The prokaryotes
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LECTURES IN MICROBIOLOGY. The Prokaryotes. Sofronio Agustin Professor. LESSON 4. Lesson 4 Topics. External Structures Cell Envelope Internal Structures Cell Shapes, Arrangement, and Sizes Classification. External Structures. Flagella Pili and fimbriae Glycocalyx. Flagella.

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The Prokaryotes

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The prokaryotes



The Prokaryotes

Sofronio Agustin



Lesson 4 topics

Lesson 4 Topics

  • External Structures

  • Cell Envelope

  • Internal Structures

  • Cell Shapes, Arrangement, and Sizes

  • Classification

External structures

External Structures

  • Flagella

  • Pili and fimbriae

  • Glycocalyx



  • Composed of protein subunits called flagellin.

  • “H” antigens used in serotyping of bacterial strains.

  • Example: Escherichia coli O157: H7

Flagellar structure

Flagellar Structure

Three components of a flagellum: filament, hook and basal body

Flagellar arrangement

Flagellar Arrangement

(a) Monotrichous (b) Lophotrichous (c) Amphitrichous (d) Peritrichous

Bacterial motility

Bacterial Motility

The rotation of the flagella enables bacteria to be motile.



Chemotaxis is the movement of bacteria in response to chemical signals. It consists of a series of tumbles and runs toward or away from source of stimuli.



  • Spirochetes have their flagella embedded in the membrane = endoflagella

  • Also called axial filament

  • Example: T. pallidum (corkscrew motility)

Pili and fimbriae

Pili and Fimbriae

  • Attachment

  • Mating (Conjugation)



Fimbriae are smaller than flagella and are important for attachment.

The prokaryotes


Pili enable conjugation to occur, which is the transfer of DNA from one bacterial cell to another (“mating”).



  • Capsule

    Protects bacteria from phagocytic cells

  • Slime layer

    Enable attachment and aggregation of bacterial cells



  • The capsule is covalently bound to the cell wall.

  • Associated with virulence in bacteria.

  • Example:

    Streptococcus pneumoniae

Slime layer

Slime Layer

  • The slime layer is loosely bound to the cell.

  • Carbohydrate rich material enhances adherence of cells on surfaces

  • Example:

    Streptococcus mutans and “plaque formation”



  • The slime layer is associated with cell aggregation and the formation of biofilms

  • Example:

    Staphylococcus epidermidis biofilms on catheter tips

Cell envelope

Cell Envelope

  • Cell wall



  • Cytoplasmic membrane

  • Cell wall-less bacteria

Cell wall

Cell Wall

  • Gram positive cell wall

    • Thick peptidoglycan (PG) layer

    • Acidic polysaccharides

    • Teichoic acid and lipoteichoic acid

  • Gram-negative cell wall

    • Thin peptidoglycan (PG) layer

    • Lipopolysaccharide layer

    • Porins

    • Periplasmic space

Peptidoglycan layer

Peptidoglycan Layer

PG is a complex sugar and peptide structure important for cell wall stability and shape.

Cell wall structures

Cell Wall Structures

Structures associated with gram-positive and gram-negative cell walls.

Cytoplasmic membrane

Cytoplasmic Membrane

  • Phospholipid bilayer

  • “Fluid mosaic” model

  • Embedded proteins for active transport

  • Enzymes for energy generation

  • Photosynthetic pigments

L forms

L Forms

Mutations can cause some bacteria to lose the ability to synthesize the cell wall and are called L forms.

Cell wall less bacteria

Cell Wall Less Bacteria

  • No peptidoglycan layer

  • Cell membrane contains sterols for stability

The mycoplasma

The Mycoplasma

  • Mycoplasma bacteria have no cell wall, which contributes to their pleomorphic shapes

  • Smallest bacteria

    (0.2 um)

  • Example:

    Mycoplasma pneumoniae

    (SEM on right)

Internal structures

Internal Structures

  • Cytoplasm

  • Genome

  • Inclusion bodies

  • Actin

  • Endospore



  • Gelatinous solution containing water, nutrients, proteins, and genetic material

  • Site for cell metabolism

Genetic structures

Genetic Structures

  • Deoxyribonucleic acid (DNA)

  • Ribonucleic acid (RNA)

  • Ribosomes

Bacterial genome

Bacterial Genome

Most bacteria contain a single circular double strand of DNA called a nucleoid.

Prokaryotic ribosome

Prokaryotic Ribosome

  • A ribosome is a combination of RNA and protein, and is the site for protein synthesis

  • Composed of large (50S) and small (30S) subunits

  • S = Svedverg unit, measures molecular size

Inclusion bodies

Inclusion Bodies

Inclusion bodies enable a cell to store nutrients and to survive in nutrient depleted environments

Bacterial cytoskeleton

Bacterial Cytoskeleton

Actin is a protein fiber present in some bacteria, which is involved in maintaining cell shape.



  • Nutrient depletion induces some bacteria (vegetative cell) to form endospores in order to survive

  • Dehydrated gel state due to calcium-protein complex

  • Dipicolinic acid (found only in spores) hardens the spore

Endospore formation

Endospore Formation

Some pathogenic bacteria that produce toxins during the vegetative stage are capable of forming spores. (e.g. Bacillus and Clostridium species)

Bacterial morphology

Bacterial Morphology

  • Coccus

  • Rod or bacillus

  • Curved or spiral

  • Cell arrangements

  • Pleomorphism

Typical shapes and arrangements

Typical Shapes and Arrangements

Scanning electron micrographs of different bacterial shapes and arrangements.

(a) Streptococcus (b) Bacillus (c) Spirochete (d) Spirillum



Some bacteria show varied shapes and arrangements called pleomorphism. Ex: Corynebacterium diphtheriae’s “Chinese letter” arrangement.

Bacterial shapes and arrangements

Bacterial Shapes and Arrangements

Cellular shapes and arrangements are useful in bacterial identification.

The dimension of bacteria

The Dimension of Bacteria

Relative size of a bacterial cell compared to other cells including viruses.



  • Phenotypic methods

  • Molecular methods

  • Taxonomic scheme

  • Unique groups

Phenotypic methods

Phenotypic Methods

  • Cell morphology - staining

  • Biochemical test – enzyme test

Molecular methods

Molecular Methods

  • DNA sequence

  • 16S RNA

  • Protein sequence

Major taxonomic groups of bacteria

Major Taxonomic Groups of Bacteria

  • The methods of classification have allowed bacteria to be classified into different taxonomic groups

  • Re: Bergey’s Manual of Determinative Bacteriology

    (Table on right)

Unique bacterial types

Unique Bacterial Types

  • Intracellular bacteria

  • Photosynthetic bacteria

  • Sulfur bacteria

  • Gliding and fruiting bacteria

Intracellular bacteria

Intracellular Bacteria

  • Intracellular bacteria must live in host cells for them to metabolize and reproduce

  • Examples:


    and Chlamydiae



Cyanobacteria are important photosynthetic bacteria associated with oxygen production.

Sulfur bacteria

Sulfur Bacteria

Green and purple sulfur bacteria are photosynthetic, do not give off oxygen, and are found in sulfur springs, freshwater, and swamps.



An example of a fruiting body bacteria in which reproductive spores are produced.



  • Associated with extreme environments

  • Contain unique cell walls

  • Contain unique internal structures



  • Archaea are found in:

  • hot springs (thermophiles)

  • high salt content areas (halophiles)

  • Example:

    Halobacterium salinarium

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