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Nanoparticles as antimicrobial agents against medically important pathogens


Priyanka Khati
Department of Microbiology, College of Basic Sciences and Humanities, GBPUA&T Pantnagar, U.S Nagar

Abstract

 Nanotechnology is gaining tremendous impetus in the present century due to its capability of modulating metals into their nano size, which drastically changes the chemical, physical and optical properties of metals. The extremely small size of nanoparticles help them to occupy a greater surface area and hence a better accessibility and targeted action.  Nanoparticles can overcome existing drug resistance mechanisms, including decreased uptake and increased efflux of drug from the microbial cell, biofilm formation, and intracellular bacteria. Nanoparticles can target antimicrobial agents to the site of infection, so that higher doses of drug are given at the infected site, thereby overcoming resistance. Antibacterial nanoparticles are also very important in the textile industry, water disinfection, medicine, and food packaging. Organic compounds which are used for disinfection have some disadvantages, including toxicity to the human body; therefore, the interest in inorganic disinfectants such as metal oxide nanoparticles (NPs) is increasing. Present review focuses the wide application of different antimicrobial nanoparticles against various pathogens infecting plants and animals as well.  Chitosan based nanoparticles are effective against broad range of gram positive and gram negative bacteria, which can be applied for medical applications due to less toxicity caused. Various carbon based nanoparticles like carbon nanotubes, fullerene and graphene oxide nanoparticles are toxic against the pathogens like Escherichia coli, Staphylococcus aureus, Candida albicans and Bulkholderia cepecia and various others which are animal and plant pathogens. Metal oxide nanoparticles like Al2O3, Fe3O4, CeO4, ZnO, MgO, TiO2 etc. are very useful against Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella sp. and Streptococcus sp. which are medicinally important pathogens. Besides this other nanoparticles like gold nanoparticles, copper nanoparticles and silver nanoparticles are also gaining their attention due to wide host range and targeted action.

Keywords: Nanoparticles, antimicrobial agents, metal nanoparticles, drug resistance

INTRODUCTION

Pathogenic microorganisms have always been a sizeable threat to plant and animal life. Infectious diseases were one of the paramount causes of mortality till the late 19th century, but the discovery of penicillin was a boon and its commercial production commenced around. For many years, antimicrobial drugs have been used to inhibit or kill bacteria and other microbes. However, microbial resistance to these drugs has developed on a very large scale over time, greatly diminishing their effectiveness, and is an ever growing problem. Over the years, antimicrobial drugs resistance has become increasingly widespread, and this has resulted in a significant threat to public health. The long list of drug resistant bacteria includes sulfonamide-resistant, penicillin-resistant, methicillin-resistant, and vancomycin-resistant Staphylococcus aureus, macrolide resistant Streptococcus pyogenes, penicillin-resistant Streptococcus pneumoniae, vancomycin-resistant Enterococcus, multidrug-resistant Mycobacterium tuberculosis, penicillin-resistant Neisseria gonorrhoeae (PPNG), Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, Shigella flexneri, Salmonella enterica, Acinetobacter baumannii, Pseudomonas aeruginosa, Vibrio cholerae, and beta-lactamase-expressing Haemophilus influenzae. Exploring other antimicrobial agents can be one option to deal with antimicrobial drug resistance. Nanoparticles owing to their small size which leads to increased reactivity and targeted action are emerging to be better antimicrobial agents.

Microbes are more unlikely to develop resistance against nanoparticles since they attack a broad range of targets which requires the microorganism to simultaneously undergo a series of mutations in order to protect themselves The size of the nanoparticle implies that it has a large surface area to come in contact with the bacterial cells and hence, percentage of interaction will be higher than bigger particles (Pal et al., 2007). The nanoparticles smaller than 10 nm interact with bacteria and produce electronic effects, which enhance the reactivity of nanoparticles.

Table1: different nanoparticles with antimicrobial properties and their mechanism of action

  Types of nanoparticles Pathogenic microorganisms Mechanism  of action Reference
          Carbon based Carbon nanotubes E. coli, Salmonella enteric, and Enterococcus faecium Cell Membrane disruption Kang et al (2007)

Dong et al (2012)

Fullerenes Salmonella, E. coli and Streptococcusspp Energy metabolism and cell membrane Bellucci., (2009)
Graphene oxide P. syringae, X. campestrispv. undulosa, Escherichia coli, Salmonella typhimurium, Enterococcus faecalis, Bacillus subtilis, F. graminearumand F. oxysporum Cell Membrane disruption Chen et al., (2014)

Krishnamoorthyet al., (2012)

                                                      Metalbased Silver nanoparticle Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella sp and Escherichia coli Sulfur-containing proteins in cell membrane, respiratory chain through free radical generation Feng et al., (2000) Sondi and Salopek-Sondi., (2004)
Zinc Oxide Escherichia coli,Pseudomonas aeruginosaSalmonella typhimurium,Staphylococcus aureusand Bacillus subtilis  Cellmembrane and intracellular contents Brayneret al., (2006)

Yamamoto et al., (2001)

Aluminum Oxide E. coli DH5α and Staphylococcus epidermidisNCIMB 12721 Generation of reactive oxygen species (ROS) Rupareliet al., (2008)

Balaet al., (2011)

Gold Nanoparticle S. aureus and E. coli Photothermal heating Grace et al., (2007) Sahaet al., (2007)
Magnesium Oxide E. coli, Bacillus megaterium and Bacillus subtilis Damage cellmembrane Koperet al., (2002)
Copper oxide B. subtilis and

Pseudomonas aeruginosa

Interact with DNA molecules and biochemical processes Rupareliet al., (2008)

Cioffiet al., (2005)

Titanium oxide S. aureus  andE. coli

 

Photocatalytic generation of strong oxidizing power Fujishimaet al., (1972) and Fujishimaet al., (1992)

 

Different Nanoparticles with Antimicrobial Effect

Several categories of nanoparticles are embraced with the antimicrobial properties, which can be exploited for the benefit of mankind. It has also been revealed that, the size and surface area of nanomaterial are important parameters affecting their antibacterial activity; that is, increasing the nanoparticles surface area by decreasing their size lead to improved interaction with bacteria. Generally, the antimicrobial activity of the nanoparticles depends on their composition, surface modification, intrinsic properties, and the type of microorganism. The probable mechanisms of their antibacterial activity were proposed as follow: inhibition of bacterial growth by impairing the respiratory chain; inhibition of energy metabolism and physical interaction with cell membrane (Fig 1). Various nanoparticles with antimicrobial properties are listed in the

 Fig 1: Overall mechanism of antimicrobial activity of nanoparticles

 Carbon based nanoparticles

 Carbon nanotubes (CNT)

CNTs are nano-sized hollow cylindrical form of carbon. Kang et al (2007) provided the first document that showed single walled carbon nanotubes (SWCNTs) had strong antimicrobial activity on Escherichia coli (E. coli). They demonstrated that SWCNTs could cause severe membrane damage and subsequent cell death. Arias and Yang (2009) investigated the antimicrobial activities of single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) with different surface groups towards rod-shaped or round-shaped gram negative and gram-positive bacteria. According to their results, SWCNTs with surface groups of -OH and -COOH indicated improved antimicrobial activity to both gram positive and gram-negative bacteria while MWCNTs with the same surface groups did not exhibit any significant antimicrobial effect. Their results showed that, formation of cell-CNTs aggregates caused to damage the cell wall of bacteria and then release of their DNA content. Dong et al (2012) also investigated the antibacterial properties of SWCNTs dispersed in different surfactant solutions (sodium dodecyl benzene sulfonate, sodium holate and sodium dodecyl sulfate) against E. coli, Salmonella enteric (S. enteric), and Enterococcus faecium. According to their results, SWCNTs exhibited antibacterial activity against both S. enterica and E. coli which was improved with the increase of nanotube concentrations. The combination of SWCNTs with surfactant solutions was also found to be low toxic to 1321N1 human astrocytoma cells, so they can be employed in biomedical applications especially for drug-resistant and multidrug-resistant microorganisms.

Fullerenes

Fullerenes are soccer ball-shaped molecules composed of carbon atoms. Fullerenes show antimicrobial activity against various bacteria, such as Salmonella, E. coli and Streptococcus spp. The antibacterial effect was probably due to inhibition of energy metabolism after internalization of the nanoparticles into the bacteria. Another bactericidal mechanism which has been proposed was the induction of cell membrane disruption. Among three different classes of fullerene compounds (positively charged, negatively charged and neutral), cationic derivatives showed the maximum antibacterial effect on E. coli and Shewanella oneidensis; while the anionic derivatives were almost ineffective. (Shvedova et al., 2012)

 Graphene oxide (GO)

A monolayer of carbon atoms which are tightly packed into a two-dimensional crystal is normally called Graphene. The antibacterial activity of graphene, in particular GO, is associated with its unique optical and electrical, mechanical and thermal properties, such as facile surface modification, high mechanical strength, good water dispersibility, and photoluminescence. GO nanosheets which could be readily dispersed in water are produced by chemically modification of the graphene with suspended hydroxyl, epoxyl, and carboxyl groups. It is documented that, membrane stress resulted from direct contact with sharp nanosheets is the major antimicrobial mechanism of GO. Both graphene and GO were shown inhibitory effect on the growth of E. coli. According to Chen et al., (2014) graphene oxide exhibit broad range of antimicrobial activities. They investigated the action against the bacterial pathogens (P. syringae and X. campestris pv. undulosa) and fungal pathogens (F. graminearum and F. oxysporum). The results showed that graphene oxide had a prominent impact on the reproduction of all four pathogens (killed nearly 90% of the bacteria and repressed 80% macroconidia germination along with partial cell swelling and lysis at 500 mg mL_1). A mutual mechanism is proposed in this work that GO intertwines the bacterial and fungal spores with a wide range of aggregated graphene oxide sheets, resulting in the local perturbation of their cell membrane and inducing the decrease of the bacterial membrane potential and the leakage of electrolytes of fungal spores. It is likely that GO interacts with the pathogens by mechanically wrapping and locally damaging the cell membrane and finally causing cell lysis, which may be one of the major toxicity actions of GO against phytopathogens. Krishnamoorthy et al (2012) also checked the antimicrobial property of GO against four different pathogenic bacteria and observed that the minimum inhibitory concentration (MIC) of graphene nanosheets against pathogenic bacteria, which was evaluated by a microdilution method was 1 μg/mL (against Escherichia coli and Salmonella typhimurium), 8 μg/mL (against Enterococcus faecalis), and 4 μg/mL (against Bacillus subtilis). The results suggest that graphene nanosheets have predominant antibacterial activity compared to the standard antibiotic, kanamycin.

Metal based nanoparticles

Metal based nanoparticles have been recently manufactured at the industrial level and have tremendous applications in medicine, water treatment and cosmetics to name a few. Many sunscreens contain these nanoparticles as well as surface coating products which are colorless and reflect UV rays more efficiently than larger particles (Oberdorster et al., 2005). Metal based nanoparticles exhibit cytotoxicity due to the electrostatic interaction with cell membrane depending on the charge on cell membrane.

 Silver nanoparticles

Silver has been in use since time immemorial in the form of metallic silver, silver nitrate, silver sulfadiazine for the treatment of burns, wounds and several bacterial infections. As early as 1000 B.C. silver was used to make water potable (Castellano et al., 2007). The 0.5% silver nitrate solution does not interfere with epidermal proliferation and possess antibacterial property against Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli (Bellinger and Conway, 1970). Silver sulfadazine is effective against bacteria like E. coli, S. aureus, Klebsiella sp., Pseudomonas sp. It also possesses some antifungal and antiviral activities (Fox and Modak, 1974). Srivathsan et al (2012) synthesized silver nanoparticles from silver nitrate with the help of metal reductant trisodium citrate by Wet Chemical method. The antimicrobial activity of the Silver Nanoparticles was investigated against the soil bacteria isolated from rhizospheric regions through in vitro and in situ. In these tests, the agar plates were used and the silver nanoparticles of various concentrations were supplemented. As results, the soil bacteria were inhibited at the low concentration of silver nanoparticles in both in vitro and in situ. These results suggest that the silver nanoparticles can be used as effective growth inhibitors in various bacteria, making them applicable to various antibacterial control systems. The nanoparticles get attached to the cell membrane and also penetrate inside the bacteria. The bacterial membrane contains sulfur-containing proteins and the silver nanoparticles interact with these proteins in the cell as well as with the phosphorus containing compounds like DNA. When silver nanoparticles enter the bacterial cell it forms a low molecular weight region in the center of the bacteria to which the bacteria conglomerates thus, protecting the DNA from the silver ions. The nanoparticles mostly target the respiratory chain and cell division finally leading to cell death. The nanoparticles release silver ions in the bacterial cells, which enhance their bactericidal activity (Song et al., 2006).

Kim et al (2007) investigated the effect of silver nanoparticle against E. coli and Staphylococcus aureus and observed that the silver nanoparticles were inhibitory to the E. coli at small concentrations whereas the growth inhibitory effect on S. aureus was mild. The free-radical generation effect of Ag nanoparticles on microbial growth inhibition was investigated by electron spin resonance spectroscopy.

ZnO Nanoparticles

ZnO nanostructure exhibits high catalytic efficiency, strong adsorption ability and are used more and more frequently in the manufacture of sunscreens, ceramics, rubber processing, wastewater treatment, and as a fungicide (Seshadri, et al., 2004). The advantage of using these inorganic oxides as antimicrobial agents is that they exhibit strong activity even when administered in small amount. ZnO nanoparticles are known to have strong inhibitory and antibacterial effects as well as a broad spectrum of antimicrobial activities (Wang et al., 2008). (Hafez et al (2014) also showed that ZnO has bactericidal activity higher than that recorded using the comparable traditional antibiotics. The antibacterial potentiality of zinc oxide (ZnO) nanoparticles (NPs), compared with conventional ZnO powder, against nine bacterial strains, mostly foodborne including pathogens, was evaluated using qualitative and quantitative assays. ZnO NP was more efficient as antibacterial agent than powder. Gram-positive bacteria were generally more sensitive to ZnO than Gram negatives. The exposure of Salmonella typhimurium and Staphylococcus aureus to their relevant minimal inhibitory concentrations from ZnO NP reduced the cell number to zero within 8 and 4 h, respectively. Scanning electron micrographs of the treated bacteria with NPs exhibited that the disruptive effect of ZnO on S. aureus was vigorous as all treated cells were completely exploded or lysed after only 4h from exposure. Promising results of ZnO NP antibacterial activity suggest its usage in food systems as preservative agent after further required investigations and risk assessments (Ahamed et al., 2011). Azam et al (2012) synthesized nanosized particles of the metal oxides (ZnO, CuO, and Fe2O3) by a sol–gel combustion route and characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy techniques. The antimicrobial activity was evaluated against both Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus and Bacillus subtilis) bacteria. Among different metal oxide nanomaterials, ZnO showed greatest antimicrobial activity against both Gram-positive and Gram-negative bacteria used. There are several mechanisms which have been proposed to explain the antibacterial activity of ZnO nanoparticles. The generation of hydrogen peroxide from the surface of ZnO is considered as an effective mean for the inhibition of bacterial growth (Yamamoto et al., 2001). It is presumed that with decreasing particle size, the number of ZnO powder particles per unit volume of powder slurry increases resulting in increased surface area and increased generation of hydrogen peroxide. Another possible mechanism for ZnO antibacterial activity is the release of Zn2+ ions which can the damage cell membrane and interact with intracellular contents (Brayner et al., 2006).

Aluminium oxide nanoparticles

Alumina nanoparticles are thermodynamically stable particles over a wide temperature range. Bala et al. (2011) synthesized alumina-silver composite nanoparticles by a simple, reproducible, wet chemical method, with the surface of the oxides modified with oleic acid. Preliminary antibacterial studies performed using disc diffusion assays against E. coli DH5α and Staphylococcus epidermidis NCIMB 12721 indicate that the composite nanomaterials have immense potential as antimicrobial agents. Mukherjee et al., (2011) investigated the antibacterial effect of alumina towards E. coli and growth inhibition was studied at varying concentrations of alumina. To ascertain whether the antibacterial activity was due to particles in the broth or due to the specific interactions with bacterial cellular components, a growth reversibility study was performed. Negligible dependence of concentration of the nanoparticles with the growth rate was observed. The growth reversibility studies indicated a significant retardation in growth of recultured E. coli cells which had prior exposure to the nanoparticles. A decrease in the extracellular protein content after nanoparticle exposure was also observed as indicated by protein assays and FTIR studies. Jiang et al. (2009) observed that dissolved metal ions were not present in a measurable quantity in the supernatant of the suspension thus ruling out the role of aluminum ions in nanoparticle mediated toxicity. They observed that the nanoparticles attached to the surface of the bacteria due to surface charge; bacterial surface was negative while alumina nanoparticles were positive at the pH studied. Aluminum oxide NPs have wide-range applications in industrial and personal care products. The growth-inhibitory effect of alumina NPs over a wide concentration range (10–1000 μg/mL) on Escherichia coli have been studied (Sadiq et al., 2009).  It is possible that the free-radical scavenging properties of the particles might have prevented cell wall disruption and drastic antimicrobial action (Sadiq et al., 2009). The alumina nanoparticles carry a positive charge on its surface at near-neutral pH. The electrostatic interaction between the negatively charged E. coli cells and the particles leads to the adhesion of nanoparticles on the bacterial surfaces (Li et al., 2004).

Gold nanoparticles

The therapeutic application of gold can be traced back to the Chinese medical history in 2500 BC. Red colloidal gold is still used in the Indian Ayurvedic medicine for rejuvenation and revitalization under the name of Swarna Bhasma (“Swarna” meaning gold, “Bhasma” meaning ash) (Mahdihassan et al., 1985). Gold also has a long history of use in the western world as nervine, a substance that could strengthen people suffering from nervous conditions. In the 16th century gold was recommended for the treatment of epilepsy. In the beginning of the 19th century it was used in the treatment of syphilis. Gold based therapy for tuberculosis was introduced in 1920s following the discovery of the bacteriostatic effect of gold cyanide towards the tubercle bacillus by Robert Koch, (Shaw et al., 1999). The major clinical uses of gold compounds are in the treatment of rheumatic diseases including psoriasis, planindromic rheutamitism, juvenile arthritis and discoid lupus erythematosus (Felson et al., 1990). Au particles are extensively exploited in organisms because of their biocompatibility (Bhattacharya et al., 2008). Gold nanoparticles (Au) generally are considered to be biologically inert but can be engineered to possess chemical or photothermal functionality. On near infrared (NIR) irradiation the Au-based nanomaterials, Au nanospheres, Au nanocages, and Au nanorods with characteristic NIR absorption can destroy cancer cells and bacteria via photothermal heating. Au-based nanoparticles can be combined with photosensitizers for photodynamic antimicrobial chemotherapy. Au nanorods conjugated with photosensitizers can kill Methicillin-resistant Staphylococcus aureus (MRSA) by photodynamic antimicrobial chemotherapy and NIR photothermal radiation (Kuo et al., 2009 and Pissuwan et al., 2009). The coating of aminoglycosidic antibiotics with gold nanoparticles has an antibacterial effect on a range of Gram-positive and Gram-negative bacteria (Saha et al., 2007). Gold nanoparticles also bind to the DNA of bacteria and inhibit the uncoiling and transcription of DNA (Rai et al., 2010).The Au nanoparticles can be used to coat a wide variety of surfaces for instance implants, fabrics for treatment of wounds and glass surfaces to maintain hygienic conditions in the home, in hospitals and other places (Das et al., 2009).

Magnesium oxide nanoparticles

Highly ionic nanosized metal oxides can be produced with extremely high surface areas and unusual crystal morphologies having numerous edge/corner and reactive surface sites (Stoimenov et al., 2002). Magnesium oxide (MgO) prepared through an aerogel procedure (AP-MgO) yields square and polyhedral shaped nanoparticles with diameters differing slightly around 4 nm, arranged in an extensive porous structure with considerable pore volume (Klabunde et al., 1996). An interesting property of AP-MgO nanoparticles is their ability to adsorb and retain for a long time (in the order of months) significant amounts of elemental chlorine and bromine (Huang et al., 2005). The AP-MgO/X2 nanoparticles exhibited biocidal activity against certain vegetative Gram-positive bacteria, Gram-negative bacteria and the spores (Richards et al., 2000). AP-MgO nanoparticles are found to possess many properties that are desirable for a potent disinfectant (Koper et al., 2002). The nanocrystals adsorb and carry a high load of active halogens due of their high surface area and enhanced surface reactivity. Atomic force microscopy and electron microscopy studies demonstrate that halogenated magnesium oxide has a very strong influence on microorganisms and their membranes in particular. Overall, the halogen such as chlorine and bromine treated MgO nanoparticles have a stronger and faster effect on the killing action of both bacteria and spores (Koper et al., 2002).

Their extremely small size allows many particles to cover the bacteria cells to a high extent which also bring halogen in an active form in high concentration in proximity to the cell (Richards et al., 2000). Standard bacteriological tests have shown magnificent activity against E. coli and Bacillus megaterium and sound activity against spores of Bacillus subtilis (Koper et al., 2002).

Copper oxide nanoparticles

Copper oxide (CuO) is a semiconducting compound with a monoclinic structure. It is the simplest member of the family of copper compounds and exhibits a range of potentially useful physical properties such as superconductivity, high temperature, electron correlation effects and spin dynamics (Tranquada et al., 1995 and Cava et al., 1990). CuO crystal also has photocatalytic or photovoltaic properties and photoconductive functionalities (Kwak et al., 2005). There is limited information available about the antimicrobial activity of nano CuO. As CuO is cheaper than silver, easily blends with polymers and relatively stable in terms of both chemical and physical properties, it finds a broad application (Xu et al., 1999). It is also recommended that highly ionic nanoparticulate metal oxides, such as CuO, may serve as potential antimicrobial agents as they can be prepared with extremely high surface areas and unusual crystal morphologies (Stoimenov et al., 2002). CuO nanoparticles were effective in killing a range of bacterial pathogens involved in hospital-acquired infections but a high concentration of nano CuO is required to achieve a bactericidal effect (Ren et al., 2009).

Copper nanoparticles have a great antimicrobial activity against B. subtilis. This may be attributed to greater abundance of amines and carboxyl groups on cell surface of B. subtilis and greater affinity of copper towards these groups.  This suggests that a release of ions into the local environment is required for optimal antimicrobial activity (Cioffi et al., 2005 and Ren et al., 2009). The exact mechanism behind bactericidal effect of copper nanoparticles is not clear.

Titanium dioxide nanoparticles

The inhibitory activity of TiO2 is due to the photocatalytic generation of strong oxidizing power when illuminated with UV light at wavelength of less than 385 nm (Chorianopoulos et al., 2011, Fujishima et al., 1972 and Fujishima et al., 1992). TiO2 particles catalyze the killing of bacteria on illumination by near-UV light. The generation of active free hydroxyl radicals (OH) by photoexcited TiO2 particles is probably responsible for the antibacterial activity. There are also studies on bactericidal activity of nitrogen-doped metal oxide nanocatalysts on E. coli biofilms and on the photocatalytic oxidation of biofilm components on TiO2 coated surfaces (Matsunga et al., 1998). The use of TiO2 photocatalysts as alternative means of self-disinfecting contaminated surfaces by further development may provide potent disinfecting solutions for prevention of biofilm formation (Liu et al., 2007).

  TiO2 in aqueous media can be used for the killing of bacteria and viruses through photocatalytic methods (Duffy et al., 2002). Chorianopoulos et al., 2011 also suggested that nanostructured TiO2 on UV irradiation can be used as an effective way to reduce the disinfection time, eliminating pathogenic microorganisms in food contact surfaces and enhance food safety. The use of UV light which is required to activate the photocatalyst and initiate the killing of the bacteria and viruses is the major drawback. In recent years, visible light absorbing photocatalysts with Ag/AgBr/TiO2 has proved to be successful at killing S. aureus and E. coli (Hu et al., 2006).

Factors Affecting Antimicrobial Effect of Nanoparticles

Chemical composition of nanoparticles

It is the principle cause of antimicrobial activity of nanoparticles. Many researchers suggest the importance of proper chemical composition or its modifications (Hetrick et al., 2008). Adams et al., (2006) compared the antimicrobial activity of three types of  nanoparticles  known  by  their  production  of  ROS, TiO2,  SiO2 and  ZnO,  against B.  subtilis and E. coli. ZnO showed highest toxicity followed by TiO2 and SiO2. Wang et al., (2010) created aerosols of 6 different nanoparticles (NiO, ZnO, Fe2O3, Co3O4, CuO, and TiO2) and compared the activity against E. coli. NiO, ZnO and CuO showed significant toxicity against the bacterial strain. The results elucidate the importance of chemical compostion of the nanoparticles for their toxicity.

Size

Size of nanpoparticle is another determining factor for its antimicrobial activity. The interaction between a living microorganism and the nanoparticle is size based. Zhang et  al., (2009)  studied  antimicrobial  activity  of  nanocomposite  powders    composed    of    titanium  oxide  and silver nanoparticles  on E. coli.  The  authors emphasized that the sizes of nanoparticles was  crucial  to  their  bactericidal  performance,  and  silver nanoparticles of less than 3 nm achieved the highest  antimicrobial  activity.

Concentration of nanoparticles

Antibacterial    activity    of    nanoparticles    directly correlates with their concentration. Kim et al., (2007) studied the concentration based effect of nanoparticles on pathogens. The researchers used the nanoparticles within a concentration range from 0.2 to 33nM against E. coli and S. aureus. The results indicate decrease in antimicrobial activity with decrease in concentrations. The nanoparticles in low concentrations sometimes show a stimulatory kind of activity towards the microorganisms, whereas the same nanoparticles in higher concentration are toxic. Babushkina et  al. (2010) demonstrated that iron nanoparticles in   the   low   concentration (0.001   and   0.01   mg/ml) posses  stimulating effect  on  the  bacterial  growth while the higher concentration (0.1 and 1 mg/ml) had inhibitory  effect  on  clinical  isolates  of S.  aureus.

 Shape of nanoparticles

The active facets on the surface on nanoparticles enable them to interact with the living microorganisms. The percent of active facets is decided by the shape of a nanoparticle and nanoparticles with high number of facets such as trunculated triangular nanoparticles show higher antimicrobial activity. Pal et al., (2007) analyzed the bactericidal activity of three different shaped silver nanoparticles: spherical, elongated   (rod-shaped),   and   truncated   triangular silver nanoplates. Number of CFUs was examined on media supplemented with nanoparticles. The silver nanoparticles with truncated   triangular facets resulted into least CFUs.

CONCLUSION

 The nanoparticles with antimicrobial properties have been reported in many studies. The small size of these nanoparticles helps in inactivation of microorganisms. This antimicrobial activity can help to fight the resistance of pathogenic microorganism towards the antibiotics and other substances. Some of this carbon based and metal based nanoparticles can be a better substitute to pesticides. The advantage of using nanoparticles over pesticides is reduced amount to be used, targeted action and hence more susceptibility. The use of these antimicrobial nanoparticles is already gaining interest in the field of medicines but still need more recognition in agricultural fields. The toxicity of nanoparticles towards the friendly flora and fauna is still a debatable issue, but a midway can still be drawn by engineering or designing the specific forms to target the candidate of interest.

Acknowledgement

All the authors cited in the literature and reference section.

Conflict of Interest

Authors declare no conflict of interest.

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