REVIEW
published: 16 November 2016
doi: 10.3389/fmicb.2016.01831
Frontiers in Microbiology | www.frontiersin.org 1 November 2016 | Volume 7 | Article 1831
Edited by:
Octavio Luiz Franco,
Universidade Católica de Brasília,
Brazil
Reviewed by:
Santi M. Mandal,
Vidyasagar University, India
Renko De Vries,
Wageningen University and Research,
Netherlands
*Correspondence:
Tikam Chand Dakal
tikam260707@gmail.com;
tikamchand.dakal@jaipur.manipal.edu
Vinod Yadav
vinodyadav@cuh.ac.in
Specialty section:
This article was submitted to
Antimicrobials, Resistance and
Chemotherapy,
a section of the journal
Frontiers in Microbiology
Received: 19 September 2016
Accepted: 01 November 2016
Published: 16 November 2016
Citation:
Dakal TC, Kumar A, Majumdar RS and
Yadav V (2016) Mechanistic Basis of
Antimicrobial Actions of Silver
Nanoparticles.
Front. Microbiol. 7:1831.
doi: 10.3389/fmicb.2016.01831
Mechanistic Basis of Antimicrobial
Actions of Silver Nanoparticles
Tikam Chand Dakal
1
*
, Anu Kumar
2
, Rita S. Majumdar
3
and Vinod Yadav
2
*
1
Department of Bio Sciences, Manipal University Jaipur, Jaipur, India,
2
Department of Biotechnology, School of Engineering
and Technology, Sharda University, Greater Noida, India,
3
Department of Microbiology, Central University of Haryana,
Mahendragarh, India
Multidrug resistance of the pathogenic microorganisms to the antimicrobial drugs has
become a major impediment toward successful diagnosis and management of infectious
diseases. Recent advancements in nanotechnology-based medicines have opened new
horizons for combating multidrug resistance in microorganisms. In particular, the use
of silver nanoparticles (AgNPs) as a potent antibacterial agent has received much
attention. The most critical physico-chemical parameters that affect the antimicrobial
potential of AgNPs include size, shape, surface charge, concentration and colloidal
state. AgNPs exhibits their antimicrobial potential through multifaceted mechanisms.
AgNPs adhesion to microbial cells, penetration inside the cells, ROS and free radical
generation, and modulation of microbial signal transduction pathways have been
recognized as the most prominent modes of antimicrobial action. On the other side,
AgNPs exposure to human cells induces cytotoxicity, genotoxicity, and inflammatory
response in human cells in a cell-type dependent manner. This has raised concerns
regarding use of AgNPs in therapeutics and drug delivery. We have summarized the
emerging endeavors that address current challenges in relation to safe use of AgNPs in
therapeutics and drug delivery platforms. Based on research done so far, we believe
that AgNPs can be engineered so as to increase their efficacy, stability, specificity,
biosafety and biocompatibility. In this regard, three perspectives research directions have
been suggested that include (1) synthesizing AgNPs with controlled physico-chemical
properties, (2) examining microbial development of resistance toward AgNPs, and (3)
ascertaining the susceptibility of cytoxicity, genotoxicity, and inflammatory response to
human cells upon AgNPs exposure.
Keywords: silver nanoparticles, multidrug resistance, antimicrobial activity, physico-chemical property,
cytotoxicity, genotoxicity, inflammatory response
INTRODUCTION
Unresponsiveness of microbes to lethal doses of structurally diverse classes of drugs with different
mechanisms of cytotoxic action is generally referred to as multidrug resistance (MDR). Multidrug
resistance of the pathogenic microorganisms to the antimicrobial drugs has become a prime
concern toward successful diagnosis and treatment of pathogenic diseases of bacterial and fungal
origin (
Desselberger, 2000). This has led to emergence and re-emergence of infectious diseases.
Indeed, exposure of antimicrobials and antibiotics to bacteria are the opportunities for microbes to
become less susceptible toward them mainly by altering the cell structure and cellular met abolism.
In this way microbes either destroy the antimicrobials and antibiotics or become unresponsive
toward them in future exposures (
Desselberger, 2000; Rai et al., 2012). Four mechanisms have been
Dakal et al. Antimicrobial Action of Silver Nanoparticles
recognized that account for antibiotic resistance in bacteria:
(a) alteration of microbial drug target proteins, (b) enzymatic
degradation or inactivation of drug, (c) decreased membrane
permeability, and (d) increased efflux of drug (
Kumar et al.,
2013
). Among all, the extrusion of antimicrobial drug by the
multidrug efflux pumps contributes maximally for MDR among
pathogenic strains (Li et al., 1997; Levy, 2002). Although,
excessive and irrational use of antibiotics is major factors in
development of resistance, the acquisition and dissemination of
drug-resistance genes and resistant bacteria have significantly
contributed to drug resistance (Davies, 1997; Levy, 2002).
Acquisition of drug-resistance generally occurs through genetic
mutations, alterations in genetic material or gaining of foreign
genetic material (Levy, 2002; Yoneyama and Katsumata, 2006).
Dissemination of drug-resistance determinants occurs within
genome via transposons or from one microorganism to another
by a number of genetic ways, for instance, through transfer of
extra-chromosomal element between Gram-positive and Gram-
negative bacteria (
Levy, 2002). Confronted by the increasing
doses of antibiotic drugs over many years, pathogens become
drug-resistant and respond to antibiotics by generating progenies
that are no more susceptible to antimicrobials therapy (Levy,
2002; Porras-Gomez and Vega-Baudrit, 2012).
Nowadays, non-traditional antimicrobial agents to
overcome MDR are increasingly gaining importance. Recently,
development of novel, efficient nanotechnological-based
antimicrobial agents against multidrug-resistant bacteria is
among one of the priority areas in biomedical research (Rai
et al., 2012
). Silver nanoparticles (AgNPs) display a broad
spectrum of antibacterial and antifungal activities (
Morones
et al., 2005; Kim et al., 2007; Panacek et al., 2009; Namasivayam
et al., 2011). Moreover, the advantage of using nanosilver
is that it is comparatively less reactive than silver ions, and
therefore, is well suited for its use in clinical and therapeutic
applications (Kim et al., 2005; Chen and Schluesener, 20 08).
The antimicrobial activity of AgNPs has been tested against
both, MDR and non-MDR strains of bacteria (Feng et al.,
2000; Morones et al., 2005; Ayala-Nunez et al., 2009; Humberto
et al., 2010; Ansari et al., 2011). In this review, we have
presented a comprehensive overview of AgNPs-induced cellular
response in b acteria and human cells. AgNPs induce, influence,
and modulate diverse range cellular, biochemical, metabolic
and inflammatory processes that account for multifaceted
antimicrobial activity of AgNPs for tackling multidrug resistance
in bacteria. Additionally, some other aspects of AgNPs-based
medicines including, physico-chemical properties of AgNPs;
cytotoxic, genotoxic and inflammatory response of AgNPs to
human cells; and application of AgNPs in therapeutics and
targeted drug delivery have also been reviewed.
MDR AND NON-MDR STRAINS:
BACTERICIDAL EFFECT OF AgNPs
MDR bacterial strains and infections caused by them are
considered as the prime reason for increased mortalit y rate,
morbidity rate and treatment cost in developing countries
(
Walker et al., 2007; Salem et al., 2015). A number of Gram-
positive and Gram-negative bacterial pathogens are known to
cause severe medical and clinical complications such as diarrhea,
urinary tract disorders, pneumonia, neonatal meningitis etc.
(
Walker et al., 2007; Salem et al., 2015). Infectious Gram-
positive bacteria include Actinomyces, Bacillus, Clostridium,
Corynebacterium, Enterococcus, Listeria, Mycobacterium,
Nocardia, Staphylococcus, Streptococcus, and Streptomyces.
Among them antibiotic-resistant bacteria are penicillin-resistant
Streptococcus pneumonia, macrolides resistant Streptococcus
pyogenes, vancomycin-resistant Enterococcus faecium (VREF),
methicillin- and vancomycin-resistant Staphylococcus aureus
(MRSA and VRSA), and multidrug-resistant Listeria and
Corynebacterium. The Gram-negative bacteria include members
of the genera Acinetobacter, Escherichia, Klebsiella, Neisseria,
Pseudomonas, Salmonella, Shigella and Vibrio. Among Gram-
negative bacteria, Vibrio cholerae and enterotoxic Escherichia
coli (ETEC) are regarded as the two most pathogenic and
dominant bacteria that cause severe secretory diarrhea, which
significant account for high mortality and morbidity (
Salem
et al., 2015). Among Gram-negative microbial pathogens
some are opportunistic microorganisms, such as Acinetobacter
baumanii, Klebsiella pneumonia, and Pseudomonas aeruginosa
that are intrinsically resistant to multiple drugs and infect
mainly immune-compromised patients (Levy, 2002). Besides
opportunistic pathogens, the strains of Salmonella typii have
also showed high frequency of drug-resistance and have become
resistance to ampicillin, chloroamphenicol, fluoroquinolones,
and some other drugs (Levy, 2002). Table 1 contains a list of
most common drug-resista nt, pathogenic bacterial strains along
with the corresponding a ntibiotic s to which the strains have
developed resistance.
AgNPs have been used alone or in combination with
antibiotics.
Namasivayam et al. (2011) evaluated and reported
the antibacterial activity of AgNPs against drug-resistant
pathogenic bacteria Bacillus subtilis, E. coli, E. faecalis,
K. pneumonia, P. aeruginosa, and S. aureus (Namasivayam
et al., 2011). Nanda and Saravanan (2009) evaluated AgNPs for
their antimicrobial activity against methicillin resistant S. aureus
(MRSA), methicillin-resistant Staphylococcus epidermidis
(MRSE), S. pyogenes, S. typhi, and K. pneumoniae. The
obser ved antibacterial activity was maximum in case of MRSA,
intermediate in MRSE and S. pyogenes, whereas the antibacterial
activity seen against S. typhi and K. pneumonia was moderate.
In order to further improve the AgNPs-based therapeutics, the
use of AgNPs-antibiotic combination against drug-resistant
pathogenic strains is recommended. AgNPs have displayed
synergistic antimicrobial effect when used in combination
with antibiotics (
Fayaz et al., 2010). The synergistic effect of
19 antibiotics and the silver–water dispersion solution was
studied by De’ Souza et al. (2006). The silver–water dispersion
solution is produced by an electro-colloidal process and the
dispersion solution contains AgNPs clusters of 15 nm diameter.
In the study, the a ntimicrobial activity of amoxicillin and
clindamycin was evaluated against some MDR strains such as
E. coli, S. aureus, S. typhi, Shigella flexneri, and B. subtilis. While
the combination of silver–water dispersion with amoxicillin or
Frontiers in Microbiology | www.frontiersin.org 2 November 2016 | Volume 7 | Article 1831
Dakal et al. Antimicrobial Action of Silver Nanoparticles
TABLE 1 | Multidrug-resistant in bacterial strains.
Bacterial strains Resistant to
GRAM-POSITIVE
Bacillus subtilis Chloramphenicol
Erythromycin Lincomycin
Penicillin Streptomycin
Tetracycline
Corynebacterium diphtheriae β-lactam antibiotics Chloramphenicol
Tetracycline
Trimethoprim
Sulfamethoxazole
Enterococcus faecium Vancomycin
Gentomicin
Listeria monocytogenes Erythromycin
Gentomicin
Kanamycin
Rifampin
Streptomycin
Sulfamethoxazole
Tetracycline
Staphylococcus aureus Methicillin
Vancomycin
Streptococcus pneumonia Penicillin
Erythromycin
Streptococcus pyogenes Erythromycin
Macrolides
GRAM-NEGATIVE
Acinetobacter baumanii Carbapenems
Imipenem
Escherichia coli Ampicillin
Cephalosporins
Chloramphenicol Fluoroquinolones
Nalidixic acid Rifampin
Sulfamethoxazole Streptomycin Tetracycline
Klebsiella pneumonia Carbapenems
Imipenem
Pseudomonas aeruginosa β-lactams
Chloramphenicol Fluoroquinolones Macrolides
Novobiocin Sulfonamides Tetracycline
Trimethoprim
Salmonella typii Amoxycilin Ampicillin Chloroamphenicol
Fluoroquinolones
Trimethoprim
Shigella flexneri Ciprofloxacin
Nalidixic acid
Vibrio cholera Fluoroquinolones Tetracycline
Different antibiotics toward which Gram-positive and Gram-negative bacteria have
developed resistance.
clindamycin had an additive effect on B. subtilis, S. aureus 6538
P strain, S. flexneri, and S. typhi, on the contrary, the AgNPs
dispersion solution in combination with amoxicillin displayed
an antagonistic effect toward methicillin-resistant S. aureus
strain (MRSA) (
De’ Souza et al., 2006). Shahverdi et al. (2007)
studied the additive effect of AgNPs antibacterial effect against E.
coli and S. auerus in presence of antibiotics such as amoxicillin,
clindamycin, erythromycin, penicillin G and vancomycin. Fayaz
et al. (2010) demonstrated synergistic effect of AgNPs against
both Gram-positive and Gram-negative bacteria in combination
with antibiotics. In case of Gram-negative bacterium S. typhi,
the potency of ampicillin-mediated cell wall lysis increases when
a combination of AgNPs and antibiotic is used (Rajawat and
Qureshi, 2012). This suggests that AgNPs must be increasing the
local concentration of antibiotics at the site of action and thus
improves their potency. Besides potency against MDR and non-
MDR bacterial strains, AgNPs a lso act as a potent, fast-acting
anti-fungal agent against a wide range of fungal genera such
as Aspergillus, Candida, Fusarium, Phoma, and Trichoderma
sp. (
Duran et al., 2007; Gajbhiye et al., 2009). AgNPs have also
synergisitc fungicidal activity against the Candida albicans,
Fusarium semitectum, Phoma glomerata, Phoma herbarum, and
Trichoderma sp. in combination a commercial antifungal agent,
fluconazole (Gajbhiye et al., 2009).
EFFECTS OF NANOSCALE AND
PHYSICO-CHEMICAL PROPERTIES ON
ANTIMICROBIAL ACTIVITY OF AgNPs
Development or synthesis of metal derived nanomaterials for
biomedical applications depends upon a number of physical,
chemical, thermal, electrical, and optical properties. Some
properties have more significance in medical applic ation while
other properties have relevance in industrial and environment al
applications. Unlike t hei r “macro” counterpart, nanoparticles
demonstrate unique and significantly effective physico-chemical
properties that make nanoparticles suitable for their intended
use in improved healthcare. Se veral studies have demonstrated
that bactericidal properties of the AgNPs are strongly influenced
by their shape, size, concentration, and colloidal state (
Pal et al.,
2007; Bhattacharya and Mukherjee, 2008; Rai et al., 2012; Nateghi
and Hajimirzababa, 2014; R aza et al., 20 16). It has been found
that reducing the size of AgNPs enhances their stability and
biocompatibility (Kim et al., 2005, 2011). Hence, it is necessary
to design appropriate sized, shaped nanoparticles with desirable
surface properties for use in a diverse range of clinical and
therapeutic int erventions.
Shape of the nanoparticles is one of the properties, which
affects other physico-chemical properties of the nanoparticles
(Burda et al., 2005). AgNPs interacts with bacteria, fungi and
viruses in a shape-dependent manner (
Panacek et al., 2009;
Galdiero et al., 2011; Tamayo et al., 2014; Wu et al., 2014;
Raza et al., 2016). Energy-filtering TEM images have revealed
alterations in the cell membrane of the gram ne gative E. coli
bacterium upon treatment with differently shaped AgNPs, both
Frontiers in Microbiology | www.frontiersin.org 3 November 2016 | Volume 7 | Article 1831
Dakal et al. Antimicrobial Action of Silver Nanoparticles
in liquid and semi-solid agar medium (Pal et al., 2007). As
compared to t h e spherica l or rod-shaped AgNPs, truncated
triangular shaped AgNPs show enhanced antibacterial action
(
Chen and Carroll, 2002; Pal et al., 2007). AgNPs with the
same sur f ace areas, however, different shapes show differential
bactericidal activity, which can be attributed to the variations in
the effective surface areas and active facets of AgNPs. Different
surface chemistries, such as foamy carbon, poly (N-vinyl-2-
pyrrolidone) (PVP), and bovine serum albumin (BSA) can also
influence AgNPs interaction with viruses, such as HIV-1 virus,
and causes their inhibition (
Elechiguerra et al., 2005). Since, both
BSA and PVP are completely encapsulated and are bounded
directly to the nanoparticle surface, there is fundamentally
no exposed surface area for AgNPs-virus interaction. On the
contrary, the foamy carbon silver nanoparticles, which display
an exposed surface area for virus attachment, display higher
cytotoxicity and cause inhibition comparatively higher than
AgNPs with BSA and PVP surface chemistry (Elechiguerra et al.,
2005). However, there is limited information available about how
shape of the nanoparticles influences AgNPs biological activity.
Another important physico-chemical property of AgNPs is
their size. In general, for nanoparticles to be effective their size
typically should be no larger than 50 nm. More precisely, silver
nanoparticles with size between 10 and 15 nm have increased
stability, biocompatibility and enhanced antimicrobial activity
(Yacaman et al., 2001). Some studies have revealed that the
antibacterial action of AgNPs is more effective against S. aureus
and K. pneumoniae when nanoparticles of smaller diameter
(<30nm) are used (
Collins et al., 2010). The antibacterial effect
of AgNPs as proposed is due to their smaller particles size
that apparently has superior penetration ability into bacteria,
especially in Gram-negative (
Morones et al., 2005). AgNPs
of 5–10 nm dimension display both bacteriostatic as well
as bactericid a l effects against S. aureus, MSSA and MRSA
(Ansari et al., 2011). Espinosa-Cristobal et al. (2009) tested
the potential of different sized AgNPs against Streptococcus
mutans, a causal organism of dental caries, and suggested that
as the AgNPs particle size diminishes, the antibacterial activity
increases. Interestingly, the attachment of AgNPs with the cell
membranes and resulting alterations in lipid bilayer lead to
increased membrane permeability, damage and cell death, a
potent antibacterial effect seemingly more pronounced when
smaller sized nanoparticles are used (
Li et al., 2013). To this end,
Pal et al. (2007) demonstrated that the surface area to volume
ratio of AgNPs and the crystallographic surface st ructures are
important factors that determine the antibacterial activity of
AgNPs.
AgNPs have been evaluated for their antiviral action mode
against HIV-1 using a number of in vitro experiments, where
at non-cytotoxic concentrations AgNPs exerted the antiviral
activity against HIV-1; however, t he mechanism underlying
their HIV-inhibitory activity remained unclear (Sun et al.,
2005). AgNPs are known to interact with HIV-1 virus via
binding to gp120 glycoprotein knobs in a size-dependent manner
(
Elechiguerra et al., 2005). Nanoparticles usually of size between
1 and 10 nm attaches to the HIV-1 virus by binding to the
disulfide bond regions of the CD4 domain present in the gp120
glycoprotein of the viral envelop (
Elechiguerra et al., 2005). Other
studies demonstrated that AgNPs ranging 5–20 nm diameter
can inhibit replication of HIV-1 (
Sun et al., 2005; Lu et al.,
2008; Suganya et al., 2 015
). In this perspective, the size of the
nanoparticles has substantial impact on antiviral potency of the
AgNPs, which can be further enhanced by optimizing AgNPs
size at nanolevel. Another case of size-dependent interaction of
AgNPs with virus is AgNPs-Hepatitis B virus (HBV) interaction
studied in a human hepatoma cell line, HepAD38 (
Lu et al.,
2008). Using UV-vis absorption titrati on assay, the in vitro
binding affinity of different sized AgNPs (10–50 nm) for HBV
DNA and extracellular virions was ascertained and the binding
caused inh ibition of HBV specific RNA and extracellular virions
synthesis (Lu et al., 2008). In this regard, it is imperative to
infer the significance of h igh binding of AgNPs for HBV DNA
and its role in preventing virions from entering into the host
cells. In this regard, in vivo studies with AgNPs are ne cessary
for designing anti-viral vaccines with high beneficial therapeutic
breakthroughs and low potential side effects.
The antibacterial effe c t is also concentration-dependent, but
the effect is independent of acquisition of drug resistance by the
bacteria.
Ayala-Nunez et al. (2009) reported a dose-dependent
antimicrobial activity of AgNPS against MRSA and non-MRSA
and found that both MRSA and non-MRSA are discouraged
in culture inoculums (conc. 10
5
-CFU per ml) at concentrations
over 1.35 × 10
−3
µg/ml. Studies on AgNPs antibacterial activity
against Gram-positive S. aureus and Gram-negative E. coli have
showed that the inhibition of the growth in case of S. aure us is less
remarkable, while E. coli is inhibited at low AgNPs concentrations
(
Kim et al., 2007). Interestingly, the Gram-positive bacteria, such
as S. aureus, P. aeruginosa, and V. cholera, are less sus ceptible
than Gram-negative bacteria, such as E. coli and S. typhi; however,
both classes of bacteria display complete growth inhibition at
higher AgNPs concentrations (>75 µg/mL) (Kim et al., 2007).
AgNPs in colloidal form, i.e., suspended nano-sized silver
particles, have shown enhanced antimicrobial potential over
AgNPs alone in a number of studies (Sondi and Salopek -
Sondi, 2004; Panacek et al., 2006; Lkhagvajav et al., 2011).
Colloidal AgNPs are synthesized using chemical reduction,
physical, biological and green method using plant extract
(Iravani, 2011; Iravani et al., 2014). The chemical reduction
method of colloida l AgNPs synthesis is the most popular method
(Figure 1). Colloidal state of AgNPs is an essential attribute for
their antimicrobial activity. On the contrary, AgNPs in liquid
system have showed only limited applications as bacteriocidal
agents be cause of their low colloidal st ability (
Kumar et a l.,
2014; Shi et al., 2014). Colloidal silver appears to be a powerful,
antibacterial therapy against infections because it serves as
a catalyst and destabilize the enzymes that pathogenic drug-
resistant bacteria, fungi, yeast, and viruses essentially need for
their oxygen utilization (Dehnavi et al., 2012; Kumar et al.,
2014; Suganya et al., 2015). For instance, colloidal AgNPs
possess enhanced bactericidal potential against drug-resistant
Gram-positive and Gram-negative bacteria and MRSA (
Sondi
and Salopek-Sondi, 2004; Panacek et al., 2006; Lkhagvajav
et al., 2011
). The enhanced bactericidal potential of the
AgNPs has been correlated to their colloid al stability in the
Frontiers in Microbiology | www.frontiersin.org 4 November 2016 | Volume 7 | Article 1831
Dakal et al. Antimicrobial Action of Silver Nanoparticles
FIGURE 1 | Schematic representation of synthesis of colloidal silver nanoparticles using chemical reduction process. Silver ions (Ag
+
) subjected to
chemical reduction to form silver atoms (Ag
0
). These atoms undergo nucleation to form primary AgNPs that further coalesce with each other to form final AgNPs.
medium. Colloidal stability of AgNPs has also been suggested
to regulate signal transduction pathways in bacteria by altering
the phosphotyrosine profile of the proteins, which leads to
growth inhibition in bacteria (Shrivastava et al., 2007). Colloidal
AgNPs synthesized using sol-gel method with size 20–45 nm
were found effective against bacterial strains such as E. coli,
S. aureus, B. subtilis, Salmonella typhimurium, P. aeruginosa,
and K . pneumoniae as well as fungal strain C. albicans at MIC
of 2–4 µg/ml (
Lkhagvajav et al., 2011). In addition, use of
γ-radiation (Shin et al., 2004), microwave irradiation (Phong
et al., 2009), Tollen’s process (Yin et al., 2002) and rational use of
mechanistic understanding (Wuithschick et al., 2013) has made
it possible to produce colloidal AgNPs with smaller size and
narrow size distribution. Recently, green synthesis has come up as
a novel synthesis procedure for producing colloidal AgNPs with
controlled size, high stability and improved antibacterial activity
(Dehnavi et al., 2012).
Mechanistic Basis of Ant imicrobial Activity
of AgNPs
Antimicrobial efficacy of AgNPs was evaluated by many
researchers against a broad range of microbes, including
MDR and non-MDR strains of bacteria, fungi, and viruses.
Nano-sized metal particles are now well-established as a
promising alternate to antibiotic th erapy because th ey possess
unbelievable potential for solving the problem associated
with the development of multidrug resistance in pathogenic
microorganisms, hence also regarded as next-generation
antibiotics (
Rai et al., 2012). In particular, the use of AgNPs
has gained much attention in this regard (Jana and Pal, 2007;
Szmacinski et al., 2008; Stiufiuc et al., 2013
). Although, AgNPs
have been proved effective against over 650 microorganisms
including bacteria (both Gram-positive and negative), fungi
and viruses; however, the precise mechanism of their mode of
antimicrobial action is not fully understood yet (
Malarkodi et al.,
2013).
Nevertheless, some fundamental modes of antimicrobial
action of AgNPs have been recognized (Figure 2, Table 2).
Use of highly sophisticated techniques such as high resolution
microscopic (AFM, FE-SEM, TEM, and XRD), spe ctroscopic
(DLS, ESR spectroscopy, Fluorescence spectroscopy, Inductively
coupled plasma-optical emission spectroscopy, UV-vis),
molecular, and biochemical techniques have provided deep
mechanistic insights about the mode of antimicrobial action
of AgNPs (
Sondi and Salopek-Sondi, 2004; Kim et al., 2007;
Pal et al., 2007; Dehnavi et al., 2012; Rai et al., 2012). The
antimicrobial action of AgNPs is linked with four well-defined
mechanisms: (1) adhesion of AgNPs onto the surface of cell
wall and membrane, (2) AgNPs penetration inside the cell and
damaging of intracellular structures (mitochondria, vacuoles,
ribosomes) and biomolecules (protein, lipids, and DNA), (3)
AgNPs induced cellular toxicity and oxidative stress cause by
generation of reactive oxygen species (ROS) and free radicals,
and (4) Modulation of signal transduction pathways. Besides
these four well-recognized mechanisms, AgNPs also modulate
the immune system of the human cells by orchestrating
inflammatory response, which further aid in inhibition of
microorganisms (Tian et al., 2007).
Adhesion of AgNPs onto the Surface of
Cell Wall and Membrane
AgNPs exposure to microorganisms causes adhesion of
nanoparticles onto the cell wall and the membrane. The
positive surface charge of the AgNPs is crucial for the adhesion
(
Abbaszadegan et al., 2015). The positive charge confers
electrostatic attraction between AgNPs and negatively charged
cell membrane of the microorganisms, thereby facilitates AgNPs
attachment onto cell membranes. Morphological changes
Frontiers in Microbiology | www.frontiersin.org 5 November 2016 | Volume 7 | Article 1831