Prokaryotic Profiles: the Bacteria and the

Archaea

                              

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Characteristics of Cells

• Eucaryotic cells: animals, plants, fungi, and protists– contain membrane-bound nucleus with DNA as

chromosomes– contain membrane-bound organelles that

compartmentalize the cytoplasm and perform specific functions

• Procaryotic cells: bacteria and archaea – no nucleus or other membrane-bound organelles

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Prokaryotic Profiles

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External Structures

• Appendages– Two major groups of appendages:• Motility – flagella and axial filaments

(periplasmic flagella)• Attachment or channels – fimbriae and pili

• Glycocalyx – surface coating

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Flagella

• 3 parts– filament – long, thin, helical structure

composed of proteins– hook- curved sheath– basal body – stack of rings firmly anchored

in cell wall

• Rotates 360o

• 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

negativelight stimuli – phototaxis

Signal sets flagella into rotary motion clockwise or counterclockwise:counterclockwise – results in smooth linear

direction – runclockwise - tumbles

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Chemotaxis in bacteria

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Fimbrae

• Fine hairlike bristles from the cell surface

• Function in adhesion to other cells and surfaces

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Pili

• Rigid tubular structure made of pilin protein

• Found only in Gram negative cells

• Functions – joins bacterial cells for DNA transfer

(conjugation)– adhesion

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Conjugation

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Glycocalyx

• Coating of molecules external to the cell wall, made of sugars and/or proteins

• 2 types1. capsule - highly organized, tightly attached2. 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

Composed of two basic layers:– cell wall and cell membrane

• Maintains cell integrity• Two generally different groups of

bacteria demonstrated by Gram stain:– Gram-positive bacteria: thick cell wall

composed primarily of peptidoglycan and cell membrane

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

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Peptidoglycan

• 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

• Consists of – a thick, homogenous sheath of

peptidoglycan 20-80 nm thick– tightly bound acidic polysaccharides,

including teichoic acid and lipoteichoic acid

– cell membrane

• Retain crystal violet and stain purple

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Gram Negative Cell Wall

• Consists of– an outer membrane containing

lipopolysaccharide (LPS)– thin shell of peptidoglycan– periplasmic space– inner membrane

• Lose crystal violet and stain red from safranin counterstain

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Cell Membrane Structure

• Phospholipid bilayer with embedded proteins – fluid mosaic model

• Functions in:– 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

• 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”

• 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

• 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

• 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

• Resting, dormant cells• Produced by some G+ genera: Clostridium,

Bacillus & Sporosarcina• Have a 2-phase life cycle – vegetative cell &

an endospore

• 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

• 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 121oC for 20-30

minutes will destroy

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3 Basic Shapes of Bacteria

• Cocci - spherical

• Bacilli - rod

• Spiral - helical, comma, twisted rod, spirochete

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Methods in Bacterial Identification

1. Microscopic morphology2. Macroscopic morphology – colony

appearance3. Physiological / biochemical

characteristics4. Serological analysis5. Genetic & molecular analysis

• G + C base composition• DNA analysis using genetic probes• Nucleic acid sequencing & rRNA analysis

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Major Taxonomic Groups of Bacteria

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

• Domain Bacteria - – 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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• 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)

Bacterial Naming Conventions

Procaryotes with Unusual Characteristics

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Rickettsias• 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• Tiny• Obligate intracellular parasites• Not transmitted by arthropods• Chlamydia trachomatis – severe eye

infection and one of the most common sexually transmitted diseases

• Chlamydia psittaci – ornithosis, parrot fever

• Chlamydia pneumoniae – lung infections

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Mycoplasmas

• 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

• Photosynthetic bacteria– Cyanobacteria– Green & purple sulfur bacteria

• Gliding, fruiting bacteria

• Appendaged bacteria– produce an extended process of the cell

wall in form of a bud, stalk or long thread

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Archaea• 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