Chapter 3

Bacterial Cell Structure1313.1 The Prokaryote ControversyList the characteristics originally used to describe prokaryotic cellsForm an opinion on the prokaryote controversy using current evidence about bacterial cells23.2 A Typical Bacterial CellDistinguish a typical bacterial cell from a typical plant or animal cell in terms of cell shapes and arrangements, size, and cell structuresDiscuss the factors that determine the size and shape of a bacterial cell.34Bacterial and Archaea Structure and FunctionProkaryotes differ from eukaryotes in size and simplicitymost lack internal membrane systemsterm prokaryotes is becoming blurredthis text will use Bacteria and Archaeathis chapter will cover Bacteria and their structures45Size, Shape, and ArrangementShape cocci and rods most commonvarious othersArrangementdetermined by plane of divisiondetermined by separation or notSize - varies6Shape and Arrangement-1Cocci (s., coccus) spheresdiplococci (s., diplococcus) pairsstreptococci chainsstaphylococci grape-like clusterstetrads 4 cocci in a squaresarcinae cubic configuration of 8 cocci

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7Shape and Arrangement-2bacilli (s., bacillus) rodscoccobacilli very short rodsvibrios resemble rods, comma shapedspirilla (s., spirillum) rigid helicesspirochetes flexible helices

78Shape and Arrangement-3mycelium network of long, multinucleate filamentspleomorphic organisms that are variable in shape

89Sizesmallest 0.3 m (Mycoplasma)average rod 1.1 - 1.5 x 2 6 m (E. coli)very large 600 x 80 m Epulopiscium fishelsoni

10Size Shape Relationshipimportant for nutrient uptakesurface to volume ratio (S/V)small size may be protective mechanism from predation

11Bacterial Cell OrganizationCommon FeaturesCell envelope 3 layersCytoplasm External structures 12

3.3 Bacterial Plasma MembranesDescribe the fluid mosaic model of membrane structure and identify the types of lipids typically found in bacterial membranes.Distinguish macroelements (macronutrients) from micronutrients (trace elements) and provide examples of each.Provide examples of growth factors needed by some microorganisms.Compare and contrast passive diffusion, facilitated diffusion, active transport, and group translocation, and provide examples of each.Discuss the difficulty of iron uptake and describe how bacteria overcome this difficulty.1314Bacterial Cell EnvelopePlasma membraneCell wallLayers outside the cell wall

15Bacterial Plasma MembraneAbsolute requirement for all living organisms

Some bacteria also have internal membrane systems1516Plasma Membrane FunctionsEncompasses the cytoplasm Selectively permeable barrierInteracts with external environmentreceptors for detection of and response to chemicals in surroundingstransport systemsmetabolic processes

17Fluid Mosaic Model of Membrane Structurelipid bilayers with floating proteinsamphipathic lipids polar ends (hydrophilic interact with water) non-polar tails (hydrophobic insoluble in water) membrane proteins1718Membrane ProteinsPeripheral loosely connected to membraneeasily removedIntegral amphipathic embedded within membranecarry out important functionsmay exist as microdomains

19Bacterial LipidsSaturation levels of membrane lipids reflect environmental conditions such as temperatureBacterial membranes lack sterols but do contain sterol-like molecules, hopanoidsstabilize membranefound in petroleum 1920Uptake of Nutrients Getting Through the BarrierMacroelements (macronutrients)C, O, H, N, S, P found in organic molecules such as proteins, lipids, carbohydrates, and nucleic acids K, Ca, Mg, and Fecations and serve in variety of roles including enzymes, biosynthesisrequired in relatively large amounts2021Uptake of Nutrients Getting Through the BarrierMicronutrients (trace elements)Mn, Zn, Co, Mo, Ni, and Curequired in trace amountsoften supplied in water or in media componentsubiquitous in natureserve as enzymes and cofactorsSome unique substances may be required22Uptake of Nutrients Getting Through the BarrierGrowth factorsorganic compoundsessential cell components (or their precursors) that the cell cannot synthesizemust be supplied by environment if cell is to survive and reproduce2223Classes of Growth Factorsamino acidsneeded for protein synthesispurines and pyrimidinesneeded for nucleic acid synthesisvitaminsfunction as enzyme cofactorsheme2324

Uptake of Nutrients

Microbes can only take in dissolved particles across a selectively permeable membraneSome nutrients enter by passive diffusionMicroorganisms use transport mechanismsfacilitated diffusion all microorganismsactive transport all microorganismsgroup translocation Bacteria and Archaeaendocytosis Eukarya only2425Passive DiffusionMolecules move from region of higher concentration to one of lower concentration between the cells interior and the exteriorH2O, O2, and CO2 often move across membranes this way2526Facilitated DiffusionSimilar to passive diffusionmovement of molecules is not energy dependentdirection of movement is from high concentration to low concentrationsize of concentration gradient impacts rate of uptake26

27Facilitated DiffusionDiffers from passive diffusion uses membrane bound carrier molecules (permeases)smaller concentration gradient is required for significant uptake of moleculeseffectively transports glycerol, sugars, and amino acidsmore prominent in eukaryotic cells than in bacteria or archaea2728Active Transportenergy-dependent processATP or proton motive force usedmove molecules against the gradientconcentrates molecules inside cellinvolves carrier proteins (permeases)carrier saturation effect is observed at high solute concentrations28

29ABC TransportersPrimary active transporters use ATPATP-binding cassette (ABC) transportersObserved in Bacteria, Archaea, and eukaryotesConsist of - 2 hydrophobic membrane spanning domains- 2 cytoplasmic associated ATP-binding domains- Substrate binding domains

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30Secondary Active TransportMajor facilitator superfamily (MFS)Use ion gradients to cotransport substancesprotonssymport two substances both move in the same directionantiport two substances move in opposite directions

31Group TranslocationEnergy dependent transport that chemically modifies molecule as it is brought into cellBest known translocation system is phosphoenolpyruvate: sugar phosphotransferase system (PTS)3132Iron UptakeMicroorganisms require ironFerric iron is very insoluble so uptake is difficultMicroorganisms secrete siderophores to aid uptakeSiderophore complexes with ferric ionComplex is then transported into cell

323.4 Bacterial Cell WallsDescribe peptidoglycan structure.Compare and contrast the cell walls of typical Gram-positive and Gram-negative bacteria.Relate bacterial cell wall structure to the Gram-staining reaction.3334Bacterial Cell WallPeptidoglycan (murein)rigid structure that lies just outside the cell plasma membranetwo types based on Gram stainGram-positive: stain purple; thick peptidoglycanGram-negative: stain pink or red; thin peptidoglycan and outer membrane35Cell Wall FunctionsMaintains shape of the bacteriumalmost all bacteria have oneHelps protect cell from osmotic lysisHelps protect from toxic materialsMay contribute to pathogenicity

36Peptidoglycan Structure Meshlike polymer of identical subunits forming long strandstwo alternating sugarsN-acetylglucosamine (NAG) N- acetylmuramic acidalternating D- and L- amino acids

37Strands Are CrosslinkedPeptidoglycan strands have a helical shapePeptidoglycan chains are crosslinked by peptides for strengthinterbridges may formpeptidoglycan sacs interconnected networksvarious structures occur38

39Gram-Positive Cell WallsComposed primarily of peptidoglycanMay also contain teichoic acids (negatively charged)help maintain cell envelopeprotect from environmental substancesmay bind to host cellssome gram-positive bacteria have layer of proteins on surface of peptidoglycan

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41Periplasmic Space of Gram + BacteriaLies between plasma membrane and cell wall and is smaller than that of Gram-negative bacteriaPeriplasm has relatively few proteins Enzymes secreted by Gram-positive bacteria are called exoenzymesaid in degradation of large nutrients4142Gram-Negative Cell WallsMore complex than Gram- positiveConsist of a thin layer of peptidoglycan surrounded by an outer membraneOuter membrane composed of lipids, lipoproteins, and lipopolysaccharide (LPS)No teichoic acids

4243Gram-Negative Cell WallsPeptidoglycan is ~5-10% of cell wall weightPeriplasmic space differs from that in Gram-positive cellsmay constitute 2040% of cell volumemany enzymes present in periplasmhydrolytic enzymes, transport proteins and other proteins43

44Gram-Negative Cell Wallsouter membrane lies outside the thin peptidoglycan layerBrauns lipoproteins connect outer membrane to peptidoglycanother adhesion sites reported4445Lipopolysaccharide (LPS)Consists of three partslipid Acore polysaccharideO side chain (O antigen)Lipid A embedded in outer membraneCore polysaccharide, O side chain extend out from the cell

4546Importance of LPScontributes to negative charge on cell surfacehelps stabilize outer membrane structure may contribute to attachment to surfaces and biofilm formationcreates a permeability barrierprotection from host defenses (O antigen)can act as an endotoxin (lipid A)4647Gram-Negative Outer Membrane PermeabilityMore permeable than plasma membrane due to presence of porin proteins and transporter proteinsporin proteins form channels to let small molecules (600700 daltons) pass

4748Mechanism of Gram Stain ReactionGram stain reaction due to nature of cell wallshrinkage of the pores of peptidoglycan layer of Gram-positive cellsconstriction prevents loss of crystal violet during decolorization stepthinner peptidoglycan layer and larger pores of Gram-negative bacteria does not prevent loss of crystal violet4849Osmotic ProtectionHypotonic environments solute concentration outside the cell is less than inside the cellwater moves into cell and cell swellscell wall protects from lysisHypertonic environmentssolute concentration outside the cell is greater than insidewater leaves the cellplasmolysis occurs50Evidence of Protective Nature of the Cell Walllysozyme breaks the bond between N-acetyl glucosamine and N-acetylmuramic acidpenicillin inhibits peptidoglycan synthesisif cells are treated with either of the above they will lyse if they are in a hypotonic solution

5051Cells that Lose a Cell Wall May Survive in Isotonic EnvironmentsProtoplasts SpheroplastsMycoplasma does not produce a cell wallplasma membrane more resistant to osmotic pressure3.5 Cell Envelope Layers Outside the Cell WallCompile a list of all the structures found in all the layers of bacterial cell envelopes, noting the functions and the major component molecules of each.5253Components Outside of the Cell WallOutermost layer in the cell envelopeGlycocalyxcapsules and slime layersS layersAid in attachment to solid surfacese.g., biofilms in plants and animals5354Capsules Usually composed of polysaccharidesWell organized and not easily removed from cellVisible in light microscopeProtective advantagesresistant to phagocytosisprotect from desiccationexclude viruses and detergents

55Slime Layerssimilar to capsules except diffuse, unorganized and easily removedslime may aid in motility

56S LayersRegularly structured layers of protein or glycoprotein that self-assemblein Gram-negative bacteria the S layer adheres to outer membranein Gram-positive bacteria it is associated with the peptidoglycan surface

5657S Layer FunctionsProtect from ion and pH fluctuations, osmotic stress, enzymes, and predationMaintains shape and rigidityPromotes adhesion to surfacesProtects from host defensesPotential use in nanotechnologyS layer spontaneously associates3.6 Bacterial CytoplasmCreate a table or concept map that identifies the components of the bacterial cytoplasm and describes their structure, molecular makeup, and functions.5859Bacterial Cytoplasmic StructuresCytoskeletonIntracytoplasmic membranesInclusionsRibosomesNucleoid and plasmids

60Protoplast and CytoplasmProtoplast is plasma membrane and everything withinCytoplasm - material bounded by the plasmid membrane

61The CytoskeletonHomologs of all 3 eukaryotic cytoskeletal elements have been identified in bacteriaFunctions are similar as in eukaryotes

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62Best Studied ExamplesFtsZ many bacteriaforms ring during septum formation in cell division

MreB many rodsmaintains shape by positioning peptidoglycan synthesis machineryCreS rare, maintains curve shape63Intracytoplasmic MembranesPlasma membrane infoldingsobserved in many photosynthetic bacteria observed in many bacteria with high respiratory activityAnammoxosome in Planctomycetes organelle site of anaerobic ammonia oxidation

6364InclusionsGranules of organic or inorganic material that are stockpiled by the cell for future useSome are enclosed by a single-layered membranemembranes vary in compositionsome made of proteins; others contain lipids may be referred to as microcompartments 6465Storage InclusionsStorage of nutrients, metabolic end products, energy, building blocksGlycogen storageCarbon storage poly--hydroxybutyrate (PHB) Phosphate - Polyphosphate (Volutin)Amino acids - cyanophycin granules6566Storage Inclusions

6667MicrocompartmentsNot bound by membranes but compartmentalized for a specific functionCarboxysomes - CO2 fixing bacteriacontain the enzyme ribulose-1,5,-bisphosphate carboxylase (Rubisco), enzyme used for CO2 fixation

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68Other InclusionsGas vacuolesfound in aquatic, photosynthetic bacteria and archaeaprovide buoyancy in gas vesicles

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69Other InclusionsMagnetosomesfound in aquatic bacteriamagnetite particles for orientation in Earths magnetic fieldcytoskeletal protein MamK helps form magnetosome chain6970RibosomesComplex protein/RNA structures sites of protein synthesisbacterial and archaea ribosome = 70S eukaryotic (80S) S = Svedburg unitBacterial ribosomal RNA 16S small subunit23S and 5S in large subunit

71The NucleoidUsually not membrane bound (few exceptions)Location of chromosome and associated proteinsUsually 1 closed circular, double-stranded DNA molecule Supercoiling and nucleoid proteins (different from histones) aid in folding

7172PlasmidsExtrachromosomal DNA found in bacteria, archaea, some fungiusually small, closed circular DNA moleculesExist and replicate independently of chromosomeepisomes may integrate into chromosomeinherited during cell divisionContain few genes that are non-essentialconfer selective advantage to host (e.g., drug resistance)Classification based on mode of existence, spread, and function

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733.7 External StructuresDistinguish pili (fimbriae) and flagella.Illustrate the various patterns of flagella distribution.7475External StructuresExtend beyond the cell envelope in bacteriaFunction in protection, attachment to surfaces, horizontal gene transfer, cell movement pili and fimbriaeflagella

76Pili and FimbriaeFimbriae (s., fimbria); pili (s., pilus)short, thin, hairlike, proteinaceous appendages (up to 1,000/cell)can mediate attachment to surfaces, motility, DNA uptakeSex pili (s., pilus)longer, thicker, and less numerous (1-10/cell)genes for formation found on plasmidsrequired for conjugation7677FlagellaThreadlike, locomotor appendages extending outward from plasma membrane and cell wallFunctionsmotility and swarming behaviorattachment to surfacesmay be virulence factors78Bacterial FlagellaThin, rigid protein structures that cannot be observed with bright-field microscope unless specially stainedUltrastructure composed of three partsPattern of flagellation varies79Patterns of Flagella DistributionMonotrichous one flagellumPolar flagellum flagellum at end of cellAmphitrichous one flagellum at each end of cellLophotrichous cluster of flagella at one or both endsPeritrichous spread over entire surface of cell

7980Three Parts of FlagellaFilamentextends from cell surface to the tiphollow, rigid cylinder of flagellin proteinHooklinks filament to basal bodyBasal bodyseries of rings that drive flagellar motor

81Flagellar Synthesiscomplex process involving many genes/gene productsnew flagellin molecules transported through the hollow filament using Type III-like secretion systemfilament subunits self-assemble with help of filament cap at tip, not base813.8 Bacterial Motility and ChemotaxisCompare and contrast flagellar swimming motility, spirochete flagellar motility, and twitching and gliding motility.State the source of energy that powers flagellar motility.Explain why bacterial chemotaxis is referred to as a biased random walk.8283MotilityFlagellar movementSpirochete motilityTwitching motilityGliding motility84MotilityBacteria and Archaea have directed movementChemotaxismove toward chemical attractants such as nutrients, away from harmful substancesMove in response to temperature, light, oxygen, osmotic pressure, and gravity

85Bacterial Flagellar MovementFlagellum rotates like a propellervery rapid rotation up to 1100 revolutions/secin general, counterclockwise (CCW) rotation causes forward motion (run)in general, clockwise rotation (CW) disrupts run causing cell to stop and tumble

86Mechanism of Flagellar MovementFlagellum is 2 part motor producing torqueRotorC (FliG protein) ring and MS ring turn and interact with statorStator - Mot A and Mot B proteinsform channel through plasma membrane protons move through Mot A and Mot B channels using energy of proton motive force torque powers rotation of the basal body and filament87Spirochete MotilityMultiple flagella form axial fibril which winds around the cellFlagella remain in periplasmic space inside outer sheathCorkscrew shape exhibits flexing and spinning movements

8788Twitching and Gliding MotilityMay involve Type IV pili and slimeTwitchingpili at ends of cell short, intermittent, jerky motionscells are in contact with each other and surfaceGlidingsmooth movements89ChemotaxisMovement toward a chemical attractant or away from a chemical repellentChanging concentrations of chemical attractants and chemical repellents bind chemoreceptors of chemosensing system 8990

91Chemotaxis In presence of attractant (b) tumbling frequency is intermittently reduced and runs in direction of attractant are longerBehavior of bacterium is altered by temporal concentration of chemicalChemotaxis away from repellent involves similar but opposite responses

913.9 Bacterial EndosporesDescribe the structure of the bacterial endospore.Explain why the bacterial endospores are of particular concern to the food industry and why endospore-forming bacteria are important model organisms.Describe in general terms the process of sporulation.Describe those properties of endospores that are thought to contribute to its resistance to environmental stresses.Describe the three stages that transform an endospore into an active vegetative cell.9293The Bacterial EndosporeComplex, dormant structure formed by some bacteria Various locations within the cellResistant to numerous environmental conditionsheatradiationchemicalsdesiccation

9394Endospore StructureSpore surrounded by thin covering called exosporiumThick layers of protein form the spore coatCortex, beneath the coat, thick peptidoglycanCore has nucleoid and ribosomes

95What Makes an Endospore so Resistant?Calcium (complexed with dipicolinic acid)Small, acid-soluble, DNA-binding proteins (SASPs)Dehydrated coreSpore coat and exosporium protect9596SporulationProcess of endospore formationOccurs in a hours (up to 10 hours)Normally commences when growth ceases because of lack of nutrientsComplex multistage process9697

98Formation of Vegetative CellActivationprepares spores for germinationoften results from treatments like heatingGerminationenvironmental nutrients are detectedspore swelling and rupture of absorption of spore coatincreased metabolic activityOutgrowth - emergence of vegetative cell98