01Overview of BacteriologyWord文档下载推荐.docx

01Overview of BacteriologyWord文档下载推荐.docx

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PROCARYOTICLIFE

TheBacteriaareagroupofsingle-cellmicroorganismswithprocaryoticcellularconfiguration.Thegeneticmaterial（DNA）ofprocaryoticcellsexistsunboundinthecytoplasmofthecells.Thereisnonuclearmembrane,whichisthedefinitivecharacteristicofeucaryoticcellssuchasthosethatmakeup,fungi,protista,plantsandanimals.Untilrecently,bacteriaweretheonlyknowntypeofprocaryoticcell,andthedisciplineofbiologyrelatedtotheirstudyiscalledbacteriology.Inthe1980'

s,withtheoutbreakofmoleculartechniquesappliedtophylogenyoflife,anothergroupofprocaryoteswasdefinedandinformallynamed"

archaebacteria"

.ThisgroupofprocaryoteshassincebeenrenamedArchaeaandhasbeenawardedbiologicalDomainstatusonthelevelwithBacteriaandEucarya.Thecurrentscienceofbacteriologyincludesthestudyofbothdomainsofprocaryoticcells,butthename"

bacteriology"

isnotlikelytochangetoreflecttheinclusionofarchaeainthediscipline.Actually,manyarchaeahavebeenstudiedasintensivelyandaslongastheirbacterialcounterparts,exceptwiththenotionthattheywerebacteria.

Figure1.ThecyanobacteriumAnabaena.AmericanSocietyforMicrobiology.Two（notuncommon）exceptionsthatprocaryotesareunicellularandundifferentiatedareseeninAnabaena:

1.Theorganismlivesasamulticellularfilamentorchainofcells.Procaryotesareconsidered"

unicellularorganisms"

becauseallthecellsinafilamentorcolonyareofthesametype,andanyoneindividualcellcangiverisetoanexactfilamentorcolony;

2.Thepredominantphotosynthetic（brightyellow-green）cellsdodifferentiateintoanothertypeofcell:

theobviouslylarge"

empty"

cellsoccasionallyseenalongafilamentaredifferentiatedcellsinwhichnitrogenfixation,butnotphotosynthesis,takesplace.

TheOriginofLife

WhenlifearoseonEarthabout4billionyearsago,thefirsttypesofcellstoevolvewereprocaryoticcells.Forapproximately2billionyears,procaryotic-typecellsweretheonlyformoflifeonEarth.Theoldestknownsedimentaryrocks,fromGreenland,areabout3.8billionyearsold.Theoldestknownfossilsareprocaryoticcells,3.5billionyearsinage,foundinWesternAustraliaandSouthAfrica.Thenatureofthesefossils,andthechemicalcompositionoftherocksinwhichtheyarefound,indicatesthatlithotrophicandfermentativemodesofmetabolismwerethefirsttoevolveinearlyprocaryotes.Photosynthesisdevelopedinbacteriaabitlater,atleast3billionyearsago.Anoxygenicphotosynthesis（bacterialphotosynthesis,whichisanaerobicanddoesnotproduceO2）precededoxygenicphotosynthesis（plant-typephotosynthesis,whichyieldsO2）.However,oxygenicphotosynthesisalsoaroseinprocaryotes,specificallyinthecyanobacteria,whichexistedmillionsofyearsbeforetheevolutionofgreenalgaeandplants.Larger,morecomplicatedeucaryoticcellsdidnotappearuntilmuchlater,between1.5and2billionyearsago.

Figure2.OpalescentPoolinYellowstoneNationalPark,WyomingUSA.K.Todar.ConditionsforlifeinthisenvironmentaresimilartoEarthover2billionyearsago.Inthesetypesofhotsprings,theorange,yellowandbrowncolorsareduetopigmentedphotosyntheticbacteriawhichmakeupthemicrobialmats.Thematsareliterallyteemingwithbacteria.SomeofthesebacteriasuchasSynechococcusconductoxygenicphotosynthesis,whileotherssuchasChloroflexusconductanoxygenicphotosynthesis.Othernon-photosyntheticbacteria,aswellasthermophilicandacidophilicArchaea,arealsoresidentsofthehotspringcommunity.

Thearchaeaandbacteriadifferfundamentallyintheirstructurefromeucaryoticcells,whichalwayscontainamembrane-enclosednucleus,multiplechromosomes,andvariousothermembranousorganellessuchasmitochondria,chloroplasts,thegolgiapparatus,vacuoles,etc.Unlikeplantsandanimals,archaeaandbacteriaareunicellularorganismsthatdonotdevelopordifferentiateintomulticellularforms.Somebacteriagrowinfilamentsormassesofcells,buteachcellinthecolonyisidenticalandcapableofindependentexistence.Thecellsmaybeadjacenttooneanotherbecausetheydidnotseparateaftercelldivisionorbecausetheyremainedenclosedinacommonsheathorslimesecretedbythecells,buttypicallythereisnocontinuityorcommunicationbetweenthecells.

TheUniversalTreeofLife

OnthebasisofsmallsubunitribosomalRNA（ssrRNA）analysis,thecontemporaryTreeofLifegivesrisetothreecellular"

Domains"

:

Archaea,Bacteria,andEucarya（Figure3）.Bacteria（formerlyknownaseubacteria）andArchaea（formerlycalledarchaebacteria）sharetheprocaryotictypeofcellularconfiguration,butotherwisearenotrelatedtooneanotheranymorecloselythantheyaretotheeucaryoticdomain,Eucarya.Betweenthetwoprocaryotes,ArchaeaareapparentlymorecloselyrelatedtoEucaryathanaretheBacteria.Eucaryaconsistsofalleucaryoticcell-types,includingprotista,fungi,plantsandanimals.

Figure3.TheUniversalTreeofLifeasderivedfromsequencingofssrRNA.N.Pace.Notethethreemajordomainsoflivingorganisms:

Archaea,BacteriaandEucarya.The"

evolutionarydistance"

betweentwoorganismsisproportionaltothemeasurabledistancebetweentheendofabranchtoanodetotheendofacomparativebranch. 

Forexample,inEucarya,humans（Homo）aremorecloselyrelatedtocorn（Zea）thantoslimemolds（Dictyostelium）;

orinBacteria,E.coliismorecloselyrelatedtoAgrobacteriumthantoThermus.

SizeandDistributionofBacteriaandArchaea

Mostprocaryoticcellsareverysmallcomparedtoeucaryoticcells.Atypicalbacterialcellisabout1micrometerindiameterorwidth,whilemosteucaryoticcellsarefrom10to100micrometersindiameter.Eucaryoticcellshaveamuchgreatervolumeofcytoplasmandamuchlowersurface:

volumeratiothanprocaryoticcells.Atypicalprocaryoticcellisaboutthesizeofaeucaryoticmitochondrion.Sinceprocaryotesaretoosmalltobeseenexceptwiththeaidofamicroscope,itisusuallynotappreciatedthattheyarethemostabundantformoflifeontheplanet,bothintermsofbiomassandtotalnumbersofspecies.Forexample,inthesea,procaryotesmakeup90percentofthetotalcombinedweightofallorganisms.Inasinglegramoffertileagriculturalsoiltheremaybeinexcessof109bacterialcells,outnumberingalleucaryoticcellsthereby10,000:

1.About3,000distinctspeciesofbacteriaandarchaeaarerecognized,butthisnumberisprobablylessthanonepercentofallthespeciesinnature.Theseunknownprocaryotes,farinexcessofundiscoveredorunstudiedplants,areatremendousreserveofgeneticmaterialandgeneticinformationinnaturethatawaitsexploitation.

Procaryotesarefoundinallofthehabitatswhereeucaryoteslive,but,aswell,inmanynaturalenvironmentsconsideredtooextremeorinhospitableforeucaryoticcells.Thus,theouterlimitsoflifeonEarth（hottest,coldest,driest,etc.）areusuallydefinedbytheexistenceofprocaryotes.Whereeucaryotesandprocaryoteslivetogether,theremaybemutualisticassociationsbetweentheorganismsthatallowbothtosurviveorflourish.Theorganellesofeucaryotes（mitochondriaandchloroplasts）arethoughttoberemnantsofBacteriathatinvaded,orwerecapturedby,primitiveeucaryotesintheevolutionarypast.Numeroustypesofeucaryoticcellsthatexisttodayareinhabitedbyendosymbioticprocaryotes.

Fromametabolicstandpoint,theprocaryotesareextraordinarilydiverse,andtheyexhibitseveraltypesofmetabolismthatarerarelyorneverseenineucaryotes.Forexample,thebiologicalprocessesofnitrogenfixation（conversionofatmosphericnitrogengastoammonia）andmethanogenesis（productionofmethane）aremetabolically-uniquetoprocaryotesandhaveanenormousimpactonthenitrogenandcarboncyclesinnature.UniquemechanismsforenergyproductionandphotosynthesisarealsoseenamongtheArchaeaandBacteria.

Thelivesofplantsandanimalsaredependentupontheactivitiesofbacterialcells.Bacteriaandarchaeaenterintovarioustypesofsymbioticrelationshipswithplantsandanimalsthatusuallybenefitbothorganisms,althoughafewbacteriaareagentsofdisease.

Themetabolicactivitiesofprocaryotesinsoilhabitatshaveanenormousimpactonsoilfertilitythatcanaffectagriculturalpracticesandcropyields.Intheglobalenvironment,procaryotesareabsolutelyessentialtodrivethecyclesofelementsthatmakeuplivingsystems,i.e.,thecarbon,oxygen,nitrogenandsulfurcycles.Theoriginsoftheplantcellchloroplastandplant-type（oxygenic）photosynthesisarefoundinprocaryotes.Mostoftheearth'

satmosphericoxygenmayhavebeenproducedbyfree-livingbacterialcells.ThebacteriafixnitrogenandasubstantialamountofCO2,aswell.

Bacteriaorbacterialproducts（includingtheirgenes）canbeusedtoincreasecropyieldorplantresistancetodisease,ortocureorpreventplantdisease.Bacterialproductsincludeantibioticstofightinfectiousdisease,aswellascomponentsforvaccinesusedtopreventinfectiousdisease.Becauseoftheirsimplicityandourrelativeunderstandingoftheirbiologicalprocesses,thebacteriaprovideconvenientlaboratorymodelsforstudyofthemolecularbiology,genetics,andphysiologyofalltypesofcells,includingplantandanimalcells.

STRUCTUREANDFUNCTIONOFPROCARYOTICCELLS

Procaryoticcellshavethreearchitecturalregions（Figure4）:

appendages（proteinsattachedtothecellsurface）intheformofflagellaandpili;

acellenvelopeconsistingofacapsule,cellwallandplasma