Prokaryotic Profiles:
the Bacteria and the
Archaea
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
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
• 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 types
•
1. capsule - highly organized, tightly attached
2. 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.
2.
3.
4.
Gram positive cells
Gram negative cells
Bacteria without cell walls
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 morphology
2. Macroscopic morphology – colony
appearance
3. Physiological / biochemical
characteristics
4. Serological analysis
5. 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 Grampositive with low G + C content
– Phylum Actinobacteria – Gram-positive
with high G + C content
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Bacterial Naming Conventions
• 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
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(pathotype)
Procaryotes with Unusual
Characteristics
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
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• Coxiella burnetti – Q fever
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
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infections
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
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sulfur reducers