華中農(nóng)業(yè)大學(xué)微生物英文版課件2

Chapter2

CellBiology

2.1Overviewofthestructureofmicrobialcells2.2Procaryoticcellwall2.3Cytoplasmicmembrane2.4Cellulargeneticinformation2.5Cytoplasmicmatrix–RibosomeandInclusions2.6Componentsexternaltothecellwall2.7Bacterialendospores2.8ComparisonoftheprokaryoticandeukaryoticcellChapteroutlineAprocaryoticcellAeucaryoticcell

Overviewofcellstructure3.TheircellwallalmostalwayscontainthecomplexpolysaccharidepeptidoglycanTheprokaryoticcell1.Theirgeneticmaterial(DNA)isnotenclosedwithinamembraneandtheylackothermembrane–boundedorganelles2.TheirDNAisnotassociatedwithhistidine4.Theyareverysmall!!Size:

Mostbacteriafallwithinarangefrom0.2to2.0umindiameterandfrom2to8uminlengthArod-shapedprokaryoteistypicallyabout1-5micrometers(μm)longandabout1μmwideMicroorganismsingeneralareverysmallandarecompletelyinvisibletothenakedeye.

Acyanobacterium8x50umsizecomparisonofmicroorganismsVisibilityscaleMetersRelativesizeofMicrobesProkaryotesEukaryotesVirusesNakedeyeLightmicroscopeElectronmicroscopeacellincreasesinsize,itssurfacearea–to–volumeratiodecreasesSurfaceareaandvolumerelationshipsincellsspirallumShape:

Bacteriahaveafewbasicshapesspherical

coccusRod-shaped

bacillusThecellwallofthebacterialcellisacomplex,semi-rigidstructurethatisresponsibleforthecharacteristicshapeofthecell.Thecellwallsurroundstheunderlying,fragileplasma(cytoplasmic)membraneandprotectsitandinternalpartsofthecellfromadversechangesinthesurroundingenvironment.Almostallprokaryoteshavecellwalls.ProkaryoticcellwallGram+Gram-SchematicdiagramofbacterialcellwallsBacteriacanbedividedintotwomajorgroups,calledgram-positiveandgram-negative.Theoriginaldistinctionbetweengram-positiveandgram-negativewasbasedonaspecialstainingprocedure,theGramstainTheGram-positivecellwallhasapeptidoglycanlayerthatisrelativelythick(ca.40nm)andcomprisesapproximately90%ofthecellwall.ThecellwallsofmostGram-positiveeubacteriaalsohaveteichoicacids.Gram-positivecellwallStructureoftheRepeatingUnitinPeptidoglycanTheseconstituentsareconnectedtoformarepeatingstructure,theglycantetrapeptide.Peptidoglycaniscomposedoftwosugarderivatives,N-acetylglucosamine(NAG)andN-acetylmuramicacid(NAM),andasmallgroupofaminoacidsconsistingofL-alanine,D-alanine,D-glutamicacid,andeitherlysineordiaminopimelicacid(DAP).PeptideandglycanunitsareconnectedinformationofthepeptidoglycansheetGram-positiveBacteriafrequentlyhaveacidicpolysaccharidescalledteichoicacidsattachedtotheircellwall.Thetermteichoicacidsincludesallwall,membrane,orcapsularpolymerscontainingglycerophosphateorribitolphosphateresidues.ThesepolyalcoholsareconnectedbyphosphateestersandusuallyhaveothersugarsandD-alanineattached.

TeichoicacidsTeichoicacidsandlipoteichoicacidsarearrangedintheoverallwallstructureofgram-positiveBacteria.TeichoicacidLipoteichoicacidTheGram-negativecellwallisathinlayerattachedtoanoutermembranevialipoproteins.Theoutermembranecontainsphospholipidonitsinnersurfaceandlipopolysaccharide(LPS)onitsoutersurface.Thespacebetweentheoutermembraneandthecytoplasmicmembraneiscalledtheperiplasmicspace.TeichoicacidsdonotoccurinGram-negativebacterialcellwalls..

OsidechainCorepolysaccharideLipidAChemicalstructureofLipopolysaccharideMolecularmodelofE.colilipopolysaccharide

Thebondsbetweenthecarbohydratesinpseudopeptidoglycanareβ1-3insteadof'β1-4asinpeptidoglycan.

CELLWALLSOFARCHAEBACTERIA

Thearchaebacteria

donotcontainpeptidoglycanintheircellwallsasoccursineubacteria.

N-acetylmuramicacidandD-aminoacidsarenotfoundinthecellwallsofarchaebacteria.(Differencesfromeubacteria)

Somearchaebacteriahavewallscomposedofpseudopeptidoglycan,whichresemblesthepeptidoglycanofeubacteriabutcontainsN-acetyltalosaminuronicacidinsteadofN-acetylmuramicacidandL.-aminoacidsinsteadoftheD-aminoacidsineubacterialcellwalls.ProtoplastFormation

Peptidoglycancanbedestroyedbycertainagentsforinstancelysozyme,thatbreaksthe1,4-glycosidicbondsbetweenN-acetylglucosamineandN-acetylmuramicacidinthemolecule.Thedifferencebetweengram-positiveandgram-negativebacteriaisduetothephysicalnatureoftheircellwalls.Ifthecellwallisremovedfromgram-positivebacteria,theybecomegramnegative.Thepeptidoglycanseemstoactasapermeabilitybarrierpreventinglossofcrystalviolet.Gram-negativepeptidoglycanisverythin,notashighlycross-linked,andhaslargerpores.Alcoholtreatmentalsomayextractenoughlipidfromthegramnegativewalltofurtherincreaseitsporosity.Forthesereasons,alcoholmorereadilyremovesthepurplecrystalviolet-iodinecomplexfromgram-negativebacteria.TheMechanismofGramStaining

ProceduresofGramStainingGrampositiveorGramnegative?Morphologyofagram-positivebacterialcell

StructureofcytoplasmicmembraneFunctionofcytoplasmicmembrane2.3CytoplasmicmembraneA.Thetypicalcytoplasmicmembraneofprokaryoticandeukaryoticcellsisalipidbilayer,asillustratedhereshowingtheorientationsofthehydrophilic(tanspheres)andhydrophobic(black)endsofphospholipidsthatmakeupthisstructure.B.Colorizedelectronmicrographof'thecytoplasmicmembrane(CM)ofthebacteriumBacillussubtilisrevealsthecharacteristicrailroadtrackappearanceofthislipidbilayer.StructureofcytoplasmicmembraneItisatypicalUNITMEMBRANE!Thecytoplasmicmembrane,ahighlyselectivebarrier,isconstructedprincipallyoflipid,withinwhichcertainproteinsareembedded.Membranescontainbothlipidsandproteins,althoughtheexactproportionsoflipidandproteinvarywidely.1.Permeabilitybarrier-preventsleakageandfunctionasgatewayfortransportofnutrientsintoandoutofthecell.2.Proteinanchor-siteofmanyproteinsinvolvedintransport,bioenergetics,andchemotaxis.3.Energyconservation-siteofgenerationanduseoftheprotonmotiveforce.

FunctionofmembraneIntracellularmembranesystemBacteriacellsdon’tcontainmembrane-enclosedorganelles.However,bacteriamayhavespecializedinvaginationsofthecytoplasmicmembrane.Theirfunctionmaybetoprovidealargermembranesurfaceforgreatermetabolicactivity.StructureofMesosomeMesosomemaybeinvolvedinwallformationduringdivisionorplayaroleinchromosomereplicationanddistributiontodaughtercells.Itmayalsobeinvolvedinsecretoryprocesses2.4CellulargeneticinformationBacterialChromosomeSupercoilingandchromosomestructureChromosomalcopynumberPlasmidsMicrographofabacteriumshowingthenucleoidregion(green)withinthecytoplasmwherethebacterialchromosomeoccursThebacterialchromosomeisacircularDNAmacromoleculeexceptinStreptomyces

whereitislinearandRhodobacter

sphaffoides,whichhastwoseparatechromosomes.

BacterialchromosomeThebacterialchromosomeisusuallyasinglecovalentlyclosedcircularmolecule.ThetermnucleoidisusedtodescribeaggregatedDNAintheprokaryoticcell.RangeofgenomesizesinvirousgroupsoforganismsandtheorganellesofeukaryaThebacterialchromosomeandsupercoiling:ExampleofE.colicellThereareover

50supercoileddomainintheE.colichromosome.ThetotalamountofDNAisabout4600kb.IfthetotalDNAisopenedandlinearized,itwouldbe1mminlength.Thethecellisonlyabout2-3umlong.SotopackagethismuchDNAintothecellrequiresthattheDNAbehighlysupercoiled.ElectronmicrographofanisolatednucleoidreleasedfromE.coli.

Chromosomecopynumber

Bacteriathatreproduceasexuallyaretypicallyhaploidingeneticcomplement.Rapidlygrowingcellscontainmorethan

1copyofthechromosome,andonlywhencellgrowthhasceaseddoesthechromosomenumberapproachonepercell.

Reproductionofabacterialcellrequiresthereplicationofthebacterialchromosome.ThemicrographshowsthesequenceofsynthesisofnewcircularloopsofdoublehelicalDNA.Bacterianormallyreproducebybinaryfission.Theinwardgrowthoftheseptumdividestheparentcelltoproducetwoequal-sizedprogenycells.Plasmidsdon’tcontainthegeneticinformationfortheessentialmetabolicactivitiesofthecell,buttheygenerallydocontaingeneticinformationforspecialfeatures.

PlasmidProkaryoticcellshavesmallextra-chromosomalgeneticelementscalledplasmids.ResistantplasmidsColplasmidsConjugativeplasmidsMetabolicplasmidsMajortypesofplasmids

2.5Cytoplasmicmatrix–RibosomeandInclusionsAlleucaryoticandprocaryoticcellscontainribosomes,whichfunctionasthesitesofproteinsynthesis.Ribosomesarecomposedoftwosubunits

Procaryoticribosomesarecalled70Sribosomes,andthoseofeucaryoticcellsareknownas80SribosomesRibosomesTheletterSreferstoSvedbergunits,whichindicatetherelativerateofsedimentationduringultra-high-speedcentrifugationWithinthecytoplasmofprocaryotic(andeucaryotic)cellsareseveralkindsofreservedeposits,knownasinclusions.Someinclusionsarecommontoawidevarietyofbacteria,whereasothersarelimitedtoasmallnumberofspeciesandthereforeserveasabasisforidentification.Amongthemoreprominentbacterialinclusionsarethefollowing:Carbonstoragepolymers–PHBandglycogenPhosphatepolymersSulfurGranulesGasVacuolesINCLUSIONSPolyhydroxybutyricacid(PHB)PHBisalipidlikecompound-oneofthemostcommoninclusionbodiesinprokaryoticorganisms.PHBiscommonlyfoundasastoragematerialanduniquetobacteria

Glycogenisastarchlikepolymerofglucosesubunits.GlycogengranulesareusuallysmallerthanPHBgranules.AVibriospeciesManymicroorganismsaccumulategranulesofpolyphosphate,whicharelargereservesofinorganicphosphatesthatcanbeusedinthesynthesisofATPPolyphosphategranule

inabacterialcellAPseudomonasspeciesThesulfurglobulesinsidethecellsofpurplesulfurbacteriumChromatiumbuderiSomebacteria,includingmanyphotosyntheticbacteria,accumulateelementalsulfurgranulesasaresultoftheirmetabolism.Gasvacuoles(blue)andstoragegranules(red)inthecyanobacteriumMicrocystisTheformationofgasvacuolesbyaquaticbacteriaprovidesamechanismforadjustingthebuoyancyofthecell.Manyaquaticcyanobacteri