The ProkaryotesThe Prokaryotes

Sofronio Agustin

Professor

Sofronio Agustin

Professor

LECTURES IN MICROBIOLOGYLECTURES IN

MICROBIOLOGY

LESSON 4LESSON 4

2

Lesson 4 TopicsLesson 4 Topics

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

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External StructuresExternal Structures

Flagella

Pili and fimbriae

Glycocalyx

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FlagellaFlagella

Composed of protein subunits called flagellin.

“H” antigens used in serotyping of bacterial

strains. Example: Escherichia coli O157: H7

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Flagellar StructureFlagellar Structure

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

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Flagellar ArrangementFlagellar Arrangement

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

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Bacterial MotilityBacterial Motility

The rotation of the flagella enables bacteria to be motile.

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ChemotaxisChemotaxis

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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EndoflagellaEndoflagella

Spirochetes have their

flagella embedded in the

membrane = endoflagella

Also called axial filament

Example: T. pallidum

(corkscrew motility)

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Pili and FimbriaePili and Fimbriae

Attachment

Mating (Conjugation)

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FimbriaeFimbriae

Fimbriae are smaller than flagella and are important for attachment.

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Pili 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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CapsuleCapsule

The capsule is covalently

bound to the cell wall.

Associated with virulence in bacteria.

Example:

Streptococcus pneumoniae

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Slime LayerSlime 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 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 EnvelopeCell Envelope

Cell wallGram-positive

Gram-negative

Cytoplasmic membrane Cell wall-less bacteria

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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

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Peptidoglycan Layer Peptidoglycan Layer

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

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Cell Wall Structures Cell Wall Structures

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

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Cytoplasmic MembraneCytoplasmic Membrane

Phospholipid bilayer “Fluid mosaic” model

Embedded proteins for active transport

Enzymes for energy generation

Photosynthetic pigments

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L FormsL 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 Cell Wall Less Bacteria

No peptidoglycan layer

Cell membrane contains sterols for stability

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The MycoplasmaThe 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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CytoplasmCytoplasm

Gelatinous solution containing water,

nutrients, proteins, and genetic material

Site for cell metabolism

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Genetic StructuresGenetic Structures

Deoxyribonucleic acid (DNA)

Ribonucleic acid (RNA)

Ribosomes

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Bacterial Genome Bacterial Genome

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

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Prokaryotic RibosomeProkaryotic 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 BodiesInclusion Bodies

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

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Bacterial CytoskeletonBacterial Cytoskeleton

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

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Endospores 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 FormationEndospore 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 MorphologyBacterial Morphology

Coccus Rod or bacillus Curved or spiral Cell arrangements Pleomorphism

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Typical Shapes and ArrangementsTypical Shapes and Arrangements

Scanning electron micrographs of different bacterial shapes and arrangements.

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

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PleomorphismPleomorphism

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

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Bacterial Shapes and Arrangements Bacterial Shapes and Arrangements

Cellular shapes and arrangements are useful in bacterial identification.

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The Dimension of Bacteria The Dimension of Bacteria

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

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ClassificationClassification

Phenotypic methods

Molecular methods

Taxonomic scheme Unique groups

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Phenotypic MethodsPhenotypic Methods

Cell morphology - staining Biochemical test – enzyme test

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Molecular MethodsMolecular Methods

DNA sequence 16S RNA Protein sequence

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Major Taxonomic Groups of BacteriaMajor 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 TypesUnique Bacterial Types

Intracellular bacteria

Photosynthetic bacteria

Sulfur bacteria

Gliding and fruiting bacteria

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Intracellular BacteriaIntracellular Bacteria

Intracellular bacteria

must live in host cells for

them to metabolize and

reproduce

Examples:

Rickettsiae

and Chlamydiae

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CyanobacteriaCyanobacteria

Cyanobacteria are important photosynthetic bacteria associated with oxygen production.

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Sulfur BacteriaSulfur 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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MyxobacteriaMyxobacteria

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

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ArchaeaArchaea

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

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ArchaeaArchaea

Archaea are found in:

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

Example:Halobacterium salinarium