Morphology and fine structure of bacteria

Bacteria:

The word bacteria is originated from Latin word bacterium meaning rod. They are unicellular prokaryotes (Greek word meaning prenucleus).

Morphology:  The study of size, shape, structure and arrangement.

• Shape of bacteria:

          Is due to presence of rigid cell wall. Few basic shapes of bacteria are.

1. Spherical bacteria called cocci meaning berries.
2. Rod Shape bacteria called bacilli meaning little staff
3. Helical curved  rods (Spiral)

               Helical curved bacteria may be of following basic shapes

1. Bacteria that look like curved rods are called vibrios  
2. Spirilla, rigid spiral shaped cells (like cork screw)     

 

1. Spirochetes flexible spiral shaped bacteria  

 

 4) Coccobacilli. Some bacilli are oval and look so much like cocci that they

are called coccobacilli  

  Besides These basic shapes few bacteria are star shaped, rectangular, and triangular.

Most bacterial cells have constant single shape and hence are monomorphic. Some bacteria are pleomorpic being variable in shape and lacking a single characteristic forms.

• Size of Bacteria

Bacteria come in many sizes

1. Spherical Bacteria Example; Cocci has size 0.5 to 2 µm
2. Rod Shape Bacteria Example; Bacilli has size 1 – 10 µm in length and 0.2 - 2µm in width.

Largest bacteria Thiomargarita nambiensis is about 0.3mm in size and coccoid in shape

World smallest bacteria is Mycoplasma genitalium 200-300nm in size and has flask shaped cells.

• Arrangement of Bacteria:

Bacterial cells are usually arranged in manner characteristics of their particular genra. Arrangement of bacteria depend upon the plane of cell division and behavior of cell after division 

Cocci: They appear in several characteristic arrangement

1. Micrococcus: Cocci that remain single are called micrococcus
2. Diplococci: Cocci that appear in pair form are called diplococci.
3. Streptococci: Cocci that remains attached in chain like arrangement are called streptococci.
4. Tetrad: Those cocci that divides into two planes and remains in group of four is known as tetrad arrangement.
5. Staphylococci: Cocci that divides in multiple planes and form grape like structure or arrangement.
6. Sarcinae: Division of cocci bacteria in three planes produced cuboidal shape.                                              

b) Bacilli have fewer grouping than cocci

1) Single bacillus:   Most bacilli appear as single rods

2) Diplobacilli : Bacilli appear in pairs after division.

3)Sterptobacilli : Bacilli occur in chains

 4) Trichomes : Are like streptobacilli but have much larger area of contact between cells.

 

5) Palisade arrangement. Cells are lined side by side like matchsticks.

• Structure of Bacteria:

Structure External to the cell wall:

a) Glycocalyx (Sugar coat)

Many bacteria secrete a sticky, viscous material that forms an extracellular coating around the cell. The material is usually a polysaccharide, however, in the case of pathogenic Bacillus anthracis, is poly-D-glutamic acid. Negative  charge on the capsule repel phagocytes.

 

If the glycocalyx is thick,covalently bound to the cell and has an organized structure, it is called a capsule. If the material is loosely bound and thinner, it is called a slime layer.  

 

Functions

 

Contributing to the organisms' pathogenicity

 

The capsule and glycocalyx allow cells to adhere to surfaces, protect bacteria from antibodies and phagocytosis, act as diffusion barriers against some antibiotics thus contributing to the organisms' pathogenicity.  In Streptococcus pneumoniae only capsulated strains are pathogenic.

Steptococcus mutans attach to dental surface and cause dental carries

 

Provide protection to organism

Against temporary drying by binding water molecules.

Block attachment of bacteriophage

 

b) Appendages

 

Many bacteria have hair like appendages that project from the cell wall. There are of two kinds of appendages:

• Flagella (singular, flagellum) and pili (singular, pilus).

 

Flagella.  Prokaryotic flagella are long, semi-rigid, helical, hollow tubular structures composed of several thousand molecules of the protein flagellin. Cells may have one or many flagella. Flagella are highly antigenic

Size and Structure;  Prokaryotic flagella is thin and 20nm in width and 10-20um in length and It is compose of three parts:

Basal body; Flagella are attached to the cell wall, cell membrane, or both by a basal body, which is a complex molecular machine that rotates the flagellum like the screw propeller of a ship. It is a ring like structure. Its composition is unknown.

Hook; it attached filament to basal body.

Filament; it is several time as long as hook cell and is composed of protein sub units called flagellin.

 

 

Arrangement:

Arrangement of flagella in bacteria is unique to species observed.

1. Monotrichous:

A single polar flagellum

Example. Pseudomonas aeruginosa

1. Lophotrichous:

A tuft/cluster of polar flagella.

 

                  Example . Pseudomonas   fluorescens.

1. Amphitrichous:

Either a single flagellum or tuft of flagella on both poles.

 

•  

 

1. Peritrichous:

Whole bacterial body is surrounded by lateral flagella.

             Example.  Salmonella  typhi

Function. They enable bacteria to propel themselves, for example, in response to a chemotactic stimulus. Bacteria having polar flagella swim in a back and forth fashion .They reverse the direction of the swimming by reversing the direction of flagella rotation.

Bacteria having lateral flagella move in a more complicated manner. Their flagella operate in synchrony to form bundle that extend behind the cell.

Bacterial movement:

Bacterial chemotaxis; most motile bacteria are capable of directed swimming toward or away from a various chemical compounds, process is called chemotaxis. Stimulus is not the chemical but concentration of chemical i.e. temporal gradient which is sensed by protein chemoreceptors

• Positive Chemotaxis:

Swimming towards a chemical called positive chemotaxis.

• Negative Chemotaxis:

              Swimming away from a chemical is called negative chemotaxis

             iii            Phototaxis Phototropic bacteria exhibit phototaxis i.e. movement towards

                              light.

  Iv       Glinding motility; some bacterial species are motile only when they are in contact with solid surface. Myxobacteria show gliding motility by its pili.

Periplasmic Flagella. Certain helical bacteria like spirochetes lack external flagella . They have gel like structure located within the cell called periplasmic flagella or endoflagella or axial filament.   

Atrichous: Bacteria without flagella.

Pili:

Pilus (hair)

“These are hollow, non-helical, filamentous structures that are thinner shorter and more numerous than flagella. They have no role in bacterial motility. They are mostly fond on Gram negative bacteria”

Functions

• Pili is involved in conjugation. One type known as F-Pilus or Sex-Pilus serves a port of entry of genetic material during bacterial mating.

 

• Pili ploy role in bacterial virulence. They help in attachment of pathogenic bacteria to attach to epithelial cell lining of respiratory of respiratory, intestinal or genitor-urinary throats and prevent the washing of bacteria hence help in the establishment of disease or infection in human. E.coli causing severe watery diarrhea, attack to lining of small intestine by Pili. Neisseria gonorrhoeae causing genitourinary tract infections attach to the lining with pili.

Difference between Fimbrae and Pili

Some Microbiologists use these two terms Pili and fiimbrae interchangeably.

FIMBRIAE	PILI
These are few hundred to few thousand in number shorter than Pili.	Only one or two in number Pili are longer than fimbriae
Major function is attachment.	Help in transfer of genetic material from one cell to another

 

Sheath

Some bacterial species those from fresh or marine environment from chain or trichomes enclosed by a hollow, tube like sheath which play a role in bacterial survival

Prosthecae

Prosthecae are semi-rigid extension of cell wall and membrane that increase the surface area of cell for nutrient absorption.

Stalks

Stalks are certain non-living ribbon like structures secreted by cells which help in attachment of cell to various surfaces.

Cell Wall:

The cell wall of bacterial cell is a complex semi rigid structure responsible for the shape of the cell.

Function of cell wall:

• It maintains the shape of bacteria.
• It prevents the cell from expending a bursting because of uptake of water because most of the bacterium live in hypotonic environment.
• It is the site of attachment of flagella.
• It is essential for bacterial growth and division. Cells whose walls have been removed are incapable of normal growth and division.

Structure and chemical composition

The bacterial cell wall is composed of a molecular network called peplidoglycan (also known as murein).The disaccharide portion is made up of monosaccharides called N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Alternating NAM and NAG molecules are linked in rows of 10 to 65 sugars to form a carbohydrate "backbone" (the glycan portion of peptidoglycan). Adjacent rows are linked by polypeplides (the peptide portion of peptidoglycan).

 

Gram-Positive Cell Walls

In most gram-positive bacteria, the cell wall consists of many layers of peptidoglycan, forming a thick, rigid structure. In addition, the cell walls of gram-positive bacteria contain teichoic acids, which consist primarily of an alcohol (such as glycerol or ribitol) and phosphate (this carries strong negative) which regulate movement of cations into and out of the cell.

 

Gram-Negative Cell Walls

 

The cell walls of gram-negative bacteria consist of one or a very few layers of peptidoglycan and an outer membrane, a bilayer consisting of phospholipid, protein, and lipopolysaccharide .Lipids portion of lipopolysaccharides is called lipid A and is referred as endotoxin and is a toxic in host blood stream or GIT. It cause fever and shock.

 

 The peptidoglycan is bonded to lipoproteins (lipids covalently linked to proteins) in the outer membrane. The periplasm, a gel-like fluid between the outer membrane and the plasma membrane is also present in Gram negative bacteria. The periplasm contains a high concentration of degradative enzymes and transport proteins. Gram-negative cell walls do not contain teichoic acids. Because the cell walls of gram-negative bacteria contain only a small amount of peptidoglycan, they are more susceptible to mechanical breakage.

 

 

Difference between Gram positive and Gram negative cell wall.

Gram negative cell wall	Gram positive cell wall
It contain less peptidoglycan almost 10% of cell wall	It contains more peptidoglycan almost 50% of cell wall.
Less thick almost 10-15 nm	More thick almost 20-25 nm
Cell wall structure is more complex	Simple structure of cell wall
Outer membrane is present i.e. composed of phospholipids, proteins and lipopolysaccharides. So it contain more lipids.	An outer membrane is absent so it contains less lipids.
Periplasmic space is present. It is a fluid is filled space between outer membrane and plasma membrane. It contains high concentration of enzymes and transport proteins.	No periplasmic space in gram positive cell wall. It contain lipotechoic acid.
Lipotechoic acid is absent	Lipotechoic acid is present

 

  Function of Cell Wall in Gram Negative Bacteria

It carries strong negative charge which is an important factor in invading phagocytosis and action of complement system. It also provides a barrier to antibiotics.

Clinical Importance

Clinically the cell wall is important because it contributes to the ability of some species to cause disease and is the site of action of some antibiotics. Chemicals that damage bacterial cell walls, or interfere with their synthesis, often do not harm the cells of an animal host because the bacterial cell wall is made of chemicals unlike those in eukaryotic cells. Thus, cell wall synthesis is the target for some antimicrobial drugs certain antibiotics, such as penicillin, destroy bacteria by interfering with the formation of the peptide cross-bridges of peptidoglycan, thus preventing the formation of a functional cell wall. Most gram-negative bacteria are not as susceptible to penicillin as gram-positive bacteria are because the outer membrane of gram-negative bacteria forms a barrier that inhibits the entry of this and other substances, and gram- negative bacteria have fewer peptide cross-bridges. However, gram-negative bacteria are quite susceptible to some beta lactam antibiotics that penetrate the outer membrane better than penicillin.

ATYPICAL CELL WALL

Mycoplasma

They have no cell wall. Their plasma membrane have lipids called sterols and enclosing the cytoplasm of cell which protect them from lysis. They are small in size and pass through bacterial filters so are sometime mistakenly considered as viruses.

Mycoplasma are opportunistic pathogens. They cause pneumonia in children.

Acid Fast Bacteria

Few bacterial genra like Mycobacterium and Nocardia have 60% of waxy material mycolic acid in their cell wall. Because of mycolic acid they are resistant to staining. They are stained by special staining techniques so are called Acid Fast Bacteria.

Archaeobacteria

Although most Archaeobacteria possess cell walls, these donot contain peptidoglycan. Their cell wall is usually composed of protein, glycoprotein or polysaccharide. A few genra like Methanobacterium have cell wall composed of pseudomurein.

Cytoplasmic Membrane                                                                                                                                                                                                               

Cytoplasmic membrane lying inside the cell wall  

Structure

Its thickness is 7.5nm. It is composed of phospholipid( 20-30%)  and proteins (60-70%).                                                                                                                       Phospholipid  form a bilayer in which most of proteins are strongly held( intergral proteins) and other types of protein that are loosely attached(peripheral proteins) and may be removed by a mild treatment of membrane such as osmotic shock.

                                                                                                                     

Function

1. Most important function of plasma membrane to act as selective barrier through which material enter and exists the cells. Larger molecules such as proteins can not pass through membrane but smaller molecules such as oxygen ,carbon dioxide and simple sugars  can pass through it.                                                                                       

 2. It also contain various enzymes that are involved in metabolism they break down nutrients and produced energy in form of ATP.                                                                                                            4.It also contain enzymes that are involved in synthesis of capsular or cell wall components.

Destruction of the Plasma Membmne by Antimicrobial Agents

 

Because the plasma membrane is vital to the bacterial cell, it is not surprising that several antimicrobial agents exert their effects at this site. In addition to the chemicals that damage the cell wall and thereby indirectly expose the membrane to injury, many compounds specifically damage plasma membranes. These compounds include certain alcohols and quaternary ammonium compounds, which are used as disinfectants. By disrupting the membrane's phospholipids, a group of antibiotics known as the polymyxins cause leakage of intracellular contents and subsequent cell death

Protoplast

            A protoplast is the portion of bacterial cell consisting of cytoplasmic membrane and cell material bounded by it.

Protoplast can be prepared from Gram positive bacteria by treating the cell with lysozyme or culturing the bacteria in the presence of penicillin.

Spheroplast.

Round ,osmotically fragile form of Gram negative bacteria by procedure similar as described in protoplast . It contain two membranes,the cytoplasmic membrane and the outer membrane.

CYTOPLASM                    

Cytoplasm is the cell material bounded by the cytoplasmic membrane and it is divided into

1.Cytoplasmic area is granular in appearance and rich in ribosomes by which are protein synthesized.

2.Chromatic area is rich in DNA.

3. Fluid portion with dissolved substances.

 Ribosomes of prokaryotes composed of two subunits 50S and 30S total are 70S. In contrast to ribosomes of eukaryotes that has sedimentation co-efficient of 80S composed of two subunits 60S and 40S. In contrast to eukaryotes, bacterial cell contains neither a distinct membrane enclosed nucleus nor a mitotic apparatus, so the structure has been designated as NUCLEOID means nucleus like. It contains a single circular DNA molecule called PLASMID.                                                                                                          Within cytoplasm of prokaryotic cells there are several kind of reserved deposits known as inclusions. These include lipid inclusion, sulphur granules, meta-chromatic granules (inorganic phosphate), polysaccharides granules, gas vacuole and magnetosomes (Fe3O4).

 CLINICAL IMPORTANCE

Several antibiotics work by inhibiting protein synthesis on prokaryotic ribosomes. Antibiotics such as Streptomycin and Gentamycin attach to 30S subunit and interfere with protein synthesis.

Other antibiotics such as erythromycin and chloremphenicol interfere with protein synthesis by attaching to 50S subunit. Because of difference in prokaryotic and eukaryotic ribosomes, the microbial cell can be killed by the antibiotic while the eukaryotic host cell remains unaffected. 

Bacterial DNA replication is inhibited by certain quinolones such as ciprofloxacin and levofloxacin. They prevent chromosomal topology by targeting DNA gyrase and Topoisomerase .

Spores  

Spore is a metabolic dormant form which under appropriate condition undergo generation to form a vegetative cell. Certain species of bacteria produce spores either within cell (endospore) or external to cell(exospores).                                                                                           

 Endospore

The process of endospore formation within a vegetative cell takes several hours and is known as sporogenesis/sporulation. These endospore are thick walled, highly durable bodies that are produce by a few genera of Gram- positive bacteria when essentials nutrients are depleted. 

Example of genera produce endospore are Bacillus, Clostridium, Sporosarcinae and few other genera.                                                                     

SHAPE AND LOCATION OF ENDOSPORE                                                                                              

There are three types of shapes and location                                                                             

Spore are elliptical and centrally located e.g.  Bacillus cerus                                                                                                                                           Spore are spherical and terminally located e.g. Clostridium tetani                                                                                                                Spore are ovoid (oval- like) and sub-terminally located.  Clostridium sub-terminate                                                                                          

Functions 

Endospore are resistant to dessication (drying) staining, radiation and disinfecting chemicals.                                                                  Composition All endospore contain large amount of DPA (dipilcolonic acid).                                                                                                                                           It is about 10% to 15% of spore weight and it occurs in combination with calcium.                                                                              Endospore remain dormant for thousands of years and returns to its vegetative state by a process called germination.  

Medical significance of sporulation

 

Some of the most notorious pathogens are spore-formers, including B. anthracis (anthrax,), Bacillus cereus (gastroenteritis,), Clostridium tetani (tetanus,), Clostridium botulinum (botulism,), and Clostridium perfringens (gas gangrene,). Spores of these organisms can remain viable for many years, and are generally not killed by boiling, but can be killed by autoclaving

                                                                                                           

 EXOSPORE   Exospore are spore external to the vegetative cell.                                                       EXAMPLE   Cell of Methylosinus species form exospores by budding at the one end of cell. 

Function. They provide desiccation and heat resistant to cell.

EXOTOXIN: Heat sensitive protein molecules manufactured during metabolism by Gram Positive and Gram Negative cells are released into host environment or in host cells categorized as

Cytotoxins:  Kills Cells

Hemolysin:  Lyse RBC

Neurotoxin : Interfere with Nerve transmission

Enterotoxin:  Effect intestinal Tract

Examples include the toxins responsible for cholera, diphtheria and botulism.