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Chapter 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• Motility (chemotaxis)• Varied arrangement (ex. Monotrichous,

lophotrichous, amphitrichous)

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Three main parts of the flagella include the basal body, hook, and filament.

Fig. 4.2 Details of the basal body in gram negative cell

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Different arrangements of flagella exist for different species.

Fig. 4.3 Electron micrograph depicting types of flagella arrangements.

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The rotation of the flagella enables bacteria to be motile.

Fig. 4.4 The operation of flagella and the mode of locomotion in bacteria with polar and peritrichous flagella.

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Chemotaxis is the movement of bacteria in response to chemical signals.

Fig. 4.5 Chemotaxis in bacteria 8

Spirochete bacteria have their flagella embedded in the membrane.

Fig. 4.6 The orientation of periplasmic flagella on the spirochete cell.

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Pili and fimbriae

• Attachment• Mating (Conjugation)

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Fimbriae are smaller than flagella, and are important for attachment.

Fig. 4.7 Form and function of bacteria fimbriae

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Pili enable conjugation to occur, which is the transfer of DNA from one bacterial cell to another.

Fig. 4.8 Three bacteria in the process of conjugating 12

Glycocalyx

• Capsule– Protects bacteria from immune cells

• Slime layer– Enable attachment and aggregation of

bacterial cells

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The capsule is tightly bound to the cell, and is associated withpathogenic bacteria.

Fig. 4.10 Encapsulated bacteria 14

The slime layer is loosely bound to the cell.

Fig. 4.9 Bacterial cells sectioned to show the typesof glycocalyces.

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The slime layer is associated with the formation of biofilms, which are typically found on teeth.

Fig. 4.11 Biofilm 16

Cell envelope

• Cell wall– Gram-positive – Gram-negative

• Cytoplasmic membrane • Non cell wall

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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 PG layer– Outer membrane– Lipid polysaccharide– Porins

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

Fig. 4.13 Structure of peptidoglycan in the cell wall

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

Fig. 4.14 A comparison of the detailed structure of gram-positive and gram-negative cell walls.

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Cytoplasmic membrane

• L-forms• Embedded proteins• Energy generation• Transport

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

Fig. 4.16 The conversion of walled bacterial cells to L forms 22

No cell wall

• No PG layer• Cell membrane contain sterols for

stability

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Mycoplasma bacteria have no cell wall, which contributes to varied shapes.

Fig. 4.15 Scanning electron micrograph of Mycoplasma pneumoniae

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Internal Structures

• Cytoplasm• Genetic structures • Storage 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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Most bacteria contain a single circular double strand of DNA called a chromosome.

Fig. 4.17 Chromosome structure 28

A ribosome is a combination of RNA and protein, and is involved in protein synthesis.

Fig. 4.18 A model of a procaryotic ribosome.

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

Fig. 4.19 An example of a storage inclusionin a bacterial cell.

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Actin is a protein fiber (cytoskeleton) present in some bacteria, and is involved in maintaining cell shape.

Fig. 4.20 Bacterial cytoskeleton

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During nutrient depleted conditions, some bacteria (vegetative cell) form into an endospore in order to survive.

Fig. 4.21 Microscopic picture of an endospore formation 32

Some pathogenic bacteria that produce toxins during the vegetative stage are capable of forming spores.

Table 4.1 General stages in endospore formation

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Cell shapes

• Coccus• Rod or bacillus• Curved or spiral• Cell arrangements• Cell size

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Scanning electron micrographs of different bacterial shapes and arrangements.

Fig. 4.23 SEM photograph of basic shapes.

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Some bacteria (ex. Corynebacterium) have varied shapes called pleomorphism.

Fig. 4.24 Pleomorphism in Corynebacterium 36

Cellular shapes and arrangements are specific characteristics that can be used to identify bacteria.

Fig. 4.22 Bacterial shapes and arrangements

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

Fig. 4.25 The dimension of bacteria 38

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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The methods of classification have allowed bacteria to be grouped into different divisions and classes.

Table 4.3 Major taxonomic groups of bacteria42

An example of how medically important families and genera of bacterial are characterized.

Table 4.4 Medically important families and genera of bacteria.

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Unique groups of bacteria

• Intracellular parasites• Photosynthetic bacteria• Green and purple sulfur bacteria• Gliding and fruiting bacteria• Archaea bacteria

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Intracellular bacteria must live in host cells in order to undergo metabolism and reproduction.

Fig. 4.26 Transmission electron micrograph of rickettsia.

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Cyanobacteria are important photosynthetic bacteria associated with oxygen production.

Fig. 4.27 Structure and examples of cyanobacteria 46

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

Fig. 4.28 Behavior of purple sulfur bacteria

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

Fig. 4.29 Myxobacterium 48

Archaea bacteria

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

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Archaea bacteria that survive are found in hot springs (thermophiles) and high salt content areas (halophiles).

Fig. 4.30 Halophile around the world