donnanequilibriainmicrobialcellwallsapHhomeostatic

ColloidsandSurfacesB:Biointerfaces24(2002)247–251Donnanequilibriainmicrobialcellwalls:apH-homeostaticmechanisminalkaliphilesKaoruTsujii*TheDEEPSTARGroup,JapanMarineScienceandTechnologyCenter,2-15Natsushima-cho,Yokosuka,Kanagawa237-0061,JapanReceived21June2001;accepted23July2001AbstractApH-homeostaticmechanismforalkaliphilicbacteriahasbeenproposedtakingtheimportantroleofanionicpolymerlayerintheircellwallsintoaccount.DonnanequilibriainthebacterialcellwallswerecalculatedtoestimatethepHvaluesinsidethepolymerlayerofthecellwallswhentheouteraqueoussolutionisinalkalinenature.Thefixedchargeconcentrationinthepolymerlayerwasestimatedtobe2–5mol/lfromthedatareportedforGram-positivebacteria,particularlyforanalkaliphilicbacteriumBacillushaloduransC-125.Thesaltconcentrationsofnearisotonicconditionsweretakenastheouteraqueoussolutions.ThepHvaluescalculatedfortheinsideofthepolymerlayer(thecellwall)weresmallerthanthoseoftheoutersolutionby1–1.5pHunits.Thisresultagreeswellwiththeexpectedvaluesestimatedfromthestabilityofcellmembranes.©2002ElsevierScienceB.V.Allrightsreserved.Keywords:Donnanequilibrium;Cellwall;Alkaliphile;pH-homeostasis;Gram-positivebacteria[1,2].ItisassumedfromthestabilityofcellmembranesthatthepHvalueisreducedbyaboutonepHunitthroughthecellwall.Anionicpolymerlayerinthecellwallshallbeparticularlyimportant,sincetheamountoftheanionicpolymerincreaseswhenthebacteriumisgrownatalkalinepH[3].ButanyquantitativeanalysisforthecontributionoftheanionicpolymerstothepH-homeostasishasnotyetbeenmadesofar.ThispaperdealswiththequantitativeestimationofpHreductionthroughthecellwallofalkaliphilicbacteria.Donnanequi-libriumtheoryofelectrolytesbetweentheanionicpolymerlayerandthebulkaqueousphasewasappliedtothecellwallsystems,andthepHreductionwassuccessfullyevaluated.*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](K.Tsujii).0927-7765/02/$-seefrontmatter©2002ElsevierScienceB.V.Allrightsreserved.PII:S0927-7765(01)00244-2K.Tsujii/ColloidsandSurfacesB:Biointerfaces24(2002)247–2512482.Themodel2.1.StructuremodelingofmicrobialcellwallsTherearetwokindsofcellwallsinmicroorgan-isms.ThecellwallsofGram-positivebacteriaconsistmainlyofpeptidoglycanlayerandanionicpolymers(teichoicacid,teichuronicacidandte-ichuronopeptides)ofabrush-likestructure.InGram-negativebacteria,ontheotherhand,theanisotropicoutermembraneconsistingofphos-pholipids(innerleaflet),proteinsandlipo-polysaccharides(outerleaflet)isthesubstitutefortheanionicpolymerlayer.Fig.1showsasche-maticillustrationofthecellwallofGram-positivebacteria.Fromthephysico-chemicalpointofview,thecellwallstructureofGram-positivebacteriacanberegardedasahydrogelbearinganionicbrush-likepolymermoleculesonitssurface,sinceacross-linkedpolymerofpeptidoglycanisakindofhydrogel.Intheanionicpolymerlayer,anumberofanionsarefixedonthepolymerchainsofteichoicacidorteichuronicacidorte-ichuronopeptide,andthesamenumberofcationsispresentintheaqueousphaseofthelayer.Upontheaboveconditionsofthemicrobialcellwalls,Donnanequilibriumofelectrolytesshouldbeat-tainedbetweentheionicpolymerlayerandthebulkaqueousphase[4,5].2.2.DonnanequilibriainthecellwallsTheDonnanequilibriumtheoryprovidesusaquantitativedescriptionofthesaltdistributionbetweenthepolymerlayerofmicrobialcellwallsandthebulkaqueousphase.Letthesodiumconcentration(=theconcentrationoffixedan-ion)intheregionofanionicpolymerlayerbeC0,andtheinitialconcentrationofaddedNaClintheaqueousphasebeC1.SomepartoftheNaClinthebulkphasemoveintothepolymerlayer,andtheequilibrationisattained.TheconcentrationchangeduetothisNaClmovementiswrittenasx.Finalsodiumandchlorideconcentrationsin-sidethepolymerlayerareC0+xandx,respec-tively,andthebothconcentrationsofNa+andCl−inthebulkphaseareC1−x,ifthevolumesofbothphasesaresame.Theequilibriumcondi-tionofsodiumchlorideinbothphasescanbewrittenas:(C0+x)x=(C1−x)2(1)Theequationiseasilysolvedas:Fig.1.StructuralmodelforthecellwallofGram-positivebacteria.Manyanionicpolymerchainsaresituatedinabrush-likeshapeonthesurfaceofapolymerhydrogel(peptidoglycan).Addedsaltinthebulkaqueousphaseenterspartlyintotheanionicpolymerlayer,andequilibrationisattained.Theequilibratedsaltconcentrationinsidethepolymerlayerismuchsmallerthanthatinbulkaqueousphasebecauseoftheveryhighconcentrationofcations(sodiumions)inthepolymerlayer.K.Tsujii/ColloidsandSurfacesB:Biointerfaces24(2002)247–251249x=C12C0+2C1(2)FinallywecancalculatetheNaClconcentra-tionratioofoutsidebulkphasetoinnerpolymerphaseasfollows:CoutCin=C1−xx=1+C0C1(3)OnecanunderstandthattheNaClconcentra-tioninbulkaqueousphaseismuchhigherthanthatinthepolymerlayerofthecellwall,whenthenumberoffixedionsonthepolymerchainsislargeenough.Eq.(3)wasderivedundertheassumptionofsamevolumeinbothbul