Bacteria.docx
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Bacteria.docx
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Bacteria
Bacteria
The bacteria (
[bækˈtɪərɪə] (help·info); singular:
bacterium)[α] arealargegroupofunicellular, prokaryote, microorganisms. Typicallyafew micrometres inlength,bacteriahaveawiderangeofshapes,rangingfrom spheres torodsandspirals. Bacteriaareubiquitousinevery habitat on Earth,growinginsoil, acidichotsprings, radioactivewaste,[2] water,anddeepinthe Earth'scrust,aswellasinorganicmatterandthelivebodiesofplantsandanimals. Therearetypically40millionbacterial cells inagramofsoilandamillionbacterialcellsinamillilitreof freshwater;inall,thereareapproximatelyfive nonillion (5×1030)bacteriaonEarth,[3] formingmuchoftheworld's biomass.[3] Bacteriaarevitalinrecyclingnutrients,withmanystepsin nutrientcycles dependingontheseorganisms,suchasthe fixationofnitrogen fromthe atmosphere andputrefaction. However,mostbacteriahavenotbeencharacterized,andonlyabouthalfofthe phyla ofbacteriahavespeciesthatcanbe grown inthelaboratory. [4] Thestudyofbacteriaisknownas bacteriology,abranchof microbiology.
Thereareapproximatelytentimesasmanybacterialcellsinthe humanflora ofbacteriaastherearehumancellsinthebody,withlargenumbersofbacteriaonthe skin andas gutflora. [5] Thevastmajorityofthebacteriainthebodyarerenderedharmlessbytheprotectiveeffectsofthe immunesystem,andafeware beneficial. However,afewspeciesofbacteriaare pathogenic andcause infectiousdiseases,including cholera, syphilis, anthrax, leprosy and bubonicplague. Themostcommonfatalbacterialdiseasesarerespiratoryinfections,with tuberculosis alonekillingabout2millionpeopleayear,mostlyin sub-SaharanAfrica. [6] In developedcountries, antibiotics areusedtotreatbacterialinfections andinagriculture,so antibioticresistance isbecomingcommon. Inindustry,bacteriaareimportantin sewagetreatment,theproductionof cheese andyoghurt through fermentation,aswellasin biotechnology,andthemanufactureofantibioticsandotherchemicals. [7]
OnceregardedasplantsconstitutingtheclassSchizomycetes,bacteriaarenowclassifiedas prokaryotes. Unlikecellsofanimalsandother eukaryotes,bacterialcellsdonotcontaina nucleus andrarelyharbour membrane-bound organelles. Althoughtheterm bacteria traditionallyincludedallprokaryotes,the scientificclassification changedafterthediscoveryinthe1990sthatprokaryotesconsistoftwoverydifferentgroupsoforganismsthat evolved independentlyfromanancientcommonancestor. Theseevolutionarydomains arecalledBacteriaand Archaea. [8]
Contents
[hide]
∙1 Historyofbacteriology
∙2 Originandearlyevolution
∙3 Morphology
∙4 Cellularstructure
o4.1 Intracellularstructures
o4.2 Extracellularstructures
o4.3 Endospores
∙5 Metabolism
∙6 Growthandreproduction
∙7 Genetics
o7.1 Bacteriophages
∙8 Behavior
o8.1 Secretion
o8.2 Bioluminescence
o8.3 Multicellularity
o8.4 Movement
∙9 Classificationandidentification
∙10 Interactionswithotherorganisms
o10.1 Predators
o10.2 Mutualists
o10.3 Pathogens
∙11 Significanceintechnologyandindustry
∙12 Seealso
∙13 Notes
∙14 References
∙15 Furtherreading
∙16 Externallinks
Historyofbacteriology
Furtherinformation:
Microbiology
AntonievanLeeuwenhoek,thefirstmicrobiologist andthefirstpersontoobservebacteriausinga microscope.
Bacteriawerefirstobservedby AntonievanLeeuwenhoek in1676,usingasingle-lens microscope ofhisowndesign. [9] Hecalledthem"animalcules"andpublishedhisobservationsinaseriesofletterstothe RoyalSociety. [10][11][12] Thename bacterium wasintroducedmuchlater,by ChristianGottfriedEhrenberg in1838.[13]
LouisPasteur demonstratedin1859thatthe fermentation processiscausedbythegrowthofmicroorganisms,andthatthisgrowthisnotdueto spontaneousgeneration.(Yeasts and molds,commonlyassociatedwithfermentation,arenotbacteria,butrather fungi. ) Alongwithhiscontemporary, RobertKoch,Pasteurwasanearlyadvocateofthe germtheoryofdisease. [14] RobertKochwasapioneerinmedicalmicrobiologyandworkedon cholera, anthrax and tuberculosis. Inhisresearchintotuberculosis,Kochfinallyprovedthegermtheory,forwhichhewasawardeda NobelPrize in1905.[15] In Koch'spostulates,hesetoutcriteriatotestifanorganismisthecauseofa disease;thesepostulatesarestillusedtoday. [16]
Thoughitwasknowninthenineteenthcenturythatbacteriaarethecauseofmanydiseases,noeffective antibacterial treatmentswereavailable. [17] In1910, PaulEhrlichdevelopedthefirstantibiotic,bychangingdyesthatselectivelystained Treponemapallidum—the spirochaete thatcauses syphilis—intocompoundsthatselectivelykilledthepathogen. [18] Ehrlichhadbeenawardeda1908NobelPrizeforhisworkon immunology,andpioneeredtheuseofstainstodetectandidentifybacteria,withhisworkbeingthebasisofthe Gramstain andthe Ziehl-Neelsenstain. [19]
Amajorstepforwardinthestudyofbacteriawastherecognitionin1977by CarlWoese that archaea haveaseparatelineofevolutionarydescentfrombacteria. [20] Thisnew phylogenetic taxonomy wasbasedonthe sequencing of 16SribosomalRNA,anddividedprokaryotesintotwoevolutionarydomains,aspartofthe three-domainsystem.[21]
Originandearlyevolution
Furtherinformation:
Timelineofevolution
Theancestorsofmodernbacteriaweresingle-celledmicroorganismsthatwerethe firstformsoflife todeveloponearth,about4billionyearsago. Forabout3billionyears,allorganismsweremicroscopic,andbacteriaandarchaeawerethedominantformsoflife. [22][23] Althoughbacterial fossils exist,suchas stromatolites,theirlackofdistinctivemorphology preventsthemfrombeingusedtoexaminethehistoryofbacterialevolution,ortodatethetimeoforiginofaparticularbacterialspecies. However,genesequencescanbeusedtoreconstructthebacterial phylogeny,andthesestudiesindicatethatbacteriadivergedfirstfromthearchaeal/eukaryoticlineage. [24] The mostrecentcommonancestor ofbacteriaandarchaeawasprobablya hyperthermophile thatlivedabout2.5billion–3.2billionyearsago. [25][26]
Bacteriawerealsoinvolvedinthesecondgreatevolutionarydivergence,thatofthearchaeaandeukaryotes. Here,eukaryotesresultedfromancientbacteriaenteringintoendosymbiotic associationswiththeancestorsofeukaryoticcells,whichwerethemselvespossiblyrelatedtothe Archaea. [27][28] Thisinvolvedtheengulfmentbyproto-eukaryoticcellsofalpha-proteobacterialsymbiontstoformeither mitochondria or hydrogenosomes,whicharestillfoundinallknownEukarya(sometimesinhighly reducedform,e.g. inancient"amitochondrial"protozoa). Lateron,someeukaryotesthatalreadycontainedmitochondriaalsoengulfedcyanobacterial-likeorganisms. Thisledtotheformationof chloroplasts inalgaeandplants. Therearealsosomealgaethatoriginatedfromevenlaterendosymbioticevents. Here,eukaryotesengulfedaeukaryoticalgaethatdevelopedintoa"second-generation"plastid. [29][30] Thisisknownas secondaryendosymbiosis.
Morphology
Furtherinformation:
Bacterialcellularmorphologies
Bacteriadisplaymanycell morphologies andarrangements
Bacteriadisplayawidediversityofshapesandsizes,called morphologies. Bacterialcellsareaboutonetenththesizeofeukaryoticcellsandaretypically0.5–5.0 micrometresinlength. However,afewspecies–forexample Thiomargaritanamibiensis and Epulopisciumfishelsoni–areuptohalfamillimetrelongandarevisibletotheunaidedeye. [31]Amongthesmallestbacteriaaremembersofthegenus Mycoplasma,whichmeasureonly0.3 micrometres,assmallasthelargest viruses. [32] Somebacteriamaybeevensmaller,butthese ultramicrobacteria arenotwell-studied. [33]
Mostbacterialspeciesareeitherspherical,called cocci (sing. coccus,from Greek kókkos,grain,seed)orrod-shaped,called bacilli (sing. bacillus,from Latin baculus,stick).Elongationisassociatedwithswimming.[34] Somerod-shapedbacteria,called vibrio,areslightlycurvedorcomma-shaped;others,canbespiral-shaped,called spirilla,ortightlycoiled,called spirochaetes. Asmallnumberofspeciesevenhavetetrahedralorcuboidalshapes. [35] Morerecently,bacteriawerediscovereddeepundertheEarth'scrustthatgrowaslongrodswithastar-shapedcross-section. Thelargesurfaceareatovolumeratioofthismorphologymaygivethesebacteriaanadvantageinnutrient-poorenvironments. [36] Thiswidevarietyofshapesisdeterminedbythebacterial cellwall and cytoskeleton,andisimportantbecauseitcaninfluencetheabilityofbacteriatoacquirenutrients,attachtosurfaces,swimthroughliquidsandescape predators. [37][38]
Manybacterialspeciesexistsimplyassinglecells,othersassociateincharacteristicpatterns:
Neisseria formdiploids(pairs), Streptococcus formchains,and Staphylococcusgrouptogetherin"bunchofgrapes"clusters. Bacteriacanalsobeelongatedtoformfilaments,forexamplethe Actinobacteria. Filamentousbacteriaareoftensurroundedbyasheaththatcontainsmanyindividualcells. Certaintypes,suchasspeciesofthegenus Nocardia,evenformcomplex,branchedfilaments,similarinappearancetofungalmycelia. [39]
Therangeofsizesshownby prokaryotes,relativetothoseofotherorganismsandbiomolecules
Bacteriaoftenattachtosurfacesandformdenseaggregationscalled biofilms or bacterialmats. Thesefilmscanrangefromafewmicrometersinthicknesstouptohalfameterindepth,andmaycontainmultiplespeciesofbacteria, protists and archaea. Bacterialivinginbiofilmsdisplayacomplexarrangementofcellsandextracellu
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