Microbiology: A Systems Approach

Chapter 4Procaryotic Profiles:

The Bacteria and Archaea

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Chapter 4

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

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

Fig. 4.25 The dimension of bacteria

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Cell shapes

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

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

Fig. 4.24 Pleomorphism in Corynebacterium

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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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Different arrangements of flagella exist for different species.

Fig. 4.3 Electron micrograph depicting types of flagella arrangements.

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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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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

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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

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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 with pathogenic bacteria.

Fig. 4.10 Encapsulated bacteria

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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

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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– Teichoic acid and lipoteichoic acid– Acidic polysaccharides– Lipids – mycolic acids - Mycobacteria

• 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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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

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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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Cytoplasmic membrane

• Fluid-Mosaic Model• Phospholipids• Embedded proteins• Energy generation• Selective barrier; semipermeable• Transport

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Internal Structures

• Cytoplasm• Genetic structures • Storage bodies• Actin• Endospore

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Cytoplasm

• Area inside the membrane• About 80% water• Gelatinous solution containing

water, nutrients, proteins, and genetic material.

• Site for cell metabolism

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Genetic structures

• Single, circular chromosome• Nucleoid region• Deoxyribonucleic acid (DNA)• Ribonucleic acid (RNA)• Plasmids• Ribosomes

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

Fig. 4.17 Chromosome structure

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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

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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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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 bacteria

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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

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Green and purple sulfur bacteria are photosynthetic, do not give off 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

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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