Chapter 3The Prokaryotic Cell

• Morphology

• Cell Structure

• Secretion Systems

• Flagella / Pili

• DNA and DNA transfer– Chapter 8

• Other

Morphology

Cell Groupings

Biofilms

Biofilm: a polysaccharide-encased community of microorganisms can grow on many surfaces (catheters, surgical devices, pipes, teeth) extremely resistant to environmental insults (antibiotics, bactericidal agents)

Biofilm ArchitectureWater channels / nutrient access

Biofilm Life Cycle

Biofilms and Pathogenesis

Vibrio cholerae biofilms protect against stomach acids

Prokaryotic Structures

Prokaryotic Structures

Cytoplasmic Membrane& Transport Systems

Simple diffusionMovement of permeable molecules alonga concentration gradient

Facilitated diffusionMovement along a concentration gradientthrough a protein channel

Active transportMovement against a concentration gradientrequires energy expenditure

Group translocationChemical alteration of molecule circumvents the concentration gradient

Active TransportMajor Facilitator Superfamily

Energy from proton motive force used to:(a) Transport nutrients into the cell(b) Expel waste products, antimicrobial drugs, etc. out of the cell

Circles = protonsDiamond = other substance

Active TransportABC Transporters

ABC = ATP-binding cassetteATP hydrolysis = energy source

1. Binding protein scavenges nutrient

2. Transporter recognizes binding protein

3. Nutrient pumped into the cell with energy from ATP hydrolysis

Transport SystemsGroup Translocation

Chemical alteration of molecule circumvents the concentration gradient

Alteration = phosphorylation

Phosphorylated nutrient is not equivalent tounphosphorylated nutrient

Energy expenditure from phosyphorylation

Transport Mechanisms

Osmosis and the Cell Wall

Simple diffusionMovement along a concentration gradient

OsmosisWater flow to eliminate a concentration gradient

Osmotic pressure on cytoplasmic membraneresults in cell expansion

Cell wall allows cell to withstand osmotic pressure

Gram-Positive / Gram NegativeCell Wall

Peptidoglycan Components

PeptidoglycanOnly found in bacteria

Alternating series of two major subunits:1. N-acetylmuramic acid (NAM)2. N-acetylglucosamine (NAG)

NAM + NAG = glycan chain

Tetrapeptide chainattached to NAMcross-linkages allow for 3D structures

gram-negative: direct cross-links gram-positive: peptide interbridge

Peptidoglycan Structure

Peptidoglycan: Drug Targets

LysozymeEnzyme found in bodily fluidsBreaks the NAM/NAG bondEffective vs. Gram-positives

B-lactam Effect

Control + Drug

Gram-Positive Cell Wall

Thick peptidoglycan

Teichoic acids negative charge

Gram-Negative Cell Wall

Thin peptidoglycan layer

Outer membraneanother lipid bilayer + proteinsLPS = outer leaflet of lipid layerlipoprotein linkage to peptidogylcan molecular barrierporins: channel-forming proteins specificity

Periplasmarea between outer membrane and cytoplasmic (inner) membranefilled with enzymes and proteins

Lipopolysaccharide (Endotoxin)

O antigenDifferences can be used to identifyspecies or strains

Lipid AHighly immunogenic

Bacteria That Lack a Cell WallMycoplasma

Sterols strengthen and stabilize cytoplasmic membrane

Capsule and Slime Layer

Capsule (glycocalyx)Gel-like layer for protection or attachmentDistinct and gelatinous

Slime layerGel-like layer for protection or attachmentDiffuse and irregular

Gram Negative Secretion Systems

Type III Secretion System

Purpose: Inject virulence factors directly into the host cell cytoplasm

Flagella

Pili

PiliHollow, helical string of protein subunits arranged as a cylinder

Function: 1. attachment (fimbrae) 2. solid media motility (twitching or gliding)

3. conjugation (F pilus or sex pilus)

Antigenic and Phase Variation

• Antigenic Variation– Altered characteristics of surface proteins– Multiple genes for surface proteins– Expression locus: site of gene expression

• Random mechanism inserts different genes into locus

• Phase Variation– Gene expression switched on and off

F Pilus and ConjugationConjugationDNA transfer from one cell to another

Transfer from F+ to F- cell

Plasmids

Plasmid-Encoded Traits

Plasmid Transfer: Conjugation

F Plasmid Integration

Plasmid Insertion SequencesAllows plasmid integration at homologous sites in the bacterialchromosome

HfrHigh frequency of recombination

Formation of F’ Cell / F’ PlasmidPlasmid can excise from Hfr cell

F’ plasmid F plasmid + small piece of chromosomal DNA

transferred via conjugationrecipients become F+

Generalized Transduction

any host gene can be transferred

common method of gene transfer

Mechanisms of DNA Transfer

Transformation: Cells must be in a specialized (“competent”) state to receive DNA

Bacterial Chromosome(s)

NucleoidIrregular, gel-like mass of the chromosome(s)10% of cell volumeSupercoiled DNA allows tight packaging

GenomicsUtilization of information from large-scale genome sequencing

Identification of virulence factors acquisition of virulence factors

gene regulatory mechanisms genetic relatedness

2002: 87 bacterial genomes sequenced

DNA TransferTransposable Elements

Allows multiple genes to move as a unit from one location (chromosome or plasmid) to another location in the cell

Transposable ElementsAcquisition of Antibiotic Resistance

Pathogenicity Islands

• Virulence-associated genes

• Gram-negative, pathogen-specific– Salmonella SPI-1, SPI-2; E. coli LEE (Pai3)

• Large (> 30 kB) distinct chromosomal units

• Lower GC content than rest of chromosome

• Unstable, flanked by insertion sequences

Bacterial Ribosomes

Ribosomes protein + rRNA components S = Svedberg unit measure of sedimentation

mRNA translation & protein synthesis important / conserved process

Differences between prokaryotic and eukaryotic ribosomes can be exploited for antimicrobial therapeutics

Prokaryotic ribosome (eukaryotic = 80S)

Bacterial Ribosomes: Drug Target

Endospores

Forms in response to nutrient deprivation

Allows cell survival in dormant state

Resistant to: heatdessicationtoxic chemicalsUV irradiation

Mainly species of Bacillus and Clostridium

Endospore Formation

SporulationOccurs when little nitrogen or carbon is present

GerminationBrief exposure to heat or chemicalsEndospore takes on water, swellsSpore coat / cortex crack openVegetative cell grows out 1 endospore = 1 vegetative cell not a means of reproduction

The Prokaryotic CellSummary

• Morphology

• Cell Structure

• Secretion Systems

• Flagella / Pili

• DNA and DNA transfer– Chapter 8

• Other