Morphology of Bacteria and Different Bacterial Parts

MORPHOLOGY OF BACTERIA

PARTS OF BACTERIAL CELL:

A study of bacterial shape and structure is known as morphology of bacteria. A bacterial cell is a prokaryotic cell which has following parts:

• Cell wall
• Cytoplasmic membrane
• Cytoplasm
• Ribosomes
• Mesosomes
• Nucleus
• Cytoplasmic inclusion bodies
• Plasmids
• Capsule/Slime layer

 

 

CELL WALL

CELL WALL COMPOSITION:

Bacterial cell wall is 10-20 nm thick and is 20-30% of dry weight. It is made up of mucopeptides. Mucopeptide consists of N—acetylglucosamine and N—acetylmuramic acid molecules alternating in chains which are linked by peptides.

DIFFERENCE BETWEEN CELL WALL OF GRAM POSITIVE & GRAM NEGATIVE BACTERIA :

The cell wall of gram positive bacteria contains teichoic acid while that of gram negative bacteria does not. The gram negative cell wall contains lipopolysaccharide which gives endotoxic activity and `O’antigen specificity to cell.

FUNCTIONS OF CELL WALL:

Cell wall provides rigidity, shape and antigenicity to the cell.

DETECTION OF CELL WALL:

Cell wall can be demonstrated by:

1. Plasmolysis

2. Microdissection

3. Differential staining in Electron microscopy

4. Mechanical rupture of cell

5. Reaction with specific antibodies

CELL WALL DEFICIENT FORMS:

Spheroplasts, protoplasts, L forms and mycoplasma are examples of cell wall deficient structures.

CYTOPLASMIC MEMBRANE

FUNCTION OF CYTOPLASMIC MEMBRANE

This is a thin semipermeable membrane which controls the flow of metabolites to & from the protoplast.

It acts as a osmotic barrier protecting the cell from external changes.

It acts as a cell boundary.

It has a role in cell growth and division.

CYTOPLASM

FUNCTION OF CYTOPLASM:

Cytoplasm which is colloidal contains organelles which perform various functions of the bacterial cell. These organelles are following:

RIBOSOMES

These are the protein synthesis sites and are smaller than ribosomes of eukaryotic cell. The sedimentation coefficient is 70s.

MESOSOMES

These are the vesicular structure formed due to invaginations of plasma membrane into the cytoplasm. They contain respiratory enzymes and carry out respiration. Thus they are analogous to mitochondria of eukaryotes.

INTRACYTOPLASMIC INCLUSIONS:

These are of many types. Metachromatic granules are an example.

METACHROMATIC GRANULES

These are also known as volutin granules and made up of polymetaphosphate. They are a reserve of energy and phosphates for cell metabolism.  These can be stained by polychrome methylene blue or Albert’s stain.

LIPID INCLUSIONS

These are storage products and can be stained by Sudan Black dye.

POLYSACCHARIDE GRANULES

These can be demonstrated by staining with Iodine.

NUCLEUS

COMPOSITION OF NUCLEUS:

Being a prokaryotic cell, the nucleus is not a defined structure in bacterial cell; instead it is made up of loosely lying chromatin material which acquires a circular compact form like a skin of thread so there is single, circular chromosome which carries genetic information. Nucleus does not contain nucleolus and nuclear membrane.

EXTRA NUCLEAR GENETIC MATERIAL

The DNA material which is present in cytoplasm but not in nucleus is known as extra nuclear genetic material. These are:

 a)Plamids/Episomes                                                 

 b) Transposons

 They are not essential for the life of bacteria but confer new properties to the bacterium.

SLIME LAYER & CAPSULE OF BACTERIA

DEFINITION & COMPOSITION OF SLIME LAYER:

These are viscid materials secreted around the bacterial cell wall.

Capsule: It is a well defined structure and is generally a polysaccharide but there are few exceptions like in anthrax bacilli capsule is made up of polypeptides.

Slime layer: It is a loose undefined structure around the cell wall.

FUNCTIONS OF CAPSULE:

Capsule serves as a protective covering against antibacterial substances such as bacteriophage, phagocytes and enzymes.

It enhances bacterial virulence.

Capsular antigen is hapten in nature and specific for the bacteria.

CAPSULAR STAINS

Negative staining with Indian ink/nigrosin

Hiss’s capsular stain

Positive staining by Muir method

DEMONSTRATION

Capsule can be demonstrated by:

(1) Negative staining with India ink.

(2) ln dark field microscopy.

(3) Quellung’s reaction.

(4) Capsular stains

EXAMPLES

Demonstration of capsule of Cryptococcus by India ink preparation.

Demonstration of capsule of pneumococcus by quellung reaction.

FLAGELLA

DEFINITION

These are long, unbranched sinuous filaments which are responsible for locomotion of

bacterial cell and are made up of flagellin protein. Flagella are of uniform diameter and their length varies from 3-20 µm.

ARRANGEMENT AND  TYPES OF FLAGELLA WITH EXAMPLES

Flagella are arranged in different manner in different bacterial cells.

Flagellar arrangement:

(a) Polar : Flagella at poles of bacteria

Monotrichous: Single polar flagellum eg. Vibrio cholerae

Lophotrichous: Tuft of flagella at one or both ends eg spirilla

Amphitrichous: Single flagellum at both the poles eg. Alkaligenes faecalis.

(b) Peritrichous: Flagella surround the whole bacterial cell eg. Salmonella typhi.

FUNCTIONS OF FLAGELLA:

• Responsible for motility of bacteria
• Are immunogenic

DEMONSTRATION OF FLAGELLA

(1) Electron microscopy

(2) Flagellar stains

(3) Dark ground microscopy

(4) Demonstration of motility

FLAGELLAR STAINS:

• Silver impregnation method
• In order to observe them with the light microscope, the thickness of the flagella are increased by coating them with mordants such as tannic acid and potassium alum, and staining them with basic fuchsin (Gray method), pararosaniline (Leifson method), silver nitrate (West method; named after Marcia West, a clinical microbiologist), or crystal violet (Difco’s method).

MOTILITY

DEFINITION

When an organism changes its position in the field with respect to another organism, it is known as motility.

BROWNIAN MOVEMENT

It is an oscillatory movement possessed by all small bodies (whether living or not) suspended in fluid. It is not actual motility of an organism.

ACTIVE & PASSIVE MOVEMENT

Active movement is when the organism flows against the flow of the fluid. Passive movement is moving passively in the direction in which the fluid is moving.

TYPES OF BACTERIAL MOTILITY WITH EXAMPLES

Slow stately motion of E coli, Actively motile salmonella and pseudomonas, Darting motility shown by vibrios, Gliding motility shown by mycoplasma, Flexion and extension, Translatory motion and Corkscrew like rotatory motion shown by spirochaetes.

DEMONSTRATION OF BACTERIAL MOTILITY:

• Wet mount – in light microscope, dark ground or phase contrast microscopes
• Hanging drop preparation – at the edge of the drop
• Semi-solid agar media – diffuse spreading growth
• Craigie’s tube
• U tube
• Swarming on solid media
• Silver impregnation method to demonstrate flagella
• Electron microscopy to demonstrate flagella

HANGING DROP PREPARATION METHOD:

1. With a toothpick, spread a small ring of Vaseline around the concavity of a depression slide. Do not use too much Vaseline.

2. After thoroughly mixing one of the cultures, use the inoculating loop to aseptically place a small drop of one of the bacterial suspensions in the centre of a coverslip.

3. Invert the depression slide, with the concavity facing down, onto the coverslip so that the drop protrudes into the centre of the concavity of the slide. Take care so that the drop does not get disturbed. Press gently to form a seal.

4. Turn the hanging drop slide over and place on the stage of the microscope so that the

drop remains hanging in the concave space, attached to the coverslip.

5. Examine the drop by first locating its edge under low power and focusing on the drop. Switch to the high-dry objective and then, using immersion oil, to the 90 to 100x objective. In order to see the bacteria clearly, close the diaphragm as much as possible for increased contrast. Note bacterial shape, size, arrangement, and motility. Be careful to distinguish between motility and Brownian movement.

6. Discard your coverslips and any contaminated slides in a container with disinfectant solution.

BACTERIAL SWARMING:

Definition of Bacterial Swarming

Swarming is due to vigorous motility of the organisms. The exact cause is not yet known. Long forms arise at the edge of the surface growth, because they have more flagella per unit length and longer flagella, which the short forms do not have.

Examples of bacterial swarming:

• Proteus mirabilis and Proteus vulgaris on blood agar.
• Clostrium tetani on neomycin blood agar.

All organisms swarm if they are streaked on a moist plate, so all plates should be dry before streaking.

FIMBRIAE

DEFINITION

These are fine, hair like appendages which project from the cell surface. They are smaller and thinner than flagella. They are made up of protein known as pilin protein. There are 8 morphological types of fimbria which have different affinities to mannose sugar.

FUNCTION S OF FIMBRIA

They are the organs of adhesion and some cause haemagglutination.

A special type of fimbria is sex pilus. These form conjugation tubes during transfer of genetic material from one cell to another in the process of transduction.

DEMONSTRATION OF FIMBRIA

Negative staining with phosphotungstic acid (PTA).

SPORE

DEFINITION

Spores are the highly resistant resting stage of the vegetative bacterial cell which help in survival of the bacteria in adverse circumstances. They are extremely resistant to the dessication, heat and chemicals.

TYPES OF BACTERIAL SPORE

• According to location: Central, Subterminal, Terminal
• According to shape: Oval, Spherical, spindle etc

DEMONSTRATION OF SPORE

 (1) Spore stains

 (2) Acid fast stains

 (3) Gram stain: Spores appear as clear spaces which do not take up stain.

Spore Stains

• Schaeffer-Fulton or Wirtz-Conklin
• Dorner’s method

PLEOMORPHISM & INVOLUSION FORMS OF BACTERIA:

• Variation in shape and size of bacterial cell is known as pleomorphism. 

• Swollen and aberrant form of bacterial cell are known as involution forms. They are seen in ageing laboratory cultures.

• Defective cell wall synthesis is often responsible for such forms.

SHAPE OF BACTERIA

CLASSIFICATION OF BACTERIA ACCORDING TO THE SHAPE:

(1) COCCI: Small, spherical, rounded or oval bacteria

(2) BACILLI: Small rods either straight or curved

(3) VIBRIO: Comma shaped bacilli

(4) SPIRALS: Bacteria in coils shapes: (a) Rigid Spirals          (b) Flexible spirals

(5) ACTINOMYCETES : Branching filaments (higher bacteria)

ARRANGEMENT OF BACTERIA

CLASSIFICATION OF BACTERIA ACCORDING TO ARRANGEMENT:

(1) In Group: Cocci arranged in groups e.g. staphylococcus

(2) In chain: Cocci arranged in chains e.g. streptococcus

(3) In pair: Cocci arranged in pairs e.g. Pneumococcus, Neisseria

(4) In chinese letter pattern: Bacilli arranged in chinese letter pattern e.g. corynebacterium diphtheriae