical with a constant width made up of a protein flagellin which has a molecular weight of 20,000 -40,000 daltons. It has a length of about 70μm and a diameter of 10-20 nm.

2. Hook: Near the cell surface the filament is attached to a slightly wider part called hook. It is 45nm long and is made up of different kind of protein.

3. Basal Body: It is located entirely within the cell envelope. The basal body consists of a small central rod inserted in to a system of rings. In Gram-negative bacteria basal body typically

Contains 2 pairs of rings. The outer pair is called L & P rings which are situated at level of outer membrane. They serve as bushing for insertion of the central body through the layer. The inner pair is called S & M ring. These are located near the level of cell membrane. The M ring is embedded either in the cell membrane or may be present just below it. The S ring is present just above the cytoplasmic membrane and possibly attached to the inner layer of peptidoglycan.

In Gram-Positive bacteria only S and M rings are present. The upper pair is not required for the support of central rod because Gram-positive bacteria posses a relatively thick and homogenous cell wall. Therefore, just S & M rings are essential for flagella functioning.

The flagellum grows at the tip rather than at its base. Flagellum monomers are synthesized within cell and later pas along the hollow core of flagellum adding to the distal end of filament. The amino acid composition of a flagellum of each species is sufficiently different from other species to confer immunological specificity. This is important in medical diagnosis of the species. The speed of flagellum motility average from 30 -50μm/second. Some bacteria can cover the distance of 20 times of their length in a second. E.g. Vibrio cholera can cover 12mm/min which is about 300 times more than its length.

Hydrodynamics of Flagella: Bacteria propel themselves by rotating their helical flagella. The movement is important to the filament at its base. S &M rings rotate relatively to each other. The cell has capacity to vary the speed and direction by rotation. Peritrichously flagellated bacteria swim straight over a moderate distance and change their direction. These interrupted changes in direction are called tumble or twiddle or twitch. Tumble is caused by a reverse rotation.

Spirochetes move by means of specialized structures called axial filament. This structure is composed of 2 groups of fibers which originate at opposite sides of the cell and over lapin the middle. Structurally and chemically the fibers of axial filament are similar to flagella and these are called endoflagella or periplasmic flagella. Unlike flagella they are covered by spirochete cell wall. The cells move by rotating around their longitudinal axis and by flexing and bending along its length.

Some species of blue green algae and Rhodo spiralles move by gliding movement.

Some bacteria possess flagella that are notably thick. These sheathed flagella are surrounded by an extension of the cytoplasmic membrane. Under certain conditions some bacteria produce polar sheathed flagella along with many peritrichously arranged unsheathed flagella. Such condition is called mixed flagellation.

Bacterial Behavior: Chemotaxis, phototaxis and other taxes

Most motile bacteria have capacity to swim towards and away from various chemicals. This phenomenon of bacterial movement is referred to as chemotaxis. Swimming towards a chemical attractant is called a positive chemotaxis where the chemical concentration acts as a attractant and similarly the movement of bacteria away from the chemicals is a negative chemotaxis where concentration of chemical act as repellant.

The movement of bacteria towards light is called phototaxis.

The swimming of bacteria in response to mangnetic field or local magnetic field is called magneto taxis.

Fimbriae and Pili: These are hollow tubes non helical that extends from cell. They play no role in motility. They are straighter, shorter and thinner than flagella. Their size ranges from 3-25nm in diameter and 0.5- 20μm in length. They are visible only with an electron microscope. With a few exceptions fimbriae are observed only in gram-negative rods. Like flagella fimbriae also originates from basal body and pierce through cell wall and capsule. It consists of a protein called pilin. They confer antigenic specificity to the cell.

Environmental conditions such as pH temperature and oxygen tension strongly affect presence or absence of fimbriae.

Pili are similar structurally but are generally longer and only one or a few pili are present on the surface to fimbriae.

Functions

They involved in conjugation in prokaryotes and in attachment to human tissue.

Fimbriae are mainly associated with adhesive properties. Thus contribute in establishment of cells by binding to cell surface in areas where they would otherwise be eliminated by movements of body fluids. E.g.: E.coli attached to the intestinal wall and urethral canal. Pili have a strong tendency to adhere to each other and cause aggregation.

Culture of E.coli and Salmonella typhimurium forms a thin layer of cells in a static liquid medium. This is a device for an improved supply of atmospheric oxygen.

Capsule

The outer most structure of many cells is a capsule. It is a slimy or mucilaginous coating synthesized at the cell membrane and extended and transported to the exterior surface through the mesh work of cell wall. Bacterial capsules vary in thickness from a fraction of micrometers to 10μmor more. Distinction between cell wall and capsule is not always clear. Capsules are not essential for life of the cell. They may be removed artificially or naturally. The presence of or absence of capsules genetically controlled but is also subjected to environmental conditions. Capsulated strains produce smooth colonies and non capsulated colonies produce rough colonies.

Composition of Capsule: Capsules are generally made up of polymers of various units. The composition varies with species. Capsules composed of a single kind if sugar is called homo polysaccharides, usually synthesized out of the cell from disaccharide by exocellular enzymes E.g.: synthesis of glucon. Capsules sometimes are composed of several kinds of sugars (hetero polysaccharides) that are activated within the cell transported across cytoplasmic membrane and polymerized outside the cell wall. E.g.: Capsule of Kleibsiella pnuemoniae.

A few capsules are polypeptides. E.g.: Bacillus anthracis, is composed of a polymer of glutamic acid.

In many cases capsular material is not highly water soluble and therefore does not diffuse away from the cell. Capsules have distinct and individual molecular structure and there fore they have the property of immunological specificity. Immunological specificity of capsules permits distinction between closely similar species of bacteria.

Capsule may contain amino sugars, sugar acids which are generally combined with water to form viscous and gelatinous slime.

Some organisms possess capsule which are not readily demonstrated. They have very thin layer of distinctive molecular structures on the cell surface.

This very thin layer on the surface of cell wall is called microcapsule. They also determine physiochemical and immunological specificity of the cell.

Functions:

Capsules protect cell against temporary drying by binding water molecule.

Capsules prevent attachment of bacteriophages.

Capsules avoid phagocytosis.

They promote attachment of bacteria to surface.

Capsules composed of compounds having an electrical charge promote the stability of bacterial suspension by preventing the cells from setting out.

Sheath:

Some bacteria from fresh water and marine environment form chains of trichome that are enclosed by a hollow tube called a Sheath. E.g.: Leptothrix ochracea. Sheaths may sometimes become impregnated with ferric or manganese hydroxides which strengthen them.

Prosthecae (Sing-Prostheca): These are semi rigid extensions of cell wall and cytoplasmic membrane and have a diameter that is always less than that of cell. These increase surface area of the cells for nutrient absorption in dilute medium. E.g.: Stella and Ancalomicrobium have several prosthecae. Caulobacter has single prostheca

Stalk: Non living, ribbon like or tubular appendages that are excreted by cells are termed stalk. E.g.: Gallionella . Stalk help in attachment.

Surface layers that are loosely distributed around the cell and diffuse into the medium are referred to as Slime layer.

Glycocalyx is a tangled mass of polysaccharide fibers that extend from the bacterial surface.

Cell Wall

The bacterial cell wall is the structure that immediately surrounds the cell membrane. The shape of the cell is imposed by the shape of the cell wall..

The important function of cell wall is to physically protect the cell. This protection is necessary because of the susceptibility of the cell membrane to physical osmotic lyses.

The cell walls of gram-negative bacteria are thin.10-15nm thick accounting 10-20%of the dry weight of cell. They have 5-15% peptidoglycan, 35% phospholipid,15% protein and 50% polysaccharide. Gram-positive cell walls are thick 25-30nm and 20-40% of total dry weight. They contain 20-80% peptidoglycan and various substances. Like protein, polysaccharides and teichoic acids. The walls contain very little lipid.

Mycobacterium and Corynebacterium and certain genera contain high amount of lipids (Mycolic acids) which is responsible for acid -fast staining. A mycolic acid derivative called cord factor is toxic and plays an important role in disease caused by C.diptheriae and M.tuberculosis.

Although archaeobacteria possess cell walls, these do not contain peptidoglycan and their cell-wall fine structure and chemical composition is different from that of eubacteria. Their walls usually composed of proteins, glycoproteins or polysaccharides. This material is called pseudomurein or pseudopeptodoglycan. The back bone consists of repeating units of N-acetylglucosamine and N-acetyltalosaminuronic acid.

Peptidoglycan: In cell walls of bacteria there is one rigid layer that is responsible for the strength of the wall. In gram-negative bacteria, additional layers are present outside this rigid layer. Peptidoglycan is a thin sheet composed of two sugar derivatives, N-acetyl glucosamine and N-acetylmuramic acid and a small group of amino acids consisting L-alanine, D-alanine, D-glutamic acid and either lysine in gram-positive bacteria and diaminopimellic acid (DAP) in gram-negative bacteria. These are connected to form a repeating structure; the glycan tetra peptide. Glycan chains formed by sugars are connected by peptide cross-links formed by amino acids. The two sugars N-acetyl glucosamine and N-acetylmuramic acid are linked by ( 1-4 glycosidic bonds.

The peptidoglycan of gram-positive bacteria is extensively cross linked than the peptidoglycan of gram-negative bacteria. The strength of the individual layers is largely determined by the extent of cross linking between the backbones.

Peptidoglycan is present only in bacteria and variations occur in different species.

Interference with bacterial ability to produce peptidoglycan result in a defective cell wall and the bacterium usually will not survive.

Lysozyme degrades preformed peptidoglycan. It occurs in saliva and tears and egg white.

Teichoic acids: It is present in gram-positive bacteria. And is a acidic polysaccharide attached to the cell wall. It consists of glycerophosphate or ribitol phosphate residues. Amino acids such as D-alanine or sugars like glucose are attached to the glycerol or ribitol. The teichoic acids are connected to either the peptidoglycan itself by a covalent bond or to plasma membrane lipids.

Teichoic acid extends the surface of peptidoglycan and because they are negatively charged help gram -positive cell wall to possess negative charge. Teichoic acids are not present in gram -negative bacteria.

Functions of cell wall:

1. Cell wall gives protection from osmolytic lyses.

2. It gives rigidity and shape to the cell.

3. It gives protection from destructive chemicals and some antibiotics.

4. It is responsible for difference in gram-staining reactions.

5. It possesses surface antigens.

6. It may elicit certain toxic symptoms of disease caused by gram-negative bacteria.

7. It provides support for anchorage of flagella.

S- layers (Surface layer or Para crystalline surface layer)

Many gram-positive and gram-negative bacteria have a regularly structured layer called s-layer on their surface. S-layer is also found among archaeobacteria. The S-layer consists of protein or glycoprotein and has crystalline appearance. They show various symmetries such as hexagonal, tetragonal depending upon no. and structure of protein or glycoprotein subunits. In gram-negative bacteria the S-layer attaches directly to the outer membrane. In gram-positive bacteria S-layer is associated with the peptidoglycan surface.

Functions of S-layer: It protects the cell against ion and pH changes, osmotic stress enzymes or predacious bacteria Bdellovibrios.

It also helps to maintain the shape and envelope rigidity of some bacterial cells.

It promotes cell attachment to surface.

It also protects some pathogens against phagocytosis and complement attack.

Adhesions: These are direct contact between the membrane and sites of export for newly synthesized lipopolysaccharides and also sites where flagella and pili are made.

Outer wall of gram-negative bacteria

Gram-negative bacteria possess an outer most membrane that surrounds a thin underlying layer of peptidoglycan. It is an impermeable barrier to prevent the escape of important enzymes to the outside of the cell and also barrier for various external chemicals and enzymes that could damage the cell. Thus membrane is rich in lipids.

The outer layer has a width and fine structure similar to a unit membrane or plasma membrane. This layer has some chemical and physical properties in common with cell membrane. Like cell membrane it is a lipid bilayer containing phospholipids and proteins but in addition it contains large amounts of unique lipid lipopolysaccharides (LPS) which replace phospholipids in the outer leaf of this unique structure. LPS has properties such that it can participate in forming a membrane.

LPS has toxic properties and is also known as endotoxin. It consists of 3 covalently linked parts. i) Lipid -A, ii) a core (R-core) and iii) O-specific polysaccharide.

Lipid-A consists of a glucose amine disaccharide whose hydroxyl groups are esterified with C12, C14 and C16. Fatty acids commonly found in lipid-A include caproic acid, lauric myristic palmitic and stearic acids.. This part determines hydrophobic properties. Attached to this and oriented towards the outer surface there is an R-core. This consists of 2- keto- 3 deoxyoctonic acids (KDO) trisaccharide combined with phosphor ethanol amine, 2 heptose molecules and the outer core layer. The core layer consists of a branch chain of glucose, galactose and N-acetyl glucosamine. This is uniform in salmonella species. The o specific side chains are attached to the core. They consist of long chain of repeating oligosaccharides. Their composition is strain a specific. They may contain galactose mannose rhamnose, obequcose, fucose colitose and other sugars. These outer hetero polysaccharides chains represent the somatic o-antigen and allow identification of strains by immunological methods.

Porins: These are proteins containing three identical subunits. Porins are transmembrane proteins and associate to form small membrane holes about 1 nm in diameter. Porins are present in the outer membrane of gram-negative bacteria and function as channels for the entrance and exit of hydrophobic low molecular weight substances. Several porins are identified. Non specific porins form water filled channels through which small substances of any type can pass. Specific porins contain special binding site for one or more substances.

Periplasmic space: A space between the plasma membrane and the outer membrane of gram-negative bacteria and sometimes a similar but a smaller gap between the plasma membrane and the cell wall in gram-positive bacteria. This space is called periplasmic space. It may be filled with a loose net work of peptidoglycan. This space is more gel than fluid filled space. The substances that occupy periplasmic space are periplasm. Periplasmic space in gram-negative bacteria range from 1-71nm and 20-40% of the total cell volume. It contains many proteins that participate in nutrient uptake. E.g.: Hydrolytic enzymes. It also contains enzymes for peptidoglycan synthesize and modification of toxic substances that could harm the cell. Gram-positive bacteria may not have a clearly visible periplasmic space and do not appear to have many periplasmic proteins rather they secrete several enzymes that ordinarily would be periplasmic in gram-negative. Such secreted enzymes are often called exoenzymes.

Cytoplasm membrane:

Cytoplasm membrane of prokaryotic cell is distinct and separable structure. Bacterial cell membrane typically consists of 3 distinct regions. The two outer layers are electron dense and are each about 2.5nm thick. The middle layer about 5 nm thick is much less electron dense and appears to consist of a bimolecular layer of lipids oriented such that hydrophilic ends are outer most and hydrophobic poles are inner most. The chemical composition of bacterial cell membrane is approximately protein 60%, lipid 30%, and carbohydrate 10%. The membrane does not contain sterols and peptidoglycan. Many of the proteins found in the membrane are enzymes or have enzyme functions.

The phospholipid bilayer has protein which is tenaciously held. These are called integral proteins. This can be recovered by detergents. Peripheral proteins are loosely attached and can be removed by mild treatment such as osmotic shock. The lipid material of the membrane has fluidity allowing the components to move around laterally. This is essential for various membrane functions and is dependent on temperature and proportion of unsaturated fatty acids present.

In eubacteria phospholipids are phosphoglycerides in which straight chain fatty acids are ester linked to glycerol. In Archaeobacteria, the lipids are poly isoprenoid branched chain lipids in which long chain branched alcohols (phytanols) are ether linked to glycerol.. Sterols are absent in prokaryotic cells. However, many bacterial membranes do contain pentalglycine steroid like molecules called hopnoids and huge quantities of hopnoids are present in ecosystem. Hopnoids are synthesized from the same precursors as steroids. They stabilize bacterial membrane.

Major classes of proteins known to be localized in the membrane include1.Permeases, 2.biosynthetic enzymes, 3.proteins that participates in ATP generation.

Functions of cell membrane:

Regulate the transport of the molecules in and out of the cell.

Secretion of extra cellular enzymes.

Respiration and photosynthesis occur in cell membrane.

Regulation of reproduction and cell wall synthesis.

Protoplast: A protoplast is that portion of bacterial cell consisting of cytoplasmic membrane and cell material bounded by it. Protoplast can be prepared by treating cells with an enzyme lyzozyme which selectively dissolves cell walls. Protoplast can also be prepared by culturing bacteria in the presence an antibiotic such ac penicillin which prevents the formation cell wall.

The protoplast is maintained in a medium that prevents them from osmotic lyses. The osmotic pressure of the medium must be sufficiently high to protect the organism from bursting.Eg.3-20% glucose, 2-5%Nacl and 10-20% serum. In the absence of cell wall, the cells burst as they continually take up water by osmosis. This can be prevented by preparing protoplast in an osmotic medium that prevents bursting of cells. The osmotic ally protected protoplast is soft and fragile and are spherical regardless of the original shape of the cell.

Spheroplast: These are round osmotically fragile form of gram-negative bacteria. This can be prepared by treating gram-negative cells with lyzozyme or culturing them in the medium containing penicillin. Although peptidoglycan of the cell wall is damaged the flexible outer membrane of the cell wall remains. Such cells are called spheroplast.

Some bacterial like organisms exists normally I wall less state resembling protoplast. Many bacteria assume protoplast form by mutation. Experimentally, bacteria temporarily in the protoplast form are referred to as L-forms.

Membrane intrusions and intracellular membrane systems:

Mesosomes: Mesosomes are intracellular membraneous localized in foldings most frequently seen in gram-positive cells and in some gram-negative cells. There are many variations in form such as vesicular type and whorl type.

Central mesosomes penetrate deep into the cytoplasm attached to the cells nuclear material thought to be involved in DNA replication and cell division. Peripheral mesosomes show a shallow penetration not being associated with nuclear material seems to be involved in export of exocellular enzymes such as penicillinase.

Mesosomes probably participate in septum formation. Mesosome increase surface area for various metabolic activities in photosynthetic bacteria. The folding serve to accommodate some centre for respiratory activity.

Currently many bacteriologists believe that mesosomes were artifacts generated during the chemical fixation of bacteria for electron microscopy. Possibly they represent part of the plasma membrane.

In cyanobacteria the photosynthetic apparatus is contained in a system of flattened membraneous sac called thylakoid which are not connected with the cell membrane.

In photosynthetic bacteria another invaginated membraneous structures chromotophores contains enzymes and pigments that perform functions similar to those of the eukaryotic chloroplast.

Cytoplasmic inclusions and Vacuoles:

Gas Vacuoles: Many aquatic prokaryotes have developed a device to counter act gravitational pull. They contain gas filled structures known as gas vacuoles. By light microscopy gas vacuoles appear densely refactile and have an irregular contour. If such cells are subjected to a sudden sharp increase in hydrostatic pressure the gas vacuoles collapse. Cells lose buoyancy and become less retractile.

Electron microscopy shows that gas vacuoles are compound organelles made up of variable number of individual gas vesicles. Each gas vesicle is a hollow cylinder 75nm in diameter with conical ends and 20-1000nm in length. The vesicle is bounded by the layer of protein 2nm thick and is banded by regular rows of subunits running at right angles to its long axis. These vesicles neither store nor accumulate gas. The composition and presence of gas in the vesicle is therefore function of dissolved gasses in the surrounding medium. Water is excluded from the interior of the vesicle in the course of their formation and growth. The functions of gas vacuoles is to enable the cells to regulate the buoyancy for the cell in order to occupy a position in water column that is optimal for their metabolic activity, light intensity, dissolved oxygen concentration or concentration of nutrients.

Gas vesicles contain only 2 different types of proteins. The major gas vesicle proteins called Gvp A. It is a small protein and makes up 97% of the total protein of gas vesicles. The second protein is called GvpC ,is larger protein but is present in smaller amounts. The function of Gvp C protein is to strengthen shell of the gas vesicle.

Gas vesicles are constructed of several copies of GvaA protein aligned as parallel “ribs” forming a water tight surface. GvpA protein folds as a beta sheet and thus gives considerable strength and rigidity to the overall vesicle structure. The ribs of GvpA protein are strengthened by GvpC protein which acts as cross linker binding several GvpA ribs together like a clamp. The final shape of gas vesicle is a function of how the GvpA and GvpC are arranged to form the vesicle.

Cytoplasmic Inclusions:

A number of bacteria characteristically store deposits of nutrient materials, usually phosphate, sulphur, carbohydrate or fat in structure called cytoplasmic inclusions or granules. Most cells have the capacity to store only one kind of material producing a characteristic inclusion which can be stained and microscopically observed.

Volutin or Metachromatic granules: Many microorganisms accumulate polyphosphate granules which are stained with basic dye such as Methylene blue. They exhibit metachromatic effect, appearing red when stained with a blue dye. In electron microscopes of bacteria they appear as extremely electron dense bodies. The polyphosphates are linear polymers of orthophosphates of varing chain lengths. Starvation of cells for almost any nutrient other than phosphate leads to polyphosphate formation. The formation of polyphosphate occurs by the sequential addition of phosphate residues to pyrophosphate ATP serving as the donor.

Poly-β-hyroxybutyrate (PHB): Aerobic bacteria under high carbon low-nitrogen culture conditions possess a lipid like material called Poly-β-hyroxybutyrate. These are readily recognized because they are easily extractable with organic solvents, readily stained by fat soluble lipid stains; blue by naphthal blue, black by Sudan black. They serve as a reserve for carbon and energy source.

Glycogen: Many prokaryotic cells synthesize and store up excess soluble carbohydrate food substances in the form of soluble polysaccharides. These are polymers of glucose and other monosaccharide units.

When cells containing glycogen are treated with iodine, they appear brown. By electron microscopy they appear as dark granules.

Sulphur Granules: In some species of aquatic and soil inhabiting bacteria, globules of pure elemental sulphur accumulate in the cell as unused food. This sulphur is often derived from the intracellular dehydrogenation (oxidation) of hydrogen sulphide or other inorganic reduced forms of sulphur. The elemental sulphur in the globules can be extracted from the cells with carbon disulphide from which the sulphur is deposited as distinctive crystals.

Chlorosomes: In green bacteria, the antenna pigments are housed in a series of cigar shaped vesicles, arranged in a cortical layer and physically distinct from the cell membrane. By electron microscopy, these are 50 n wide 100-150 nm long, being enclosed by a single layered membrane 3-5 nm thick. The photosynthetic pigments are entirely contained within them.

Carboxysome: A number of photosynthetic bacteria and chemotrophic bacteria contain structures termed polyhedral bodies, 50-500 nm wide with polygonal profiles, which are surrounded by a monolayer membrane about 3.5 nm wide and which have a granular content. They contain enzyme for carbon dioxide fixation. These have called carboxysomes.

Magnetosomes: In a group of bacteria known as magnetotactic the sensing organelle termed magnetosomes are present. These are uniformly shaped enveloped crystals of magnetite, a ferro-magnetic mineral, within the cell. These often arranged in a string. Magnetosomes mainly orient the cells in a magnetic field and thereby determine the direction in which they swim.

Cyanophycin: Many of cyanobacteria accumulate nitrogenous reserve material known as cyanophycin. When cultures approach the stationary phase cyanophycin granules, which have a distinct appearance in electron microscope have copolymers of arginine and aspartic acid. This material represents 87% of the cellular dry weight.

Cytoplasm:

Cell material bounded by the cytoplasmic membrane is not a homogenous substance. It is divided into i) cytoplasmic area, ii) the chromatic area, iii) the fluid portion with dissolved substances.

Some ribisomes are free in the cytoplasm and others are associated to the inner surface of the cytoplasmic membrane. The ribosomes have a sedimentation co-coefficient of 70 Svedberg units and are composed of 2 subunits; 50S and 30S units. Polyribosomes or polysomes are an aggregation of varying sizes. Some polysomes consist of chains of ribosomes strung along the thread of connecting substance which may be mRNA. Ribosomes largely consists of rRNA and some mRNA and thus responsible for the synthesis of specific proteins Ribosomes are 16 x18 nm in size and contain 80-85% of bacterial RNA. Bacterial cells contain about 5000-50,000 ribosomes. Ribosomes are sites of protein synthesis. Disruption of ribosomes disrupts the life. Antibiotics like erythromycin and streptomycin are effective against ribosome.

Cytoplasm consists of various substances or sub cellular particles that are mainly proteins and nucleoproteins with some lipid and other materials. All the particulate matter in the cytoplasm is surrounded by and suspended in an aqueous fluid or semi fluid ground substance or matrix.

The matrix is a complex mixture containing in a solution a variety of ions, amino acids some proteins, lipoproteins, peptides purines, pyrimidines sugars, vitamins nucleoproteins co-enzymes. Functionally these are i) precursors, ii) sources of energy iii) waste products of the cell.

The matrix may also contain tRNA and enzymes in solutions. Besides it also contains granules or globules of inert stored substances.

Nuclear material:

The information for bacterial function is housed on a single circular molecule of DNA. Most cells have only one copy of chromosome. Before cell division, the chromosome will duplicate.

The size of the chromosome varies according to species. The DNA fibril seen in bacterial cells is 1500µm thin & 3nm thick, flexible, circular filament. The amount of DNA represents approximately 4X 106 base pairs with a molecular weight of about 2.7 x 109.

DNA is highly charged molecule, since adjacent bases are linked by phosphate groups, each with an ionized hydroxyl group. The resulting negative charges are neutralized by polyamines such as spermine and spermedine and by Mg2+ as well as basic proteins.

The circular DNA in bacteria is described as the bacterial chromosome or nucleoid or the chromatin body or the nuclear equivalent.

Additional genetic information may be found as plasmids in many bacteria. These are small, circular pieces of DNA that can replicate independently of the chromosome.

Plasmid DNA may give bacterium the capacity to synthesize new products. Though it is not required for growth, it carries information for antibiotic resistance or ability to produce toxins or ability to produce surface appendages.

Spores and Cysts

Endospore:

These are unique to bacteria. They are thick –walled, highly refractile bodies that are produced one per cell. Sporulation is a response initiated by depletion of an essential nutrient in the bacterial environment. Spores require several hours to be produced and must be completely formed before it encounters adverse conditions such as heat, cold and radiation.

Endospores are extremely resistant to desiccation, staining, disinfection, radiation and heat. The degree of heat resistance varies with the bacterial species. Most can resist heat treatment 80oC for at least 10 minutes.

Only five genera produce endospores.

|Genus |Cell Shape |Relation to oxygen |

|1.Bacillus |Rods |Aerobes ,Some are facultative anaerobes |

|2.Clostridium |Rods |Strict anaerobe |

|3.Desulfatomaculum |Rods |Strict anaerobe |

|4.Sporolactobacillus |Rods |Aerotolerant |

|5.Thermoactinomycetes |Mycelial |Aerobes |

All endospores contain large amounts of dipicolinic acid (DPA). It accounts for 10-15% of dry weight. It occurs in combination with large amounts of calcium & is probably located in the core. The calcium & DPA complex may play a role in heat resistance.

The endospore has a much more complex structure. Spores have many more defined enclosing layers. The outer most layers, when present, are called exosporium. Within the exosporium there is a wall covering known as spore coat. It represents 30-60% of dry weight of the spore and largely composed of proteins which accounts for 80% of total spore protein. Proteins have an unusually high content of cystein and hydrophobic amino acids. Cortex is present beneath the spore coat. It is composed of a multi laminated structure similar to cell wall, the primary constituent being the peptidoglycan. The endospore cortex peptidoglycan is a unique peptidoglycan containing 3 repeating N-acetylglucosamine muramic acid dimmer differing with respect to substitutions and lactic acid moiety muramic acid. A muramic acid lactum subunit without any attached amino acids and alanine subunit pairing only an L-alanine residues and a tetrapeptide subunit bearing the sequence of L-alanine ,D-glutamic acid, mesodiaminopimellic acid ,D-alanine. These subunits represent 55%, 15%, and 30% respectively. There is very little cross linkage between tetrapeptide chains.

Inside the cortex the cytoplasm is enclosed by both cell wall and cell membrane. Thus the major difference between the spore and vegetative cells are primarily confined to the exterior of the spore cell wall. The DNA base composition ranges from 38-42%.

The core of mature endospore differs greatly from the vegetative cells. Besides having abundant calcium dipicolinate content; the core is in a partially dehydrated state. The core of a mature endospore contains only 10-3-% of the water content and the core cytoplasm is a gel. The pH of the core cytoplasm is about one unit lower than that of the vegetatitive cell and contains high levels of core-specific proteins called small acid-soluble spore proteins (SASP). These are made during sporulation process and have at least two functions. SASP bind tightly to DNA core and protect it from potential damage from UV light, desiccation and dry heat. It also functions as a carbon and energy source for the out growth of a new vegetative cell from the endospore, a process called germination.

Endospore Formation

a) Vegetative cell containing 2 nuclear bodies

b) Condensation of nuclear material

c) Beginning of transverse wall formation

d) Completion of transverse wall, the fore spore with its nuclear material is now cut –off from vegetative cell

e) Engulfment of the fore spore

f) Synthesis of spore cortex

g) Synthesize of spore coat

h) Liberation of endospore

Endospore Germination

The change from a dormant spore to an active vegetative cell involves 3 stages.1) Activation, 2) Germination and 3) Outgrowth.

1) Activation: Activation of all spores occurs within a few minutes if the spores are heated in an aqueous fluid at about 65oC for 15-16 minutes. These spores are called heat activated or heat shock. Certain chemicals such as L-alanine, inosine, glucose and some reducing agents may also bring about the activation of endospores under natural conditions. Activation is very slow and occasional. Activation is reversible process.

2) Germination: when activated spores are placed under favorable conditions germination takes place. Germination is an irreversible process and occurs only after activation. This process is rapid and expressed by loss refractability, loss of resistance to heat and other deleterious substances and unmasking of metabolic activity. Germination of activated spores requires a chemical trigger. The specific substances are sources of nitrogen, phosphorus, carbon, sulphur, energy, L-alanine, glucose and various inorganic anions and cat ions. On subjecting spores to physical treatments the spore coat may also initiate germinations only if the nutrients required for macromolecular synthesis are present. On completion of germination the spore will proceed to grow into a vegetative cell.

3) Out growth: Out grows involves an initial swelling of the spore within its spore coat accompanied by rapid synthesize of a vegetative cell wall. The newly formed vegetative cell that emerges from spore coat elongates and proceeds to undergo the first vegetative division.

The location, size and shape of endospore vary in different species. Endospore may be an elliptical, spherical, or ovoid in shape. Endospore may be present at the tip of the cell or in the center of the cell or sub terminally.

E.g. : B.cereus – Central and elliptical

Clostridium tetani – Terminal and spherical

Bacillus polymyxa – central and spindle

Bacillus laterosporus – Lateral and spindle

Variations in endospore morphology: (1, 4) central endospore; (2, 3, 5) terminal endospore; (6) lateral endospore

Exospores: They are produced external to the vegetative cell by budding at one end of the cell. They lack dipicolinic acid and are not resistant to heat and desiccation. E.g.: Methane oxidizing genus Methylosinus.

Cyst: Members of the genus Azatobacter produce cyst. These are dominant thick walled desiccation resistant forms. These are developed by differentiation of vegetative cell. Structurally and chemically these are different from the endospore. The whole vegetative cell transforms into the cyst.

Myxospores: These are produced by the genera Myxococcus and Sporocytophaga from the rod shaped vegetative cells.

Bacterial Growth and Reproduction:

The term growth as commonly applied to bacteria and other microorganisms usually refers to changes in the total population rather than an increase in size or mass of individual organism. Bacteria reproduce by variety of mechanisms. The most common means of bacterial reproduction is binary fission. One cell divides in to two by formation of transverse septum. Thus the single bacterium increase in population by geometric progression (1(2(4(8(16(32(2n).

Some bacteria reproduce by budding in which a small bud develops at one end of the cell and this bud is finally separated from the parent cell.

E.g. Rhodopseudomonas acidophila.

Bacteria reproduce by fragmentation of filaments into small bacillary or coccoid cells. Each of this can grow into a new organism. E.g. Nocardia species.

Streptomyces and related bacteria reproduce by spores formed at the tip of hyphae.

Bergey’s Manual of systematic Bacteriology

It is a compendium of standard and molecular information on all recognized species of prokaryotes and contains a number of dichotomous keys and other systematic information useful for identification purposes. The latest edition of Bergey’s Manual of systematic Bacteriology, published in five volumes has incorporated many of the concepts that have emerged from ribosomal RNA sequencing studies and blends with a wealth of conventional taxonomic information.

A second major source is a multi-volume treatise called The Prokaryotes.

In combination, Bergey’s Manual and The Prokaryotes offer microbiologists the foundations as well as details of microbial taxonomy and phylogeny as we know it today and are typically the first sources the practing taxonomist seeks out when beginning the characterization of a newly isolated prokaryote.

Organization of Bergey’s Manual of systematic Bacteriology

Taxonomic Rank and Section Representative Genera

Volume-1.The Archaea,Cyanobacteria,Phototrophs

And Deeply Branching Genera

The Archaea

Kingdom Crenarchaeota

Section I-Thermoprotei, Sulbolobi, and Barophiles Thermoproteus, Sulfolobus

Kingdom Eukarychaeaota

Section II The Methonogens Methanobacterium

Section III-The Halobacteria Halobacterium, Halococcus

Section IV-The Thermoplasm Thermoplasma,Picrophilus

Section V-The Thermococci Thermococcus,Archaeoglobus

Deeply Branching Genera

The Bacteria(Eubacteria)

Section VI—Aquifex and Relatives Aquifex,Hydrogenobacter

Section VII-Thermotogas and Geotogas Thermotogo,Geotogo,

Thermodesulfobacterium

Section VIII-The Denois Deinococcus

Section IX-Thermi Thermus,Magnetobacterium

Section X-Chrysiogenes Chrysiogenes

Section XI-The Chlroflexiand Herpetosiphons Chlroflexus,Herpetosiphon

Section XII-Thermomicrobia Thermomicrobium

Section XIII-Prochloron and Cyanobacteria Procholron,Synechococcus,

Oscillatoria,Anabaena,Nostoc,

StigonemaPleurocapsa

Section XIV-Chlorobia Chlorobium,Pelodictyon

Volume 2.The Proteobacteria

The Bacteria

Kingdom Proteobacteria

Section XV-The α- Proteobacteria Rhodospirillum, Rickettsia, Rhizobium, Brucella, Nitrobacter

Section XVI-The β- Proteobacteria Neisseria,Nitromonas,Thiobacillus

Section XVII- The γ-Proteobacteria Chromatium,Escherichia,

Klesiella, Proteus, Salmonella,

Shigella,Yersinia,Haemophilus,

Pseudomonas, Azotobacter,Vibrio

SectionXVIII-The δ- Proteobacteria Desulfovibrio,Bdellovibrio,

Myxococcus

Section XIX- The Є-Protobacteria Campylobacter,Helicobacter

Volume 3.The low G +C Positives

Section XX-The Clostridia and Relatives Clostridium,Desulfotomaculum

Section XXI-Mollicutes Mycoplasma,Ureplasma,

Spiroplasma,Acholeplasma

Section XXII-The Bacilli and Lactobacilli Bacillus,Lactobacillus,

Streptococcus, Enterococcus,

Listeria, Staphylococcus

Volume 4 .The High G+C Positives

Section XXIII- Class Actinobacteria Actinomyces,Micrococcus,

Arthrobacter, Cornybacterium,

Mycobacterium, Streptomyces,

Nocardia,, Actonoplanetes,

Frankia, Bifidobacterium

Volume 5. The Planctomycetes,Spirochaetes,

Fibrobacters,Bacteroids and Fusobacteria

Section XXIV-The Planctomyceyes,Chlamydia and Relatives Planctomyces,Chlamydia

Section XXV- The Spirochetes Spirochaeta,Borrelia,Leptospira,

Serpulina

Section XXVI-The Fibrobacters Fibrobacter

Section XXVII- The Bacteroides Bacteroides,Prevotella,

Porphyromonas

Section XXVIII- The Flavobacteria Flavobacterium

Section XXIX- The Sphingobacteria,Flexibacteria Shingobacterium,Flexibacter,

Cytophaga

Section XXX-The Fusobacteria Fusobacterium

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Pictures and Diagrams: sources:

1. BACTERIA ULTRA STRUCTURE: micro.

2. BACTERIA SHAPES AND ARRANGEMENTS: en. AND

3. FLAGELLAR ARRANGEMENTS: bcmd.edu

4. FLAGELLA STRUCTURE:

5. FLAGELLAR MOVEMENTS: pc.maricopa.edu

6. BACTERIAL CHEMOTAXIS: ucordova.

7. FIMBRIAE: microwunderkammer.

8. PEPTIDOGLYCAN: bcmd.edu

9. TETRAPEPTIDE LINKS:

10. TEICHOIC ACID: people.rit.edu

11. TEICHOIC ACID STRUCTURE: heart-

12. OUTER WALL OF GRAM NEGATIVE BACTERIA: ws.collin.edu

13. LIPOPLYSACCAHRIDE STRUCTURE:

14. CELL MEMBRANE: micro.magnet.fsu.edu

15. ENDOSPORE: faculty.ksu.edu.sa

16. ENDOSPORE FORMATIOM: biology-pictures.

17. ENDOSPORE MORPHOLOGY: en.

18. BACTERIA REPRODUCTION: biology.

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Pass word: bacteria

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