In spite of the advancement of medical and pharmaceutical sciences, the chemotherapy is still a major problem for delivery drugs to specific site of interest against various life-threatening infectious diseases. Most of the drugs having high toxicity, leads to several side effects, reducing the quality of life. The use of conventional microbicidal agents against the infections has associated inadequate therapeutic index, low drug bioavailability, development of multiple drug resistance and adverse systemic side effects. In this concern, antimicrobial silver nanoparticle has emerged as potent efficient agent against infection due to its ultra small controllable size as high surface area and increased reactivity with active functional structure. The surface ligand coating of silver nanoparticle incorporated drug as drug delivery vehicle enlightens its sustained release with reduced side effects when administered into the body. This review also focuses on the mechanism of action of the silver nanoparticle system as antimicrobial drug targeting.
Keywords:Silver nanoparticles; Drug delivery vehicle; Infection; Toxicity; Drug resistance; Mechanism of action
When microorganisms such as viruses, bacteria and parasites enter into the body, innate immunity functions through phagocytes mainly macrophages as a first line of defense against these infectious agents and checks before they develop an overt infection. If these first defenses become hampered, the adaptive immune system mediated by lymphocytes becomes activated producing a specific reaction to each infectious agent for its eradication and further preventing by memorizing infectious agent to cause disease later. During infection, the serum concentration of C-reactive protein (CRP) becomes elevated.
This CRP, in turn, binds to C-protein of microorganisms’ cell wall promoting binding of complement to facilitate their uptake by phagocytes such as macrophages through chemo taxis opsonisation. The destruction of microorganisms’ cell wall in the phagolysosomal compartment of macrophages by proteolytic lysozyme facilitates an attack on the cell membrane by the complement system.
Interferon’s (IFNs) compose a group of proteins that are significant in viral infections. When host cells become infected by viruses, they may produce IFNs. Different types of cell when become infected, produce IFN-α or IFN-β whereas T-lymphocytes produced in the thymus when become activated by antigen release IFN-γ. These IFNs, altogether, function on uninfected cells to induce a state of antiviral resistance.
In cell mediated immunity, antigen-presenting cells present processed antigen to helper T cells, which represent central to the development of immune responses. These events can help B cells, produced in bone marrow and fetal liver, to make antibodies and modulate the activities against infections of varieties of other effectors cells, including natural killer cells, granulocytes, macrophages, cytotoxic T cells and antibody-dependent cytotoxic cells. Many of these effects become mediated by lymphokines, as well as cytokines, specifically macrophages, though both T and B cells may be influenced by suppressor T cells.
In the development of the state of diseases, micro-organisms proliferate in the phagolysosomes of the host macrophages through escaping proteolytic effect of lysozyme and suppressing protective host immune responses. Therefore, infectious diseases, caused by intra or extra cellular infection, biofilm or
medical device-mediated infections, have demonstrated a global
public health hazards causing millions of deaths every year.
Though applications of antibiotics in the 20th century
established a sensible reduction in public illness and death
caused by the infectious microorganisms, the development
of antimicrobial drug resistance  has created an emerging
problem in public health. The development of new antibiotics
and chemical modifications of existing antimicrobial drugs can
not only resolve the problem of microbial resistance but also
requires a more long term effective metallic nanotechnology in
medicine against infectious diseases .
Silver nanoparticles (AgNPs) in suspension release silver
ions [3-5] which are highly toxic to microorganisms. This
microbicidal activity depends on the structure, dimension,
size, concentration, ionic strength, coating, temperature, and
time on the dissolution behavior of AgNPs [5-7]. Additionally,
this material with surface coating can be used as a carrier for
delivering a wide range of therapeutic components such as
drugs, antibodies and pharmaceuticals in several biomedical
applications . Controlled release of biologically active silver
from nanosilver can be regulated by different ligand coatings
in preventing silver ions from releasing and increasing half-life
in systemic circulation for passive targeting [5,7,9]. Moreover,
surface functionalizations on this AgNP can be made by
decorating various ligands such as sugars, proteins, peptides
and genetic materials for active targeting which require further
investigations on their therapeutic efficacies. Taken together,
this review focuses that ligand coated drug incorporated AgNPs
may be useful as potent oral therapeutic tool in reducing toxicity,
enhancing release, improving solubility and bioavailability,
and providing better formulation for synergistic effect against
pathogenic microbial diseases.
AgNPs incorporated nanodrug conjugate is prepared
chemically by reduction of metal salt precursor i.e. 0.01M AgNO3
with 0.01M NaBH4 reducer followed by the addition of 0.001mg
drug and kept under stirring for 2h . The homogeneous
slurry is spun at 4000 rpm for 25min to get a pellet which is
lyophilized and used for further investigations
Chitosan coated silver nanodrug conjugate is prepared by
ionotropic gelation method. Silver nanodrug conjugate is mixed
with 100mg chitosan and 1% acetic acid. Sodium triphosphate
solution is then added drop wise and kept stirring for 2h. The
obtained slurry is lyophilized and used for further studies.
Citrate-capped AgNPs are synthesized following the method
 while 5mM sodium citrate and 25μM tannic acid are added
into the 250μM AgNO3 solution.
The mixture is stirred continuously and refluxed until the
solution turns to light yellow. Citrate AgNPs are purified by ultra
filtration, washed and stored at 4 ᵒC for future use. The other types
of ligands such as mercaptopropionic acid, mercaptohexanoic
acid, mercaptopropionic sulfonic acid, polyvinylpyrrolidone,
polyethylenimine, mono, di or poly -ethylene glycol, and sugars
are also used to coat AgNPs [12,13].
Since time immemorial, silvers are familiar for their broad
spectrum of antimicrobial activities. The antimicrobial potential
of AgNPs has been increased by reducing their size to less than
10nm with modified surface dimension  which is useful to
combat also multidrug resistance where multidrug resistant
proteins and P-glycoprotein are responsible for effluxing
drugs . Various investigations have sought to establish a
mechanism of action of AgNPs against pathogenic microbes.
Under ambient condition, O2 molecules may chemically adsorb
on the 111 facet of AgNP surface and oxidize the surface Ag atom
to form Ag+ ions while O2 molecules are incompletely reduced to
reactive oxygen species (ROS) such as 02.-, .OH.
The released positive silver ions can bind negatively
charged cell membrane to interfere membrane integrity .
Furthermore, the endocytosed or pinocytosed AgNPs may
exhibit a Trojan-horse-type effect to release silver ions in the
cytoplasm to interact with organelles e.g. mitochondria .
The intracellular Ag+ ions released from AgNPs can bind with
thiol groups (-SH) of proteins and enzymes located on the
cellular surface, causing cellular membrane destabilization
and mitochondrial ATP synthesis breakdown resulting ROS
generation that induces oxidative stress leading to irreversible
damage to DNA replication [17-19]. Ag+ ions may also adhere to
the membrane wall, causing holes through which they also can
penetrate cell-inside microbes and interact with intracellular
components as well as protein containing sulphur.
Several investigators studied the levels of aspartate
aminotransferase (AST) and alanine aminotransferase (ALT),
the markers for hepatotoxicity, in plasma after oral exposure of
AgNPs and demonstrated that AST and ALT levels do not increase
despite the higher oral exposure (>500mg/kg bw) of AgNPs
(~60nm) for 28-day except mild enhancements of alkaline
phosphatase and cholesterol levels signifying no indication of
acute hepatotoxicity [20-23].
Several attempts were made to evaluate immunotoxic
responses after oral exposure of AgNPs. Some reports
demonstrate that not only serum levels of IgG and IgM but also
proliferations of T-or B-cells isolated from spleen and mesenteric
lymph nodes in response to lipopolysaccharides or concanavalin
A were not significantly altered by the exposure of AgNPs.
Furthermore, oral AgNPs exposure did not affect the levels of cytokines in the supernatants of the stimulated T- and B-cells
as well as the activities of natural killer cells isolated from the
spleen. Therefore, these results suggest that nonspecific immune
responses do not occur in vivo by the oral AgNPs administration
Conventional antimicrobial therapies using antibiotics and
other agents have raised an issue on drug resistance acquired by
the infectious microbes in several diseases. Though it has been
emphasized on the development of new antibiotics and chemical
modification of existing drugs to solve the problem, metallic
nanoparticles have emerged as new potential antimicrobial
agents due to their ultra-small size, high surface to volume
ratio and unique physicochemical properties stemmed from
interactions with microorganisms including cellular uptake
and aggregation leading to toxicity and microorganism killing
. Ligand-dependent Ag+ release with drug may offer potent
synergistic antimicrobial activities not only for drug but also for
AgNPs due to their short carbon chain and weak binding atom of
oxygen. Therefore, the optimization of the surface ligands such
as coordination atoms, carbon chain lengths and terminal groups
is very important to prepare nanoparticles for commercial
applications against infectious diseases.