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International Journal of Nanomedicine 2017:12 1227–1249
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REVIEW
open access to scientific and medical research
Open Access Full Text Article
http://dx.doi.org/10.2147/IJN.S121956
The antimicrobial activity of nanoparticles:
present situation and prospects for the future
Linlin Wang
1,
*
Chen Hu
2,
*
Longquan Shao
2
1
Department of Stomatology, Hainan
General Hospital, Haikou, Hainan,
2
Department of Stomatology,
Nanfang Hospital, Southern Medical
University, Guangzhou, People’s
Republic of China
*These authors contributed equally
to this work
Abstract: Nanoparticles (NPs) are increasingly used to target bacteria as an alternative to
antibiotics. Nanotechnology may be particularly advantageous in treating bacterial infections.
Examples include the utilization of NPs in antibacterial coatings for implantable devices and
medicinal materials to prevent infection and promote wound healing, in antibiotic delivery
systems to treat disease, in bacterial detection systems to generate microbial diagnostics, and in
antibacterial vaccines to control bacterial infections. The antibacterial mechanisms of NPs are
poorly understood, but the currently accepted mechanisms include oxidative stress induction,
metal ion release, and non-oxidative mechanisms. The multiple simultaneous mechanisms
of action against microbes would require multiple simultaneous gene mutations in the same
bacterial cell for antibacterial resistance to develop; therefore, it is difficult for bacterial
cells to become resistant to NPs. In this review, we discuss the antibacterial mechanisms of
NPs against bacteria and the factors that are involved. The limitations of current research are
also discussed.
Keywords: antimicrobial activity, nanoparticles, oxidative stress, antimicrobial resistance
Introduction
Bacterial infections are a major cause of chronic infections and mortality. Antibiotics
have been the preferred treatment method for bacterial infections because of their
cost-effectiveness and powerful outcomes. However, several studies have provided
direct evidence that the widespread use of antibiotics has led to the emergence of
multidrug-resistant bacterial strains. In fact, super-bacteria, which are resistant to
nearly all antibiotics, have recently developed due to abuse of antibiotics. Studies
have shown that these bacteria carry a super-resistance gene called NDM-1.
1
The
major groups of antibiotics that are currently in use have three bacterial targets:
the cell wall synthesis, translational machinery, and DNA replication machinery.
Unfortunately, bacterial resistance can develop against each of these modes of action.
The mechanisms of resistance include expression of enzymes that modify or degrade
antibiotics, such as β-lactamases and aminoglycosides,
2
modification of cell compo-
nents, such as the cell wall in vancomycin resistance and ribosomes in tetracycline
resistance,
3
and expression of efflux pumps, which provide simultaneous resistance
against numerous antibiotics.
4
Most of the antibiotic resistance mechanisms are
irrelevant for nanoparticles (NPs) because the mode of action of NPs is direct contact
with the bacterial cell wall, without the need to penetrate the cell; this raises the hope
that NPs would be less prone to promoting resistance in bacteria than antibiotics.
Therefore, attention has been focused on new and exciting NP-based materials with
antibacterial activity.
Correspondence: Longquan Shao
Department of Stomatology, Nanfang
Hospital, Southern Medical University,
Guangzhou 510515, People’s
Republic of China
Tel +86 20 6278 7153
Email shaolongquan@smu.edu.cn
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Most bacteria exist in the form of a biofilm, which often
contains diverse species that interact with each other and their
environment. Biofilms are specifically microbial aggregates
that rely on a solid surface and extracellular products, such
as extracellular polymeric substances (EPSs).
5
Bacteria move
reversibly onto the surface, but the expression of EPSs renders
the attachment irreversible. Once the bacteria are settled, syn-
thesis of the bacterial flagellum is inhibited, and the bacteria
multiply rapidly, resulting in the development of a mature
biofilm. At this stage, the bacteria are stuck together, forming
a barrier that can resist antibiotics and provide a source of
systemic chronic infections. Thus, biofilms are a serious health
threat.
6,7
Moreover, the bacteria within biofilms can produce
superantigens to evade the immune system. Therefore, despite
the abundance of antimicrobial drugs and other modern anti-
bacterial agents, bacterial infections remain a major issue. The
chronic infections related to planktonic bacteria and biofilms
are always difficult to cure because of their inherent resistance
to both antimicrobial agents and host defenses. In particular,
biofilms are less restrained by antibacterial agents than the
respective planktonic bacteria are.
8
Nanomaterials are materials that have at least one dimen-
sion (1–100 nm) in the nanometer scale range or whose basic
unit in the three-dimensional space is in this range.
9
NPs in
particular have demonstrated broad-spectrum antibacterial
properties against both Gram-positive and Gram-negative
bacteria. For example, ZnO NPs were found to inhibit
Staphylococcus aureus, and Ag NPs exhibit concentration-
dependent antimicrobial activity against Escherichia coli
and Pseudomonas aeruginosa.
10
However, the detailed
antibacterial mechanisms of NPs have not been thoroughly
explained, and the same types of NPs often present contrasting
effects. The antimicrobial mechanism of action of NPs is gen-
erally described as adhering to one of three models: oxidative
stress induction,
11
metal ion release,
12
or non-oxidative mech-
anisms.
13
These three types of mechanisms can occur simul-
taneously. Certain studies have proposed that Ag NPs prompt
neutralization of the surface electric charge of the bacterial
membrane and change its penetrability, ultimately leading to
bacterial death.
14
Moreover, the generation of reactive oxygen
species (ROS) inhibits the antioxidant defense system and
causes mechanical damage to the cell membrane. According
to existing research, the major processes underlying the
antibacterial effects of NPs are as follows: 1) disruption
of the bacterial cell membrane; 2) generation of ROS; 3)
penetration of the bacterial cell membrane; and 4) induction
of intracellular antibacterial effects, including interactions
with DNA and proteins.
This review focuses on the mechanisms of bacterial
resistance and the antibacterial activity of NPs. Investigation
of the antibacterial mechanisms of NPs is very important for
the development of more effective antimicrobial materials.
Bacterial resistance to NPs
The primary reason why NPs are being considered as an
alternative to antibiotics is that NPs can effectively prevent
microbial drug resistance in certain cases. The rampant use
of antibiotics has led to the emergence of numerous hazards
to public health, such as superbugs that do not respond to any
existing drug and epidemics against which medicine has no
defense.
15
The search for new, effective bactericidal materi-
als is significant for combatting drug resistance, and NPs
have been established as a promising approach to solve this
problem.
16–18
However, NPs can also promote the emergence
of bacterial resistance in certain cases.
19
In this section, we
present the positive and negative aspects of the interactions
between NPs and drug-resistant bacteria.
Mechanisms of antimicrobial resistance
Bacterial resistance has become a serious problem due to the
massive application of antibiotics, which are used prophy-
lactically or remedially without proper medical indications;
the inappropriate selection of alternate antimicrobials; and
the frequent switching between antimicrobial treatments.
Drug-fast and multidrug-resistant bacteria have multiple
causes that can all be summarized as an interaction of intrinsic
and extrinsic factors. The latter factors mainly include the
sustained “selection pressure” of antibiotics and ecological
changes in the human microenvironment.
In the following sections, we describe the mechanisms
of the intrinsic factors from two different angles.
From the angle of genetics at the DNA level
We first analyze the internal causes at the gene level.
Resistance can be divided into intrinsic resistance and acquired
resistance according to the source of the resistance genes.
Intrinsic resistance can be caused by spontaneous mutation
of existing or exogenous genes, whereas acquired resistance
is caused by acquisition of resistance genes from another
organism. The emergence of multidrug resistance (MDR) in
particular is a result of the acquisition of different types of
drug resistance genes by the same bacterial cell.
20
In general,
intrinsic resistance is subordinate and less important. There
are three ways by which resistance can be transferred
and spread between bacteria: plasmids,
21,22
transposons,
23
and integrons.
24
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The antimicrobial activity of nanoparticles
From the angle of biochemistry at the protein level
Certain resistance mechanisms are a result of changes in
proteins, including specific types of enzymes and major
targets on the surface of cells. The main mechanisms with
respect to biochemistry are 1) the alteration of targets;
2) the generation of inactivated enzymes or passivated
enzymes; 3) the use of active efflux pump systems;
25,26
4) the
presentation of obstacles to antibiotic permeation; 5) the
formation of biofilms;
27
and 6) the emergence and elimination
of a specific protein, such as BamA
28
or KatG,
29
which can
affect infection through unknown mechanisms. Certain
bacteria show antimicrobial resistance through only one of
the mechanisms listed earlier, but two or more mechanisms
can also be combined in one type of bacterium, including
7) induction of an antagonist through metabolic pathways
or 8) increased production of a competitive inhibitor
counteracting the antibiotic.
15
Therefore, in the pre-NP era, three methods used to
overcome antibiotic resistance were the development of
new drugs, high-dose administration of an antibiotic,
6
and
the combination of multiple antibiotics.
30,31
However, the
production of novel antibiotics could not keep up with the
mutation of bacteria, and intolerable toxicity always accom-
panied high-dose treatment. These treatment strategies also
led to antibiotic misuse and the emergence of multidrug-
resistant strains.
The effects of NPs on microbial
resistance
Positive side: as an effective therapeutic method to
combat microbial resistance and multidrug-resistant
mutants
Increasing numbers of NP variants and NP-based materials
have been used as a new line of defense against microbial
resistance and MDR.
32,33
Different types of NPs have different
mechanisms for combating microbial resistance. Various
antibacterial mechanisms of NPs according to the metabolic
process involved are presented in the “Antibacterial
mechanisms of NPs” section.
One of the accepted relationships between nanomateri-
als and antibacterial activity is as follows: “Nanomaterials
as antibacterial complements to antibiotics are highly
promising and are gaining large interest as they might fill
the gaps where antibiotics frequently fail.”
16
In addition,
nanomaterials can complement and support traditional
antibiotics “as a good carrier.”
34
This section focuses on the
distinct features and complementary advantages of using
NPs/nanotechnologies as antibacterial agents compared with
traditional antibiotics, which can be summarized as follows:
1) overcoming the existing antibiotic resistance mechanisms
that are listed in the “Antibacterial activity of NPs” section
including the disruption of bacterial membranes and the
hindrance of biofilm formation,
17
2) combatting microbes
using multiple mechanisms simultaneously,
17
and 3) acting
as good carriers of antibiotics.
Overcoming the existing antibiotic resistance
mechanisms
Most types of NPs can overcome at least one of the common
resistance mechanisms mentioned in the “Antibacterial
activity of NPs” section (including the disruption of bacterial
membranes and the hindrance of biofilm formation).
17
These
effects are a result of the bactericidal mode of NPs, which
is based on their specific physicochemical properties.
35
In contrast to traditional antibiotics, NPs have characteristic
dimensions ,100 nm. Their uniquely small size results in
novel properties, such as greater interaction with cells due to
a larger surface area-to-mass ratio and versatile and control-
lable application.
6
The mechanisms by which NPs disrupt bacterial
membranes are described in detail in the “Antibacterial
process of NPs” section; rather, the interaction of NPs with
cell barriers (including cell walls and membranes) and the
synthesis of bacterial proteins are considered in this section.
The bacterial cell membrane is difficult to change through
only a few genetic mutations because of its highly conserved
nature, which further reduces the probability of bacterial
drug resistance.
In addition to the disruption of bacterial membranes,
hindrance of biofilm formation is an important mechanism,
as biofilms play an important part in the development of
bacterial resistance.
36
The unique composition and structure
of bacterial biofilms provide shelter or protection to the
embedded microorganisms, helping them to escape from
most antibiotics. In addition, bacterial biofilms are “a breed-
ing ground” for frequent resistance mutations and the
exchange and alteration of these mutations among different
bacterial cells.
15
Studies have shown that many NPs can prevent or over-
come biofilm formation, including Au-based NPs,
37
Ag-based
NPs,
38
Mg-based NPs,
39
NO NPs,
40,41
ZnO NPs,
7
CuO NPs,
42
Fe
3
O
4
NPs,
43
and YF NPs.
44
Greater prevention of biofilms is
achieved by a smaller size and higher surface area-to-mass
ratio, and the particle shape of NPs also has a remarkable
effect on biofilm destruction (eg, NPs with a rod like shape
are more effective than NPs with a spherical shape).
41
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Combatting microbes using multiple mechanisms
simultaneously
The antimicrobial mechanism of traditional antibiotics is
usually relatively simple, which is partly why bacterial
resistance has emerged. In contrast to traditional antibiotics,
NPs combat microbes via multiple mechanisms that are
simultaneously active. The advantage of these simultaneous
mechanisms is obvious: a microbe is unlikely to have
multiple mutated genes, so it is much more difficult to
develop resistance to NPs.
Acting as good carriers of antibiotics
NPs not only can combat bacterial and microbial resistance
themselves, as mentioned earlier, but also can act as a
“medium and carrier” of antibiotics. However, the mecha-
nisms of NP-based drug delivery are different from those
presented earlier.
Several types of NPs are currently used for drug delivery:
liposomal NPs,
45
solid lipid (SL) NPs,
46,47
polymer-based
NPs, polymer micelles, inorganic nanodrug carriers (includ-
ing magnetic NPs, mesoporous silica NPs, carbon nano-
materials, and quantum dots), terpenoid-based NPs,
48
and
dendrimer NPs.
49
As a carrier for the delivery of antibiotics, the main
advantages of NPs compared with conventional delivery
systems are as follows:
Size: The ultra-small and controllable size of NPs
is suitable for conducting antimicrobial operations and
combating intracellular bacteria.
50
The treatment of infections caused by intracellular
pathogens and strains with drug resistance is more complex
using antibiotics
51
because of antibiotics’ poor membrane
transport. Drugs of average size thus have little effect on
intracellular microbes. A modified treatment method using
drug-loaded NPs as intermediaries has been proposed to
overcome this limitation. The size of most types of NPs is so
small that they are easily phagocytosed by host phagocytes.
Moreover, the structures of many types of NPs are suitable
for carrying drugs (such as liposomal NPs, whose walls are
composed of one or more lipid bilayers surrounding sphere-
shaped NPs),
52
and the flexibility of NPs to enter host cells
via endocytosis makes it possible for most of the drug to be
released intracellularly.
Protection: NP carriers can help to increase the serum
levels of antibiotics and protect the drugs from resistance
by target bacteria.
Within NP carriers, drugs are protected from detrimental
chemical reactions; thus, the potency of the drugs can be
maintained. In addition, protection from the resistance mech-
anisms of the target bacteria is an important mechanism.
6
Increased efflux and decreased uptake of antibiotics in
bacterial cells (such as in P. aeruginosa and E. coli) are two
common and important reasons for resistance to traditional
antibiotics. However, researchers have shown that many NPs
can overcome these mechanisms,
18
inhibiting drug resistance.
For example, in the gastrointestinal tract, dendrimers can
inhibit P-glycoprotein-mediated efflux.
49
Precision and security: NP carriers can help to target
antibiotics to an infection site and thereby minimize systemic
side effects.
It is difficult to encourage high-dose drug absorption
at the desired site while preventing side effects (including
drug toxicity) when using conventional antibiotics without
a carrier. NP-based antibacterial drug delivery systems
deliver the drug to the site of action and therefore reduce
the side effects. The undesired adverse effects of antibiotics
on the body are specifically weakened because of the higher
dose delivered to the site of infection.
Targeted NP-based drug delivery consists of passive
targeting or active targeting. Passive targeting is achieved
through enhanced permeation and retention at the infection
site, and active targeting is achieved though surface modifi-
cation of NPs, allowing the NP-based drug delivery system
to selectively recognize specific ligands on the cells at the
infection site. Active targeting includes receptor targeting,
magnetic targeting, and temperature targeting.
Vancomycin strongly inhibits Gram-positive bacteria.
However, vancomycin has strong ear and kidney toxicity.
One way to improve treatment would be to increase drug
delivery to the desired location, thus limiting the amount of
drug reaching organs where it is unnecessary. With the help
of NP carriers, vancomycin-modified mesoporous silica NPs
(MSNs is a subset of Van) were designed, which made it
possible to detect and kill pathogenic Gram-positive bacteria
selectively over macrophage-like cells.
52
An effective and crucial strategy frequently employed to
achieve “target therapy” is to first target macrophages with
NPs because most active bacteria at infection sites can be
targeted and swallowed by macrophages. The drugs in the
NPs are then released in the macrophages in which bacteria
are present.
53
Controllability: Sustained and controllable release of
antibiotics can be achieved flexibly.
With a conventional delivery method, the blood drug
level is maintained for a short time in a relatively large
range that can exceed the maximal tolerated dose or fail to
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The antimicrobial activity of nanoparticles
reach the lowest effective dose. As a result, repeated dos-
ing is indispensable, with associated side effects. With the
appropriate NP carrier or method of drug release, the blood
concentration of the medicine at the infection site can be
sustained at the required effective level for a long time,
resulting in good stability, reduced frequency of medication,
improved patient compliance, and reduced patient pain.
Compared with free drug at the same concentration, drug
delivered via an NP carrier has a much more prominent
inhibitory effect on cellular growth, along with prolonged
drug release.
54
Moreover, NPs can be activated by different
types of controllable stimulatory factors (such as chemical
agents, a magnetic field, light, pH, and heat).
55–57
In ocular remedies delivered using an appropriate SL NP
carrier that can prolong the retention time in the pre-corneal
area, the release of levofloxacin and other drugs is sustained
and controlled, producing a better curative effect compared
with conventional ophthalmic solutions.
58,59
Combination: Multiple drugs or antimicrobials can be
packaged within the same NP, and NPs can be combined with
other constructs to improve the agents’ antibacterial properties.
In this article, two levels of “combination” are described.
On the one hand, when faced with a single type of NP
containing multiple antibacterial agents, it is difficult for
bacteria to be resistant or to develop resistance because the
probability of a cell containing multiple resistance mutations
is very small. In addition, the simultaneous combination of
different drugs results in higher efficiency due to the joint
action of multiple mechanisms.
On the other hand, two or more types of NPs can be
used in combination for enhanced antibacterial effects
and prevention of resistance.
6
When used alone, different
types of NPs have distinct disadvantages. For example, the
disadvantages of liposomes are their short shelf life, poor
stability, low encapsulation efficacy, rapid removal by the
reticuloendothelial system, cell interactions or adsorption,
and intermembrane transfer. The disadvantages of SL NPs
are an unpredictable gelation tendency and inherent low
incorporation rates.
49
Hybrid NPs can maximize the strengths
while minimizing the weaknesses of the individual types of
NPs. For example, studies have shown that superior efficacy
of in vivo cellular delivery can be achieved by lipid–polymer
hybrid NPs compared with delivery without polymeric NPs
or by liposomes.
60
In addition, a prolonged effective time can be achieved
through the “combinatorial” method, which can effectively
and significantly reduce the possibility of the development
of resistance in bacteria.
61
The abovementioned advantages may unite in diverse
combinations with different emphases in the process of
actual application.
Negative side: as a promoter of drug resistance
With more and more research available, knowledge of the
effects of NPs is beginning to develop from a single, positive
angle, even as researchers try their best to be unbiased.
While most of the studies mentioned earlier have shown
evidence that the use of NPs as an antibacterial agent can
effectively reduce the rate of resistance, the existence of a
“pushing hand” is undeniable under certain experimental
conditions, such as “in water along with the proper
temperature and pH”.
19
This phenomenon has been investigated. One study
19
reported that the conjugative transfer of plasmids (such as RP4,
PK2, and pCF10) could be promoted by aluminum NPs, which
further resulted in the dissemination of MDR among bacteria
not only of the same species but also across genera. The factors
related to this promotion are as follows: 1) the extent of dam-
age to the bacterial cell membranes resulting from oxidative
stress caused by the aluminum NPs; 2) the concentrations of the
aluminum NPs and mating cells; 3) the suitable environment,
including the temperature and pH, which affects the transfer in
water; and 4) the selectively promoted expression of specific
genes (such as trfAp, trfA, and trbB) that are important for the
transfer and replication of RP4 plasmids.
The negative effects also require attention to prevent the
promotion of MDR, which may result in further hazards to
public health and the environment.
The application of NPs
Resurgent interest in NPs has been stimulated by the
appearance of drug-resistant bacteria and the increasing
rate of hospital infection outbreaks. Due to their excellent
antimicrobial resistance properties, NPs have been widely
used in many fields. Each type of NP has its own advantages
and localization. Parameters including the mean particle
size, shape, the specific surface area, and surface curvature
affect the antibacterial activity and mechanism. In fact,
the application of NPs in fighting bacteria has decreased
bacterial infection. In the following sections, the antibacterial
applications of NPs are discussed in detail.
Antibacterial application of NPs
Antibacterial coating of implantable devices
There are two types of human implantable devices with
antimicrobial coatings; the first type is fully implantable
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