Prokaryotic Profiles: the Bacteria and the Archaea

Characteristics of Cells

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Eucaryotic cells: animals, plants, fungi, and protists

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contain membrane-bound nucleus with DNA as chromosomes

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contain membrane-bound organelles that compartmentalize the cytoplasm and perform specific functions

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Procaryotic cells: bacteria and archaea

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no nucleus or other membrane-bound organelles

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Prokaryotic Profiles

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External Structures

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Appendages

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Two major groups of appendages:

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Motility – flagella and axial filaments (periplasmic flagella) Attachment or channels – fimbriae and pili Glycocalyx – surface coating

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Flagella

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3 parts

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filament – long, thin, helical structure composed of proteins

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hook- curved sheath

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basal body – stack of rings firmly anchored in cell wall Rotates 360 o 1-2, or many distributed over entire cell Functions in motility

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Flagellar Function

Guide bacteria in a direction in response to external stimulus: chemical stimuli – chemotaxis; positive and negative light stimuli – phototaxis Signal sets flagella into rotary motion clockwise or counterclockwise: counterclockwise – results in smooth linear direction – run clockwise - tumbles

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Chemotaxis in bacteria

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Fimbrae

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Fine hairlike bristles from the cell surface Function in adhesion to other cells and surfaces

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Pili

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Rigid tubular structure made of pilin protein Found only in Gram negative cells Functions

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joins bacterial cells for DNA transfer (conjugation)

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adhesion

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Conjugation

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Glycocalyx

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Coating of molecules external to the cell wall, made of sugars and/or proteins 2 types 1.

2.

capsule - highly organized, tightly attached slime layer - loosely organized and attached

– – –

Functions Attachment and formation of biofilms inhibits killing by white blood cells protect cells from dehydration and nutrient loss

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Biofilms

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Biofilm on a Catheter

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The Cell Envelope

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Composed of two basic layers:

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cell wall and cell membrane Maintains cell integrity Two generally different groups of bacteria demonstrated by Gram stain:

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Gram-positive bacteria: thick cell wall composed primarily of peptidoglycan and cell membrane

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Gram-negative bacteria: outer cell membrane, thin peptidoglycan layer, and cell membrane

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Peptidoglycan

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Unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments Provides strong, flexible support to keep bacteria from bursting or collapsing because of changes in osmotic pressure

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4 Bacterial Groups Based on Cell Wall Composition

1. Gram positive cells 2. Gram negative cells 3. Bacteria without cell walls 4. Bacteria with chemically unique cell walls

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Gram Positive Cell Wall

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Consists of

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a thick, homogenous sheath of peptidoglycan 20-80 nm thick

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tightly bound acidic polysaccharides, including teichoic acid and lipoteichoic acid

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cell membrane

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Retain crystal violet and stain purple

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Gram Negative Cell Wall

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Consists of

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an outer membrane containing lipopolysaccharide (LPS)

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thin shell of peptidoglycan periplasmic space inner membrane Lose crystal violet and stain red from safranin counterstain

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Cell Membrane Structure

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Phospholipid bilayer with embedded proteins – fluid mosaic model Functions in:

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providing site for energy reactions, nutrient processing, and synthesis transport into and out of the cell

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Cell Membrane Structure

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Cytoplasm

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Dense gelatinous solution of sugars, amino acids, & salts 70-80% water serves as solvent for materials used in all cell functions

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“Chromosome”

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Single, circular, double-stranded DNA molecule that contains all the genetic information required by a cell DNA is tightly coiled around protein, aggregated in a dense area called the nucleoid

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Plasmids

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Small circular, double-stranded DNA Free or integrated into the chromosome Duplicated and passed on to offspring Not essential to bacterial growth & metabolism May encode antibiotic resistance, tolerance to toxic metals, useful enzymes & toxins Used in genetic engineering- readily manipulated & transferred from cell to cell

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Ribosomes

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Made of 60% ribosomal RNA & 40% protein Consist of 2 subunits: large & small Procaryotic differ from eucaryotic ribosomes in size, and number of proteins Site of protein synthesis All cells have ribosomes

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Ribosomes

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Endospores

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Resting, dormant cells Produced by some G+ genera: Clostridium, Bacillus & Sporosarcina Have a 2-phase life cycle – vegetative cell & an endospore

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Sporulation

-formation of endospores

Germination

- return to vegetative growth Hardiest of all life forms Withstand extremes in heat, drying, freezing, radiation & chemicals

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Sporulation Cycle

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Endospores

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Environmental resistance linked to high levels of calcium & dipicolinic acid Dehydrated, metabolically inactive Thick coat Longevity verges on immortality, 25-250 million years Pressurized steam at 121 o C for 20-30 minutes will destroy

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3 Basic Shapes of Bacteria

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Cocci - spherical Bacilli - rod Spiral - helical, comma, twisted rod, spirochete

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Methods in Bacterial Identification

1. Microscopic morphology 2. Macroscopic morphology – colony appearance 3. Physiological / biochemical characteristics 4. Serological analysis 5. Genetic & molecular analysis

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G + C base composition

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DNA analysis using genetic probes Nucleic acid sequencing & rRNA analysis

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Major Taxonomic Groups of Bacteria

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Domain Archaea – primitive, adapted to extreme habitats and modes of nutrition Domain Bacteria -

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Phylum Proteobacteria – Gram-negative cell walls Phylum Firmicutes – mainly Gram positive with low G + C content Phylum Actinobacteria – Gram-positive with high G + C content

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Bacterial Naming Conventions

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species –a collection of bacterial cells which share an overall similar pattern of traits in contrast to other bacteria whose pattern differs significantly strain or variety – a culture derived from a single parent that differs in structure or metabolism from other cultures of that species (biovars, morphovars) type – a subspecies that can show differences in antigenic makeup (serotype or serovar), susceptibility to bacterial viruses (phage type) and in pathogenicity (pathotype)

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Procaryotes with Unusual Characteristics

Rickettsias

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Very tiny, gram-negative bacteria Most are pathogens that alternate between mammals and fleas, lice or ticks Obligate intracellular pathogens Cannot survive or multiply outside of a host cell Cannot carry out metabolism on their own

Rickettsia rickettisii

– Rocky Mountain spotted fever

Rickettsia prowazekii

– epidemic typhus

Coxiella burnetti

– Q fever

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Chlamydias

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Tiny Obligate intracellular parasites Not transmitted by arthropods

Chlamydia trachomatis

– severe eye infection and one of the most common sexually transmitted diseases

Chlamydia psittaci

parrot fever – ornithosis,

Chlamydia pneumoniae

infections – lung

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Mycoplasmas

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Naturally lack a cell wall Membranes stabilized by sterols, resistant to lysis Extremely small Range in shape from filamentous to coccus or doughnut shaped

Mycoplasma pneumoniae

– causes atypical pneumonia in humans

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Variations in the Shape of Mycoplasma

pneumoniae

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Free-living Nonpathogenic Bacteria

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Photosynthetic bacteria

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Cyanobacteria

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Green & purple sulfur bacteria Gliding, fruiting bacteria Appendaged bacteria

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produce an extended process of the cell wall in form of a bud, stalk or long thread

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Archaea

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Constitute third Domain Appear more closely related to Domain Eucarya than to bacteria Contain unique genetic sequences in their rRNA Have unique membrane lipids & cell wall construction Live in the most extreme habitats in nature, extremophiles Includes: methane producers, hyperthermophiles, extreme halophiles, and sulfur reducers

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