Similarities and Differences between Silver Ions and Silver in Nanoforms as Antibacterial Agents

17 Pages·2017·3.64 MB·English

International Journal o f
Molecular Sciences
Review
Similarities and Differences between Silver Ions and
Silver in Nanoforms as Antibacterial Agents
AnnaKe˛dziora1,*,MateuszSperuda1,EvaKrzyz˙ewska2 ID,JacekRybka2,AnnaŁukowiak3
andGabrielaBugla-Płoskon´ska1,*
1 DepartmentofMicrobiology,InstituteofGeneticsandMicrobiology,UniversityofWrocław,51-148Wrocław,
Poland;[email protected]
2 DepartmentofImmunologyofInfectiousDiseases,HirszfeldInstituteofImmunologyandExperimental
Therapy,PolishAcademyofSciences,53-114Wrocław,Poland;[email protected](E.K.);
[email protected](J.R.)
3 InstituteofLowTemperatureandStructureResearch,PolishAcademyofSciences,50-422Wrocław,Poland;
[email protected]
* Correspondence:[email protected](A.K.);[email protected](G.B.-P.);
Tel.:[phone](A.K.);[phone](G.B.-P.)
Received:2December2017;Accepted:16January2018;Published:2February2018
Abstract: Silverisconsideredasantibacterialagentwithwell-knownmodeofactionandbacterial
resistance against it is well described. The development of nanotechnology provided different
methodsforthemodificationofthechemicalandphysicalstructureofsilver,whichmayincreaseits
antibacterialpotential. Thephysico-chemicalpropertiesofsilvernanoparticlesandtheirinteraction
withlivingcellsdifferssubstantiallyfromthoseofsilverions. Moreover,thevarietyoftheformsand
characteristicsofvarioussilvernanoparticlesarealsoresponsiblefordifferencesintheirantibacterial
modeofactionandprobablybacterialmechanismofresistance. Thepaperdiscussesindetailsthe
aforementionedaspectsofsilveractivity.
Keywords: silverions;silvernanoparticles;silvernanocompounds;nanotechnology;modeofaction;
resistanceofbacteria
1. Introduction
Silverionsasantibacterialagentshavebeenknownforages. Adetailedhistoryofsilverusage
waswelldocumentedintheliterature[1,2]. Silverionsfromdissolvedsilvernitrate(lapis,AgNO )
3
and silver sulfadiazine are agents with proved efficacy against Gram-positive and Gram-negative
bacteria[3–6].
The dynamic development of nanotechnology in recent years has provided possibilities for
fabricating various forms of silver nanoparticles (AgNPs) [7]. Their most important feature is the
highly developed surface area of small-size particles, which allows to increase the antimicrobial
efficacyandbioavailabilityofmaterialsusedinthebiologyandbiomedicalsector[8]. Wecanobserve
a “nanotechnology race” of nanoproducts applications [7] in the biomedical sector too. In 2017
Shengetal.[9]reviewedthatover1000articlesconcernedtheeffectsofnanoparticlesonbacteriaand
over90%ofthemwerepublishedafter2008. Atpresent,moreandmorepublicationsconcerningthe
synthesisandantibacterialactivityofsilvernanoformsarebeingpublished[10–32].
Therefore,themaingoalofthismanuscriptistoemphasizethevarietyofsilvernanomaterials,
theirdiversityinphysico-chemicalpropertiesandfinally,thehighpossibilityfordifferentinteractions
withcells—especiallywithbacterialpathogens. Wecomparedthemodeofactionofsilverionsand
silvernanocompositesagainstbacteria(Gram-positiveandGram-negativebacterialcells),analysed
thelimitationofusageofsilvermaterialsconsideringthebacterialresistancetothem,determinedthe
Int.J.Mol.Sci. 2018,19,444;doi:10.3390/ijms19020444 www.mdpi.com/journal/ijms
Int.J.Mol.Sci. 2018,19,444 2of17
gapofknowledgeandshowedfutureperspectives. Increasingamountofsilverintheenvironment,
havingitssourceinhumanactivities,especiallynanotechnologyproducts,causeair,waterandearth
pollution. Wewouldliketoemphasizethesubstantialdifferencesbetweenvariousformsofsilver
nanoparticleswhichcannotbeconsideredasidenticalmaterials. Asinthecaseofotherbiocidesthe
borderbetweenbenefitsandtoxiceffectisverynarrow[33].
2. PropertiesofSilverMaterialsvs. AntibacterialActivity
Silverionsarerelativelyreactive. Thebindingofsilverionsintheformofinsolubleprecipitates
(AgCl),orduringtheinteractionswithproteins(e.g.,albumin),causesasignificantdecreaseofits
antibacterialefficacy,whichisveryimportantinthecaseofAg+bioapplications.
AgNPscanexistintheformofmetallicsilverwithatomsstronglyconnectedwitheachother[34].
ThetypicalAgNPsareusuallyofafewtoadozennmindiameter[35]andcantakedifferentshapes
(spherical,irregularorplanar)[9,20,24–28,36–38]thatdependonthemethodsofproduction. Oneof
themostpopular“bottomup”eco-friendlymethodisbiologicalsynthesis,usinglivingcellsofbacteria,
fungiandplantstoobtainsilvernanoparticles[9,15–17,19,24,27,37,38].
ItisworthtounderlinethatAgNPsneverperformaloneduetotheirtendencytoaggregateas
aresultoftheinteractionsbetweensilveratoms. Topreventaggregationorganic(e.g.,citricacid)or
inorganiccarriers(stabilizers,suchassilica,grapheneortitaniumdioxide)areused. Fromthepoint
ofviewofnanotechnology,theunaffectedphysico-chemicalpropertiesofAgNPs(especiallylarge
surfaceareatovolumeratio)arecrucialtomaintaintheirantibacterialefficacy. Varietyofsynthesis
methods(includingchemicalsandreactionparameters)determinethevarietyofphysico-chemical
propertiesofnanomaterialsandtheirbiologicalactivity. InTable1theinfluenceofimportantchemical
andphysicalpropertiesonthatactivityisshown.
Int.J.Mol.Sci. 2018,19,444 3of17
Table1.Overviewofcertainnanocompositesofsilver:theirphysico-chemicaldescriptionandbiologicalactivity.
Nanocomposite
SilverNanoparticles SilverNanoparticles SilverAmountin FormofCompound
(NamedAccording TypeofSynthesis AntibacterialActivity References
Size Shape Nanocomposites (IfApplicable)
totheReference)
Antibacterialeffectwasdose-dependent.Testedsilver
nanoparticlesweremoreeffectiveagainstGram-negative
Silvernanoparticles 10–15nm spherical,polyhedral n/a n/a chemical bacteriathanGram-positive; [18]
MoA:bindingtothecellwallandpenetratingit;modulation
ofcellularsignalling
Increasedantibacterialactivityofantibioticsinthepresence
variable:most
AgNPs 5–30nm n/a n/a biological ofAgNPs; [19]
spherical
MoA:bindingtoproteins(bythiolgroups)andDNA
39nm(spherical),40nm InhibitionofEscherichiacoligrowthonmediumwithsilver
variable:most
(triangular),133–192nm, nanoparticles;
Silvernanoparticles spherical,triangular, n/a n/a chemical [20]
diameter:16nm MoA:damageofbacterialcellmembraneonmultiple
rod-shaped
(rod-shaped) locations,formationofirregularpits
InhibitionofE.coligrowthat6µMconcentrationofNano-Ag;
MoA:changesinexpressionofgenesencodingenvelope
Nano-Ag 9.3±2.8nm spherical n/a n/a chemical proteins(accumulationofenvelopeproteinprecursors), [21]
destabilizationofoutermembrane,disturbanceofproton
motiveforce
InhibitionofE.coligrowthat50–60µg/mLconcentrationof
silvernanoparticles;
Silvernanoparticles 12nm spherical n/a n/a chemical MoA:damageofmembranes,incorporationofsilver [22]
nanoparticlesintomembranes,formingpits,disturbancesin
permeability
InhibitionofGram-negativeandGram-positivebacteria
growthat75µg/mLconcentrationofsilvernanoparticles;
icosahedral,twinned, agglomeratedinside MoA:bindingtocellmembrane,permeabilitychanges,
Silvernanoparticles 16±8nm,21±18nm n/a chemical [23]
decahedral thecarbonmatrix disturbancesinrespirationprocess,penetrationofthe
bacterialmembranes,interactingwithDNA,releasing
silverions
nanotexture,rutile, InhibitionofS.aureus(MRSA)andE.coligrowthat15–75µM;
Ti/Ag notspecified notspecified 1.93–6.08%[m/m] biological [24]
anatase MoA:notspecified
0.5gnanosilver/diatomitekillsabove99%ofE.coliwithin
Nanosilver/diatomite 1–20nm sphericalparticles 0.537%[m/m] notspecified chemical 30min; [25]
MoA:notspecified
InhibitionofE.coli,Klebsiellapneumoniae,Bacilluspumilusand
chitosan/alginate
Chitosan-AgNps 8–28nm spherical 1%[m/m] chemical Staphylococcusaureusgrowth; [26]
nanofibers
MoA:notspecified
Int.J.Mol.Sci. 2018,19,444 4of17
Table1.Cont.
Nanocomposite
SilverNanoparticles SilverNanoparticles SilverAmountin FormofCompound
(NamedAccording TypeofSynthesis AntibacterialActivity References
Size Shape Nanocomposites (IfApplicable)
totheReference)
Inhibitionofmultidrug(MDR)pathogens:Acinetobacter
15–160nm(mean sphericaland baumannii,E.coli,PseudomonasaeruginosaandSalmonella
AgNps n/a notspecified biological [27]
diameter60±10nm) irregular entericagrowthat25–50µgconcentration;
MoA:notspecified
graphenesheets
31.5–42nm(mean InhibitionofE.coligrowth
GO-L-cys-AgNps diameter35.34±0.2nm) spherical notspecified functionalizedwith chemical MoA:damagesofthecellmembrane [28]
L-cysteine
InhibitionoftheGram-positiveandGram-negativebacteria
AgNPs 6–26nm,4.24–23.22nm spherical n/a foam biological growthat676.9mg/Lconcentration; [29]
MoA:notspecified
100nm,30nmdiameter InhibitionofE.coliandS.aureusgrowthat0.05mg/L;
Ag-NPs Spherical,rod-like n/a oilmicroemulsion chemical [30]
200–300length MoA:notspecified
InhibitionofE.coli,S.aureus,P.aeruginosagrowthat
AgNPs 15nm spherical n/a n/a biological 50µg/mL; [31]
MoA:notspecified
Increasedantibacterialactivityofampicillin,erythromycin
variable:sphericalor andchloramphenicolinthepresenceofAgNPs(E.coli,
AgNPs 5–40nm n/a n/a biological [32]
rod-like SalmonellaTyphi,S.aureus,Micrococcusluteus);
MoA:notspecified
n/a—notapplicable;MoA—Modeofantibacterialactionofsilver;AgNPs—silvernanoparticles;GO—grapheneoxide;MDR—multi-drugresistant;MRSA—Methicillin-resistantS.aureus.
Int.J.Mol.Sci. 2018,19,444 5of17
3. ModeofAntibacterialActionofSilver(MoA)
Lietal.[39]provedthatsilverionshavesimilarmodeofactiontosilvernanoparticlesbutstronger
antibacterialactivitythanAgNPs.
The antibacterial activity of silver ions (Ag+) is directly proportional to the environmental
concentrationofsilverions. Duetotheoligodynamiceffect,silvershowshighantibacterialefficacy
eveninlowconcentrations. Jungetal.[40]comparedtheantibacterialactivityofsilverionsobtained
invariouswaysandshowedthatsilverionsproducedinanelectrolyticwayarebetterantibacterial
agentsthanthoseobtainedbydissolvingthesilvercompounds.
Theantibacterialmodeofactionofsilverionsisconnectedwith: (i)interactionwiththebacterial
cellenvelope(destabilizationofthemembrane—lossofK+ionsanddecreaseofATPlevel,bondedwith
phospholipids),(ii)interactionwithmoleculesinsidethecell(e.g.,nucleicacidsandenzymes),(iii)the
productionofreactiveoxygenspecies(ROS)[41]. Theinteractionofsilverionswithbacterialinner
membraneisoneofthemostimportantmechanismsofAg+toxicity[42]. Jungetal.[40]provedthat
theaccumulationofAg+inthebacterialcellenvelopeisfollowedbytheseparationofthecytoplasmic
membrane (CM) from the cell wall in both Gram-positive and Gram-negative bacteria [40,43].
Sütterlinetal.[41]showedthataminimalbactericidalconcentration(MBC)ofAg+forGram-positive
bacteriawasmorethan32timeshigherthantheMBCvaluesfortheGram-negativebacterialcells.
According to reference [40], carboxyl groups (–COOH) in glutamic acid and phosphate groups in
teichoicacidaremostlyresponsibleforbindingofsilverions.Ontheotherhand,Randalletal.[44]
suggestedthatthedamagecausedbyAg+intheinnermembrane(IM)isoneofthemostimportant
mechanismsinstaphylococci. Ithasbeenprovedthatsilverionsenterbacteriacellswithin30min
of exposition and bind to cytoplasm components, proteins and nucleic acids [40,45]. As it can be
seen from the TEM (transmission electron microscopy) pictures, shown in Figure 1, both types of
bacterialcells(Gram-positiveandGram-negative)treatedwithAg+ werelysedandtheleakageof
cytoplasmcouldbeobservedinallcases.Jungetal.[40]suggestedthatsilverionsinducean”activebut
nonculturable”state(ABNC)inbacteriacells. StressinducedbyAg+ causedthatbacteriamaintained
themetabolismandphysiologybutstoppedthegrowth,thereforethenumberofviablecellsdecreased
intheperformedinvitrotests.
OneofthedifferencesbetweenthemodeofAg+actionagainstGram-positiveandGram-negative
bacteriaregardsthewayofsilveruptakeintothecell. SilverionsenterGram-negativecellsviamajor
outer membrane proteins (OMPs), especially OmpF (and its homolog OmpC) [21,43], which is a
39 kDa transmembrane protein with trimeric β-barrel structure. Each monomer of OmpF is built
by sixteen transmembrane, antiparallel β-strands assembled with each other via hydrogen bonds.
Those strands form a stable β-sheet which afterwards folds into a cylindrical tube with a channel
function. Besidesporinandiontransporteractivity,OmpFisinvolvedinthetransportofothersmall
molecules (e.g., drugs) across the bacterial outer membrane (OM) [46,47]. The importance of the
OmpF/OmpCroleinthemechanismofresistancetosilverhasbeendiscussedrepeatedlyinafew
publishedpapers[43,48–50]. Sometimestheresultsoftheconductedexperimentswerequitedifferent.
Radzigetal.[48]claimedthatE.colilackingOmpF(orOmpC)intheOMwas4–8timesmoreresistant
toAg+ orAgNPsthanE.coliwhichpossessedthoseproteins. Inanotherstudy, Randalletal.[43]
provedthatprolongedexposuretosilverionscausedmissensemutationsinthecusSandompRgene.
ThelatterresultedinthelossoffunctionofOmpRprotein(whichisatranscriptionfactorofOmpF
and OmpC) and, finally, in the lack of OmpF/C proteins in the OM. E. coli BW25113 without the
mentionedOMPsischaracterizedbyalowpermeabilityoftheOMandahighlevelofresistanceto
Ag+. Thosefeatureswereobservedonlyinthesituationwhenbothproteinswerenotpresentinthe
OM[43]. Yenetal.[49]standinoppositiontotheresultsshownabove. Intheirresearch,regardlessof
thepresenceorabsenceofOmpF/OmpCinthebacterialOM,theyobservednochangesinbacterial
sensitivitytosilverions. Lietal.[50]testedtheantibacterialactivityofsilver-coatedcarbonnanotubes
on Salmonella Typhimurium and observed reduced expression of the ompF gene after exposure to
thesenanoparticles.
Int.J.Mol.Sci. 2018,19,444 6of17
Int. J. Mol. Sci. 2018, 19, x 6 of 17

Figure 1. Internal morphology of S. aureus (A) and E. coli (B) observed via TEM (a,b) untreated
Figure1.InternalmorphologyofS.aureus(A)andE.coli(B)observedviaTEM(a,b)untreatedbacteria,
bacteria, (c,d) bacteria treated with Ag+ (0.2 ppm) during 2 h. Black and white arrows indicate
(c,d)bacteriatreatedwithAg+(0.2ppm)during2h.Blackandwhitearrowsindicatepeptidoglycan
peptidoglycan and cytoplasmic membrane, respectively (A) and outer membrane, peptidoglycan and
andcytoplasmicmembrane,respectively(A)andoutermembrane,peptidoglycanandcytoplasmic
cytoplasmic membrane (B). Arrowhead indicate separation of the cell membrane from the cell wall.
membrane(B).Arrowheadindicateseparationofthecellmembranefromthecellwall. Reprinted
Reprinted from [40] with American Society for Microbiology Publishing Group permission.
from[40]withAmericanSocietyforMicrobiologyPublishingGrouppermission.
Another molecular mechanism of antibacterial toxicity of silver ions is connected with their
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rpersoutletisni nbitohseydnetnhaestuisr a[t4i5o]n. Mofothreeorvibero,s oitm heasn abteivene sptrruovcteudr ethaantd siinlvheibr iitoionns oinftperroatceti nwbitiho snyuncthleeisci sa[c4id5]s.
Mfoormreionvge r,biotnhdass bweeitnhp proyvreimditdhiantes ilbvaesreiso.n Isni nttherea cctownsitehqunuenclceei,c aDcNidAsf ocromndinegnbseosn dasnwd itrhepplyicraimtioidni nies
binahseibs.itIend t[h5e2]c.o nsequence,DNAcondensesandreplicationisinhibited[52].
TThhee aannttiibbaacctteerriiaall mmooddee ooff aaccttiioonn ooff ssiillvveerr nnaannooppaarrttiicclleess rreemmaaiinnss ssttiillll uunncclleeaarr aanndd iiss tthhee ssuubbjjeecctt ooff
ddiissccuussssiioonn.. AAl olotto offs csiceinecnecere rpeoprotsrtssu sguggesgtesstths atthtahte tmhee cmhaenchisamniosmft ooxfi ctiotyxiociftAy goNf PAsgiNssPism iisla srimtoislailrv etor
isoilnvse,rd uioentso, tdhueel ifteo ctyhcel eloiffe siclyvcelre noafn osiplvaertri cnleasnaonpdartthicelierst raannds fothrmeira ttioranntsofosrimlvaetrioionn sto[ 2s2i,l2v3e,5r 3i,o5n4]s.
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wfoallyo:w(iin)gw withayt: h(ei) cewllithen vtheelo pceell( ee.ngv.,emlopeme b(era.gn.,e ,mpeemptbidraongel,y cpaenp,tiFdioggulryeca2n),, (Fiii)guwreit h2)s, ig(ini)ifi wcaintht
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Int.J.Mol.Sci. 2018,19,444 7of17
Int. J. Mol. Sci. 2018, 19, x 7 of 17

Figure 2. Accumulation of silver nanoparticles in P. aeruginosa cells (silver nanoparticle concentration
Figure2.AccumulationofsilvernanoparticlesinP.aeruginosacells(silvernanoparticleconcentration
75 µg/mL, silver size: 10 nm). Reprinted from [23] with Copyright Clearance Center permission.
75µg/mL,silversize:10nm).Reprintedfrom[23]withCopyrightClearanceCenterpermission.
One of the most important ways of silver antibacterial activity is the induction of ROS
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anaaetmroobsepsh (eirni cthoex ylagcekn oafn doxtyhgeeann)t iibs avceteryri alolwac.t iIvt iitsy iomfpsiolvrtearnnt atnhoapt aArtgiNclePss acgaanin isntdauncaee raonb aesdd(iintitohnealla, ck
exoogfeonxoyuges,n R)iOsvS egreynloerwa.tiIotnis. iAmsp noartnaonptathrtaitcAlegs—NPsuscchan aisn TdiuOce2 oanr ZadndOit—iownaitl,he lxaorggeen souursf,aRceO aSrgeeans earnatdio n.
higAhslyn arneoacptaivrteic cleast—alysuticch saitseTs iOca2no prrZondOu—cew RitOhSl airng ethseu rpfarceeseanrceea soafn UdVh iglihglhyt rdeaucet ivtoe cpahtoaltyotciactsailtyetsicc an
propproedrtuiecse [R62O]S, ailnsoth tehep rpehseontocceaotaflUytVicl RigOhtS dgueenetoraptihoont obcya tsaillvyetirc fporrompse rctainesno[6t2 b],e aelxsocluthdeepdh [o6t3o]c. atalytic
ROLSokg eent earla. t[i5o4n] pbyoisnitlevde rofuotr mthse chainghn oatnbtiebeaxccteluridael dac[t6i3v]i.ty of AgNPs and claimed that the effective
concentLraotkioentsa lo.f[ 5A4]gpNoPins teadndo uAtgth+ e(uhnigdhera nateirboabcitce rciaolnadcittiivointy) owfeAreg NatP nsaannodmcloaliamr eadndth amtitchreomeffoelcatri ve
levceolns,c ernetsrpaeticotinvseolyf.A RgaNi Pest aanld. A[5g6+] (ruenvdieewreader otbhiact csoinlvdeitri onna)nwoepraeratitcnleasn ommaoyl abrea nadlsom iucrsoemd oalgaarilnesvte ls,
murletsidpreuctgi vreelsyi.stRaanit e(tMalD.R[5)6 ]braecvteieriwa,e dbotthha tGsrilavmer-pnoasnitoivpea r(tmicleetshimciallyinb-reeasilsstoanuts eSd. aaguareinuss—tmMuRltSidAr,u g
Strreepstiosctoacnctus(M pyDogRe)nbesa)c atenrdia G,braomth-nGergaamti-vpeo (sEit. icvoeli,( mK.e pthniecuimllionn-iraees, iPst.a anetruSg.ianuorseau).s — MRSA,Streptococcus
pyoMgeannedsa)la entd aGl. r[6a4m] -pnreogvaetdiv teh(eE c.hcaolrig,eK-.dpenpeeunmdoennita em,Pe.cahearnuigsimno osaf) A. gNPs efficacy. They noticed the
Zeta poMteanntidaal loeft Eanl.t[e6r4oc]opcrcouvs efdaetchaelisc,h Parrogtee-udse vpuelngdareins tamndec AhagnNisPms oofnA thgeN lPesveefl:fi −ca1c5y, .−T2h6e, y−3n2o.2ti cmedVt, he
resZpeetcatipvoeltye,n wtiahlicohf Etenstteirfoyc tohcceu cshfaaregcael icse,lPlsr oatneuds pvaurltgiaclreiss aanndd AingdNicPasteodn tthhaet lGevraeml:-−po15si,t−iv2e6 E,.− fa3e2c.a2lims V,
celrle aspcceuctmivuellay,tew mhiochret eAstgifNyPths etchhaanr gGercaemlls-naengdatpiavret icPl.e svuanlgdariinsd. iScoanteddi tehta taGl. r[a2m2]- pcolasiitmiveedE t.hfaaetc aallissoc ell
negacactiuvmeluyl actheamrgoerde sAilgvNerP s(stihlvaenr Gnraanmoc-noemgpaotisvietePs.),v uinlg caorims.pSaornisdoine ttoa l.p[o2s2i]ticvlealiym cehdatrhgaetda lssiolvneer giaotnivs,e ly
shocwhasr gheigdhs ailnvteibra(scitlevreiarln eafnfiocacocym apso wsietells. ), incomparisontopositivelychargedsilverions,showshigh
anTtihbea clitfeerciyalcleef fiofc ancaynoapsawrteilcll.es and nanocomposites has an important influence on the antimicrobial
mode oTfh aecltiifoency calnedo fenffaincaocpya ratincdle sdaenpdenndans oocnom thpeo seitnevsihroasnmaneinmtaplo crtoanndtiitnioflnuse.n Gceitoipnotuher aent tiaml. ic[r6o5b] ial
obsmerovdeed othfaatc stpiohneraicnadl AegffiNcPasc y(3–a5n dnmd)e, puesnedd saso an dtihsienfeencvtairnot ninm menotuatlhcwoansdhi,t uionndse.rgGoi ttriapnosufroremtaatli.on[6 5]
(agogbrseegravteidonth) aatfsteprh eursicaagleA. gTNheP si(n3n–e5rn dmia),muesetedr aosfa dAigsiNnfPesc tiannctrienamseodu tthow 5a0s–h2,0u0n dnemrg oantrda ncshfeomrmicaatli on
tra(nasgfgorremgaattiioonn )ofa AftegrNuPssa tgoe A. gTChl eafitnern uersadgiea wmaeste orbosefrAvegdN, wPshaint ccraena pserdovteo th5e0 –io2n0i0zantmiona onfd mcehteamllicic al
silver from nanoparticles to silver ions. Biotransformation of silver nanoparticles depending on
environmental condition is often observed. Mokhena et al. [26] observed that size and shape of
Int.J.Mol.Sci. 2018,19,444 8of17
transformationofAgNPstoAgClafterusagewasobserved,whatcanprovetheionizationofmetallic
silver from nanoparticles to silver ions. Biotransformation of silver nanoparticles depending on
environmental condition is often observed. Mokhena et al. [26] observed that size and shape of
nanoparticles changed after heating at 90 ◦C. After 3 h of heating, the size of AgNPs increased
from 28 to 30 nm and spherical particles became irregular: their shape changed from spherical to
rod-like. Heatingfor48hresultedinamixtureofrod-like(76–121nm)withlessabundantspherical
(28–50nm)nanoparticles. Ramanetal.[27]indicatedthatduringtheeco-friendlyproductionofsilver
nanoparticles their size and shape were pH and temperature dependent. McGillicuddy et al. [66]
reportedthatAgNPsreleasedfromconsumerproductstotheenvironmentduringtheirlifecyclehave
variedproperties,althoughtheirdeterminationisdifficult.
ThetheoryaboutantibacterialmodeofactionofAgNPsisalsoconnectedwiththeoxidation
ofAgNPs. Itismorethanlikelythatthesurfaceofsilvernanoparticlesisoxidized[57]. Thesmaller
thesizeoffabricatedparticles,thehigheroxidecontentduetolargersurfaceareatovolumeratio.
The presence of oxide on the surface ensures high antibacterial activity of AgNPs, most probably
duetothehigherconcentrationofROS[55]generated. Xiuetal.[67]showedthattoxicityofAgNPs
dependsonthepresenceofO andisconnectedwithsilverionsrelease.Theytestedglycol-thiol-coated
2
(PEG) AgNPs with different sizes. The oxidative dissolution of AgNPs was observed only under
aerobicconditionsandPEGcoatingdidnotsecurenanoparticlesfromthatphenomenon. Inanaerobic
conditions, tested PEG-AgNPs did not show the antibacterial activity upon E. coli K-12 due to
nonoccurrenceofdissolvedsilverions. Xiuetal.[67]claimedthatamongotherthesizeandshape
or AgNPs coating has an influence on extent and duration of Ag+ release into solution. It was
shown that the release of Ag+ ions from nanosilver in aqueous solutions corresponds to the mass
leachedordissolvedofoneortwooxidizedmonolayersfromitssurfacedependingonnanosilver
size. Theantibacterialactivity(againstE.coli)ofnanoparticles(size<10nm,whereAg Olayerwas
2
removed),wassignificantlylowerthanthatofas-preparedparticleswithoxidizedsurface[68].
Raietal.[56]andDuránetal.[69]reportedthatantibacterialefficacydependsonthesize,shape,
concentrationanddosesofusedAgNPs. Moronesetal.[23]observedthehighestefficacyforAgNPs
withasizebelow10nm. Paletal.[20]showedtheshape-dependentefficacyofAgNPs,whichresults
fromthedifferentsurfaceareabetweensphericalandtriangularAgNPs,withthelatterformexhibiting
muchhigherefficacy. Shengetal.[9]reviewedindetailsthedose-responseofbacteriatoAgNPsand
noticedhighdifferencesintheefficacyconcentrationsofdifferentAgNPsandvariouscellresponse.
Everynanocompositehasadifferentphysical(size,shape,amount)andchemical(presenceof
othercompounds, oxidationstate)properties[9,21,28](Table1). Therefore,wespeculatethatthey
should be considered as separate forms, with different properties and different ways of bacterial
toxicity. Thus, the mode of action of particular silver nanoparticles (nanoforms, nanocomposites)
could be the result of the binding strength, type of target, time of interaction, oxidation level, etc.
Moreover, the carrier’s compound can enhance the antibacterial activity due to changes of the
physico-chemical properties as well as mechanical mode of action (for example, TiO in anatase
2
formpossessphotoactivityandgraphene-basedstructuresmightcausemechanicaldamagesofthe
cell,overwrapbacteriaorincreasethesurfacearearesultinginstrongerinteractionwithcells[28,62].
Whenbioactivityofmetalnanoparticlesisconsidered,itisworthtomentionthatthesematerialscan
change their properties and toxicity depending on the time and conditions of storage. Both these
parameters, as well as surface chemistry of AgNPs, influence the evolution of the nanoparticles’
properties over time. While stored, different processes may occur, such as oxidation, dissolution,
agglomeration,cappingagentdegradationorattachmentofAg+ionstocontainerwalls[70]. Allthe
changeshaveasignificantinfluenceontheparticlestoxicity[54,70]indicatingstrong”aging”effects.
Therefore,contradictorytoxicityresultsmightbeobservedintheliteratureforidenticalAgNPsagainst
thesamebacteria.
AcomparisonofthemodeofactionofsilverionsandsilvernanoparticlesagainstGram-negative
andGram-positivebacteriawithshortinteractiondescriptionispresentedinFigure3.
Int.J.Mol.Sci. 2018,19,444 9of17
Int. J. Mol. Sci. 2018, 19, x 9 of 17

FFiigguurree 33.. AA ccoommppaarriissoonn ooff tthhee ssiillvveerr iioonnss ((AA)) aanndd ssiillvveerr nnaannooppaarrttiicclleess’’ ((BB)) mmooddee ooff aaccttiioonn ttoo GGrraamm--
nneeggaattiivvee ((lleefftt)) aanndd GGrraamm--ppoossiittiivvee ((rriigghhtt)) bbaacctteerriiaa.. ((11)) PPoorree ffoorrmmaattiioonn;; mmeettaabboolliitteess aanndd iioonnss lleeaakkaaggee
((sshhoowwnn aassp lpulsuas nadnmd inmuisnuins thine fithgeu rfeigaubroev ea)b(o2v)eD) e(n2a)t uDreantiaotnuroaftsiotrnu cotuf rsatlrauncdtucryalt oapnlads mcyictopprloatseminisc;
epnrzoyteminess; ineancztiyvmateios ni.n(a3c)tIinvaacttiiovna.t io(3n) ofInreascptiivraattoiorny cohfa inreesnpziryamtoersy. (4c)hIanicnr eeansezyomfienstr. a(c4e)l luIlnacrrreeaascet ivoef
oinxtyrgaecenllsupleacri erse(aRcOtivSe) coonxycegnetnr atsipoenc.i(e5s) I(nRtOerSa)c ticoonncweinthtraritbioons.o m(5e). (I6n)tIenrtaecrtaiocnti onwwithit hrnibuocsloemicea.c id(6s).
(I7n)teInrahcitbiiotnio wniotfhs niguncalelitcr aancisddsu. c(t7i)o Inn.hibition of signal transduction.
4. Current Limitations and Future Prospects of Silver Materials Usage
4. CurrentLimitationsandFutureProspectsofSilverMaterialsUsage
The general resistance of bacteria to heavy metals is presented in Figure 4. The mechanism of
The general resistance of bacteria to heavy metals is presented in Figure 4. The mechanism
resistance to heavy metals is connected with the locking of uptake metal into the cell or the
of resistance to heavy metals is connected with the locking of uptake metal into the cell or the
detoxification of the metals inside. The resistance of bacteria to silver may be divided into an
detoxification of the metals inside. The resistance of bacteria to silver may be divided into an
endogenous and exogenous mechanism. The first one (endogenous) is connected with a loss of
endogenousandexogenousmechanism.Thefirstone(endogenous)isconnectedwithalossofspecial
special proteins (OmpC/F) and an up-regulation of efflux mechanism (Cus system) as an effect of two
point mutation after long-term exposition of bacteria (e.g., E. coli) to silver ions (AgNO3). The
Int.J.Mol.Sci. 2018,19,444 10of17
proteins(OmpC/F)andanup-regulationofeffluxmechanism(Cussystem)asaneffectoftwopoint
mInut.t Ja. tMioonl. Sacfit. e2r01l8o, n19g,- xt ermexpositionofbacteria(e.g.,E.coli)tosilverions(AgNO ). Theendoge1n0o ouf s17
3
mechanismofbacterialresistancetosilverionswasprovenbyRandalletal.[43]. Theyshowedthat
endogenous mechanism of bacterial resistance to silver ions was proven by Randall et al. [43]. They
silverprovideselectivepressuretoenrichapopulationofsilverresistantbacteria. After6daysof
showed that silver provide selective pressure to enrich a population of silver resistant bacteria. After
exposuretosubinhibitoryconcentrationsofsilverions,theresistantstrainofE.coliBW25113could
6 days of exposure to subinhibitory concentrations of silver ions, the resistant strain of E. coli BW25113
beselected.
could be selected.
TheexogenouswayisassociatedwithSilproteinslocatedinthecellmembranesandresponsible
The exogenous way is associated with Sil proteins located in the cell membranes and responsible
foreffluxofsilverionsoutofthebacterialcell. Thedescriptionofthosemechanismislocatedbelow
for efflux of silver ions out of the bacterial cell. The description of those mechanism is located below
(Figure4).
(Figure 4).

Figure 4. Diversity of bacterial mechanisms of resistance to heavy metals [71].
Figure4.Diversityofbacterialmechanismsofresistancetoheavymetals[71].
The chromosome-encoded mechanism of resistance to silver is strictly connected with the
presTehnecec horfo emffolusxo mpue-menpcso wdeitdhmine bcahcatneirsiaml mofermesbisrtaannecse [t4o3s]i. lDveurei stos tirtisc ftulynccotinonnesc, tseidmwilaitrhittihese ipnr epsreontecein
osfeeqffluuenxcpeus,m spusbwstirtahtien sbpaecctiefriicailtym aenmdb rsaunbecsel[l4u3l]a.rD luoecatotioitns,f ualnl cetifoflnusx, spimumilaprsit iweserine pclraostseiifniesde qiuneton cfeivs,e
susubpsterraftaemsiplieecsi:fi AciBtyCa n(AdTsPu-bbcienldluinlagr cloacsasteitoten),, aMllFefSfl (umxapjourm fpasciwlitearteorc lsausspiefirefdaminitloy)f,i vReNsDup (errefsaimstailniecse:-
AnBoCdu(AlaTtPio-bni-nddivinisgiocnas),s eMttAe)T,ME F(mSu(mltaidjorrufga cainlidta ttoorxiscu cpoemrfapmouilnyd), RexNtrDus(rioesni)s taanndce S-nMoRd u(lsamtiaolnl -mdiuvlitsiidornu),g
MreAsTisEtan(mceu) l[t7id2]r.u Tghea npdumtopxsi cofc eoamchp omuenndtioenxterdu ssiuopne)rfaanmdilyS MwRere( samlraelaldmy ufoltuidndru ign trheesi sctealln ocef )G[r7a2m].-
Tnheegaptuivme pbsacotefrieaa c[5h0]m. Ternatniospnoedrtesrus preesrpfaomnsiliyblew feorre thael reexatdruysifoonu nodf dirnugtsh,e decteelrlgoefntGs,r baimoc-indeegsa, tdivyees
baanctde,r iimap[5o0r]t.anTtrlayn, shpeoarvtye rmserteaslpso innstoib elextfroarcetlhluelaerx tsrpuascioe nbeolfondgru tgos t,hdee rteesrigsetanntsc,e-bnioodciudleast,iodny-desivaisnido,n
imsuppoerrtfaanmtliyly,.h Teahvey amctievtaitlys ionft otheoxster apceulmluplasr dseppaecnedbse loonn gthteo pthreotorens-imstoatnivcee- nfoodrcuel a[t7io2n,7-3d]i.v Tishioonse
sutrpaenrsfpaomrtielyr.s hTahvee athctei vaibtyilitoyf toth coaspetuprue mtopxsic dcoempepnodusndosn frtohme pcyrototopnla-smmo tainvde tfhoer cpeer[i7p2l,a7s3m],icT shpoascee.
trRaNnsDp oprutemrsp hcaovnesitshtse oafb tihlirteyet soucbaupntiutrse: stuobxsictrcaotem-bpionudnindgs tfrraonmspcoyrttoepr l(alsomcataendd inth tehep ienrnipelra msmeimcbspraancee).,
RpNeDrippluasmmpicc omnesmistbsraonfeth frueseiosnu bpurontietsi:ns (uMbFstPr)a aten-dbi onudtienrg mtreamnsbproanrtee rfa(clotocra t(eOdMinF)t.h Tehien nfoerrmmeedm cbormanpele),x
pseereipmlass tmo ibcem spemanbnriannge bfoutshio mnepmrobtreainne(sM [7F3P,)7a4n].d RoNuDte trrmanesmpobrrtaenres wfaicttho rh(igOhM sFp)e.cTifhiceitfyo rtmo etodxcico mcaptiloenxs
sebeemlosntgo tboe thspe aHnMninEg-RbNotDh (mheemavbyr-amneetsa[l7 e3f,f7l4u]x.)R sNubDfatrmanilsyp, osurtcehrs aws CithushCigFhBAsp—ecoifincei toyf tfoewto HxicMcEa-tRioNnsD
bpeluomngpst,o rtehsepoHnMsibEl-eR fNorD b(ahcetaerviya-lm reestiasltaenfflceu xto) scuobpfpaemr ialyn,ds usiclhvears iConuss C[7F5B–A77—]. o neoffewHME-RND
pumpTs,hree soppoenrsoinbl ecufosCrFbBacAt eorifa lar ebsaicsttearnicael tcohcroomppoesroamned esnilcvoedreios ntsh[e7 5s–y7s7te].m of active, extracellular
tranTsphoerot poef rAogn(Ic)u asnCdF BCAu(oI)f wahbiachct ecroinasliscthsr oomf CousosAm,e CeunscBo, dCeusstCh e(psryostteeimns, osfuabcutnivites, oefx RtrNacDel eluffllaurx
trpaunmsppo)r tanodf Aag p(Ie)raipnldasCmui(cI )chwahpiecrhocnoen, sCisutssFo f[7C5u,7s8A]., TChues Btr,aCnuscsrCip(tpioront eoifn sg,esnuebsu ennictsodoifnRgN CDuseCfflFuBxA
pduemppen)dasn dona tpheer eipxtlarascmelilcuclahra pcoenrocnene,trCatuiosFn [o7f5 h,7e8a]v.yT-mheettaral niosncrsi pantido nthoef ogcecnuersreenncceo odfi nogpeCrounsC cuFsBRAS.
dTehpee nodpseroonnt hgeatehxetrrsa cgeelnluelsa rofc oCnucseRn t(rraetsipoonnosfeh reeagvuyla-mtoert)a alniodn CsuasnSd (thhiestoidcicnuer rkeinncaeseo)f wopheicrhon recguusRlaSt.e
Tthhee oepxeprroenssgioanth oefr sthgee nefefsluoxf CpuumsRp(’sr ecsopmonpsoenreengtus laafttoerr) raenadctiCouns wSi(thhi stthide isnteimkuinlaanset )[w79h,8ic0h]. rTehguusl aftaer,
CusCBA is the only known chromosomal-encoded pump responsible for silver resistance [75,80].
The function and structure of each CusCFBA subunit and CusF is well known. The substrate-
binding transporter, CusA, is located in the inner membrane. This homotrimer consists of 1047 amino
acids and contains 12-transmembrane α-helices [75,76,80]. Its activity depends on proton-motive