Nanoparticles as a Platform for Antimicrobial Drug Delivery

The hasty developments in nanotechnology will have intense impact on various areas of therapeutic applications like drugs and bioactive delivery, tissue engineering, and other application. This review mainly emphasized on the nanoparticle systems for delivery of antimicrobial drug. Various antimicrobial agents have been used to kill or inhibit the growth of microbes such as bacteria, fungi and viruses. As the pharmacological actions of these drugs have been well established, feeble delivery and degradation could lead to insufficient therapeutic index and cause side effects. Nanoparticles have many advantages like small and controllable size, large surface area to mass ratio, and functionalizable structure. These properties make the nanoparticles, a suitable delivery system of antimicrobials. Along with this, it also overcomes the limitations intra-additional antimicrobial therapeutics. In last few years, development and fabrication of antimicrobial agent in nanoparticle systems has evolved as an innovative and promising alternative that increases therapeutic effectiveness and overcoming the undesirable side effects. In this chapter, we focused on the current progress and challenges in producing nanoparticle system for antimicrobial drugs delivery for the treatment of microbial diseases.


Introduction
Microbial infections remain foremost causes of morbidity and mortality in hospitals around the world. The innovation of miracle drugs called antibiotics in the 20 th century leading a histrionic reduction in death and illness from these infectious diseases. Plentiful antimicrobial agents have been used to eradicate the microbes. Despite the fact the therapeutic efficiency of available drugs and treatment has been well proven, incompetent and deprived delivery of antimicrobial agents could consequence in insufficient therapeutic index and causing adverse effects like nausea, vomiting, irritation, scaling and gut microbial flora reduction. Along with developing multidrug resistant microbial strains, however, variations in the society, environment, technology and developing microorganisms are contributing to the emergence of new diseases and development of antimicrobial resistance (1). The continuing presence of antibiotic resistance in pathogenic and opportunistic microorganisms forces the pharmaceutical companies and the researcher community to continuously develop novel antibacterial drug, drug targets and delivery system to improve the activity against multidrug-resistant bacteria. In the current scenario, the progress of molecular nanostructures through precise particle size and shape is of prominent interest in biomedical applications like delivery of antimicrobial agents. Nanoscale materials/polymers have emerged up as novel antimicrobial agents and delivery system because of high surface area to volume ratio and distinctive properties (2; 3). Due to various metallic and polymeric nanoparticles (4) have confirmed antimicrobial activities, the development of novel applications in this field makes them an attractive alternative to antibiotics.
Earlier, before discovery of chemotherapeutics, many herbs and various inorganic antimicrobials such as silver and copper etc. were used to cure microbial infections. In the recent times, the advances in the field of nanoscience and nanotechnology has provide opportunities for the nanosized inorganic and organic particles which are leading applications as improvements in industrial, medicine and therapeutics, synthetic textiles and food packaging products (5; 6). Nano-technological applications in medicine have generated an entirely new field of technology that is set to bring momentous advances in the management of various 32 Nanoparticles as a Platform for Antimicrobial Drug Delivery diseases (7).
In earlier decades, the application of nanotechnology, particularly the use of nanoparticles for antibiotic/drug delivery, has produced noteworthy impact in management of infectious diseases. Various systems for delivery of nanoparticles like gels, liposome, polymeric nanoparticles, dendrimers, and inorganic nanoparticles, have acknowledged momentous attention. Antimicrobial molecules loaded into nano-carriers by different methods like chemical synthesis, cross-linking, physical encapsulation, adsorption, to show an improved pharmacokinetic profile and therapeutic index when related to their free form (8). Innumerable nanoparticle-based drug delivery systems have been approved for clinical use to treat a variety of infectious diseases, and a number of formulations are now under various stages of pre-clinical and clinical tests (9). Due to advancement in fabrication, green synthesis multifunctional nanoparticles, various inventive approaches have emerged, more work on improving on nanoparticle therapeutic efficacy against microbial infection. In this chapter, we discuss about nanoparticle approaches having significant potential to improve upon current treatments. Researches and investigations were made in this arena provide wonderful prospects for alternative and more effective antimicrobial approaches that amend the pharmacokinetics properties of antibiotics and produce novel antibiotics with novel inhibition mechanisms.

Nanoparticles
The focus of pharmaceutical research is being steadily shifted from the development of new chemical entities to the development of novel drug delivery system of existing drug molecule to maximize their effect in term of therapeutic and patient protection (10). Various novel drug delivery system like transdermal patches, Microspheres, Nanoparticles, Resealed erythrocytes, Ocusert, Depots, Liposomes, Niosomes, Buccal films etc., offer several advantages over the conventional drug delivery systems (11). Moreover, the development of novel drug delivery systems is going to be the outmost need of pharmaceutical industry especially after enforcement of product patent. Nanoparticles attract an increasing interest in contemporary drug research because they could be used as a very efficient drug delivery system. Nanotechnology is evolving as a rapidly growing field with its application in various fields of science and research for the purpose of producing new substances and molecules at the nanoscale level (12). The word "nano" is used to show one billionth of a meter or 10 -9 . The term "Nanotechnology" was first used by a Japanese scientist Professor Norio Taniguchi in 1974 to define precision manufacturing of materials at the level of nanometer. Nanotechnology deals with novel technology that provides a platform for development and exploration of biological systems, and offers various stimulus models for bio-assembled components. Nano-structured materials whichever inorganic or organic exhibited numerous features like optical and catalytic. These properties mainly depend on the size and shape of prepared nanoparticles.
Nanoparticles are defined as particulate dispersions or solid particles or groups of atoms in the size range of 1-100nm. "Nano" is a Greek word identical to dwarf means extremely small. The use of nanoparticles is getting impulse in the present century as they having defined chemical, optical and mechanical properties.
Nanoparticles, first developed around 1970, they were initially devised as carriers for vaccines and anticancer drugs. The drugs may be enclosed inside the sphere of the nanoparticle or linked to the surface. Once they are at the target site, the drug payload may be released from the nanoparticle by diffusion, swelling, erosion or degradation. Active systems are also possible, e.g. drug release in response to the input of external energy such as targeted ultrasound, light or magnetic field. Nanoparticles have been successfully used in nano-chemistry to enhance the immobilization and activity of catalysts, (13) in medical and pharmaceutical nano-engineering for delivery of therapeutic agents, (14) in chronic disease diagnostics, and in sensors (15). Nanoparticles have been also used in clothing and in the food industry to limit bacterial growth (16; 17).

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Bulk material should have constant physical properties regardless of its size, but at the nanoscale size-dependent properties are often observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant.  For bulk materials larger than one micrometer (or micron), the percentage of atoms at the surface is insignificant in relation to the number of atoms in the bulk of the material. The interesting and sometimes unexpected properties of nanoparticles are therefore largely due to the large surface area of the material, which dominates the contributions made by the small bulk of the material.  Nanoparticles of usually yellow gold and gray silicon are red in color; gold nanoparticles melt at much lower temperatures (nearly 300 o C for 2.5 nm size) than the gold slabs (1064 o C).  Suspensions of nanoparticles are possible since the interaction of the particle surface with the solvent is strong enough to overcome density differences, which otherwise usually result in a material either sinking or coating in a liquid.  Nanoparticles also often possess unexpected optical properties as they are small enough to connect the electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution.

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The high surface area to volume ratio of nanoparticles provides a tremendous driving for drug diffusion.  Moreover, nanoparticles have been found to impart some extra properties to various day to day products. For example the presence of titanium dioxide nanoparticles imparts what we call the self-cleaning effect, and the size being nano-range, the particles cannot be observed. Zinc oxide particles have been found to have superior UV blocking properties compared to its bulk substitute. This is one of the reasons why it is often used in the preparation of sunscreen lotions, and is completely photostable.

Advantages of Nanoparticles
Nanoparticles can offer significant advantages over the traditional delivery mechanisms in terms of various aspects (18). Nanoparticles have capability of both passive and active drug targeting due to its particle size and surface characteristics. With the use of nanoparticles, there is increase in drug therapeutic efficacy and reduction in side effects due to altering organ distribution of the drug and subsequent clearance of the drug. Nanoparticles also control and sustain the release of the drug. On the basis of choice of matrix constituents, nanoparticles release and degradation properties can be easily modulated. Nanoparticles also have an advantage of Site-specific targeting. It can be achieved by using targeting ligands to surface of particles. Nanoparticles used for various routes of administration including oral, nasal, parenteral, intraocular etc. Along with these nanoparticles have high stability, capability to transport both hydrophilic and hydrophobic molecules, high ratio of surface area to mass, high reactivity, prolonged systemic circulation, precise drug targeting and concurrent delivery of multiple drugs (18).

Limitations of Nanoparticles
In spite of these advantages, nanoparticles do have limitations.
 For example, their small size and large surface area can lead to particle-particle aggregation, making physical handling of nanoparticles difficult in liquid and dry forms.  In addition, small particles size and large surface area readily result in limited drug loading and burst release. Table 1 state about the different parameters for characterization nanoparticles and instruments used for these characterizations.

Nanoparticles as Platform for Antimicrobials Delivery
These characteristic physicochemical properties have advantages in delivering and targeting to the target site, consequently minimizing adverse effect. Additionally, these could move in cells through endocytosis and consequently release the antibacterial agent to eliminate infections. In fact, a number of researches on nanoparticles based antibiotics have been developed and reviewed.

Metallic Nanoparticles
The continuous growing resistance of microbes to antibiotics causes severe health complications in current years. To confrontation with these problems, modern researcher identified the novel application by combining nanotechnology with intrinsic antimicrobial activity of the metals. Metallic nanoparticles are being utilized in many field of science with engineering comprising medical fields also and are still attract the scientists to invent new dimensions for their respective worth which is generally credited to their corresponding small sizes. In present scenario, the research in nanoparticle field is mainly focused on its synthesis or green synthesis not much on its applications. Research mainly focused on green synthesis because of disadvantages of physical and chemical methods like considerable environmental defect, laborious and expensive (19). The metallic nanoparticles are most promising because of good antibacterial properties due to their large surface area to volume ratio, which is coming up as the current interest in the researchers due to the growing microbial resistance (20). Various Metals like copper, zinc, titanium, magnesium, gold (21) and metal oxides have been widely studied for their antimicrobial activities. Metal oxide nanoparticles, recognized due to its highly potent antibacterial effect, include iron oxide (Fe 2 O 3 ), titanium oxide (TiO 2 ), copper oxide (CuO), and zinc oxide (ZnO). These nanoparticles mainly synthesis by either reducing the particle size by mechanical/ ball milling, chemical etching, microwave decomposition, thermal/ laser ablation and particle sputtering by chemical precipitation, microwave hydrothermal method, electrochemical precipitation, hydrothermal synthesis, vapor deposition, solvo-thermal method, atomic and molecular condensation, sol gel process, spray pyrolysis, laser pyrolysis, aerosol pyrolysis or by green synthesis using bacteria, plant extract and fungus (22). In the Table 2, 3 and 4, we briefly discussed about various metallic nanoparticle, their mechanism of action, application and limitation and toxicity.     Membrane disruption and Reactive oxygen species production Interact with DNA molecules and intercalate with nucleic acid strands.
Disrupt biochemical processes. 43 9. Aluminum oxide Increases diffusion and accretion inside the microbial cells Create spit formation, increases perforation, and causes membrane disorganization and also causes microbial cell death 51 Gold nanoparticles (4-5 nm) Inflammation, accretion of gold in lungs, and an increased number of macrophages 53

Polymeric Nanoparticles
Polymeric nanoparticles with biodegradable and biocompatible polymers are good candidates as particulate carrier for antimicrobial agent and drug delivery and there has been considerable interest in the use of nanoparticles as potential delivery systems. Numerous investigations have shown that nanoparticles can not only improve the stability of therapeutic agents against enzymatic degradation, but by modulating polymer characteristics, they can also achieve desired therapeutic levels in target tissues for the required duration for optimal therapeutic efficacy (54, 1). Depending on their composition and intended use, they can be administered orally, parenterally, or locally (54). Different nanoparticles manufacturing methods were described allowing modification of physicochemical characteristics such as size, structure, morphology, surface texture and composition to meet different requirements.
Nanoparticles act on microbes by releasing antibiotics, antimicrobial peptides, and antimicrobial agents or by contact-killing like quaternary ammonium compounds etc. Cationic groups (with long hydrophobic chains) are capable to lysis the membrane to penetrate and burst the bacterial membrane and lead to death (52,55).
Lots of the nanoparticle therapeutics in research are polymeric nanoparticles, which have been comprehensively explored as platform for drug delivery. Polymeric nanoparticles are fabricated from biocompatible and biodegradable polymers. Polymeric nanoparticles are formulated by a self-assembly process using block-copolymers comprising of two or more polymer chains with different hydrophilicity. Polymeric nanoparticles have been framed to encapsulate either hydrophilic or hydrophobic drug molecules, as well as macromolecules such as proteins, peptides and nucleic acids (56).
The main issues in this field are the loading efficiency, stability of bioactive agent during preparation and release, release profiles and surface modification. Particles size and surface property (surface charge and hydrophobic or hydrophilic property) are primary factors for the in-vivo fate of nanoparticles. Surface modification of nanoparticles has been achieved mainly by two methods:  Surface coating with hydrophilic polymers/surfactants  Development of biodegradable copolymers with charged functional group or hydrophilic segments.

Methods of Preparation of Polymeric Nanoparticles
Method of preparation are mainly depends upon the physico-chemical characteristic of polymers and the drugs. They are classified as-  Gelatin Enhanced delivery and significant higher expression level.

Chitosan
Enhanced delivery and significant higher expression level.
PLGA Enhanced delivery and significant higher expression level.
Poly-alkyl cyano acrylate Improve absorption /permeation.   Nanoparticles as a Platform for Antimicrobial Drug Delivery

Techniques to Determine Antibacterial Activity of Nanoparticles
Various experimental methods and assays are available for measuring the antibacterial efficacy of nanoparticles. Each methods are unique and having their advantage and disadvantage. These methods/assays are (85)

Conclusions
In summary, it can be concluded that nanotechnology developed as a new approach in pharmaceutical sciences and research grasps great promise for overcoming complications related to delivery of antimicrobial and infection curing approaches. Various antimicrobial substances are unusable due to their instability, physicochemical limitations, enzyme degradation and cytotoxicity. Along with these problems, many microbes and their strains were getting resistant to the available antibiotics become as problem that surges the necessity to develop novel antibacterial substances. These issues were reasons for emergence of nanotechnology that provide many option for antimicrobials. The small size of the nanoparticles is very suitable for delivery of antibacterial substances.