The Prokaryotes

Sofronio AgustinProfessor

LECTURES IN MICROBIOLOGY

LESSON 4

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Lesson 4 Topics

External Structures Cell Envelope Internal Structures Cell Shapes, Arrangement, and Sizes Classification

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External Structures

Flagella

Pili and fimbriae

Glycocalyx

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Flagella

Composed of protein subunits called flagellin.

“H” antigens used in serotyping of bacterial strains.

Example: Escherichia coli O157: H7

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Flagellar Structure

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

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Flagellar Arrangement

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

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Bacterial Motility

The rotation of the flagella enables bacteria to be motile.

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Chemotaxis

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.

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Endoflagella

Spirochetes have their flagella embedded in the membrane = endoflagella

Also called axial filament

Example: T. pallidum (corkscrew motility)

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Pili and Fimbriae

Attachment

Mating (Conjugation)

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Fimbriae

Fimbriae are smaller than flagella and are important for attachment.

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Pili

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

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Glycocalyx

CapsuleProtects bacteria from phagocytic cells

Slime layerEnable attachment and aggregation of bacterial cells

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Capsule

The capsule is covalently

bound to the cell wall.

Associated with virulence in bacteria.

Example:

Streptococcus pneumoniae

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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”

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Biofilms

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

Example:Staphylococcus epidermidis biofilms on catheter tips

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Cell Envelope

Cell wallGram-positive

Gram-negative

Cytoplasmic membrane Cell wall-less bacteria

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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

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Peptidoglycan Layer

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

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Cell Wall Structures

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

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Cytoplasmic Membrane

Phospholipid bilayer “Fluid mosaic” model Embedded proteins for active transport Enzymes for energy generation Photosynthetic pigments

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L Forms

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

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Cell Wall Less Bacteria

No peptidoglycan layer

Cell membrane contains sterols for stability

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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)

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Internal Structures

Cytoplasm Genome Inclusion bodies Actin Endospore

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Cytoplasm

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

Site for cell metabolism

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Genetic Structures

Deoxyribonucleic acid (DNA)

Ribonucleic acid (RNA) Ribosomes

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Bacterial Genome

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

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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

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Inclusion Bodies

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

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Bacterial Cytoskeleton

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

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Endospores

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

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Endospore Formation

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

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Bacterial Morphology

Coccus Rod or bacillus Curved or spiral Cell arrangements Pleomorphism

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Typical Shapes and Arrangements

Scanning electron micrographs of different bacterial shapes and arrangements.

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

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Pleomorphism

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

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Bacterial Shapes and Arrangements

Cellular shapes and arrangements are useful in bacterial identification.

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The Dimension of Bacteria

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

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Classification

Phenotypic methods Molecular methods Taxonomic scheme Unique groups

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Phenotypic Methods

Cell morphology - staining Biochemical test – enzyme test

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Molecular Methods

DNA sequence 16S RNA Protein sequence

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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)

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Unique Bacterial Types

Intracellular bacteria

Photosynthetic bacteria

Sulfur bacteria

Gliding and fruiting bacteria

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Intracellular Bacteria

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

Examples:

Rickettsiae and Chlamydiae

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Cyanobacteria

Cyanobacteria are important photosynthetic bacteria associated with oxygen production.

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Sulfur Bacteria

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

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Myxobacteria

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

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Archaea

Associated with extreme environments Contain unique cell walls Contain unique internal structures

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Archaea

Archaea are found in:

hot springs (thermophiles) high salt content areas (halophiles)

Example:Halobacterium salinarium