Possible Mosquito Control by Silver Nanoparticles Synthesized by Soil Fungus (Aspergillus niger 2587) ()
1. Introduction
Mosquito vectors are solely responsible for transmitting diseases such as malaria, dengue, chikungunya, Japanese encephalitis, and lymphatic filariasis. Anopheles species are the most important species as they are capable vector for malaria parasites. About 3.3 billion people—half of the world’s population—are at risk of malaria. In 2010, there were about 216 million malaria cases (with an uncertainty range of 149 million to 274 million) and an estimated 655,000 malaria deaths (with an uncertainty range of 537,000 to 907,000). Increased prevention and control measures have led to a reduction in malaria mortality rates by more than 25% globally since 2000 and by 33% in the WHO African Region [1].
Culex mosquitoes are painful and persistent biters and are responsible for filariasis. Lymphatic filariasis is a neglected tropical disease. More than 1.3 billion people in 72 countries worldwide are threatened by lymphatic filariasis, commonly known as elephantiasis. Over 120 million people are currently infected, with about 40 million disfigured and incapacitated by the disease [2].
Aedes mosquitoes on the other hand are also painful and persistent biters. Ae. aegypti is responsible for spreading dengue. The incidence of dengue has grown dramatically around the world in recent decades. Over 2.5 billion people—over 40% of the world’s population—are now at risk from dengue. WHO currently estimates there may be 50 - 100 million dengue infections worldwide every year [3]?
The problem has a complex face and it has to be handled carefully. It is essential to control mosquito population so that people can be protected from mosquito borne diseases. These diseases can be controlled by targeting the causative parasites and pathogens. It is easier to control vectors than parasites. The chemical control was one of the most widely used conventional methods for mosquito control since chemical pesticides are relatively inexpensive usually produces immediate control. Generally, the chemical control is carried out by the indoor residual spraying of insecticides such as dichloro diphenyl trichloro ethane, hexa chlorocyclo hexane, benzene hexa chloride, melathion and synthetic pyrothroid. But, the development of resistance against these chemicals in various mosquito populations has been reported.
It is known that larvicides play a vital role in controlling mosquitoes in their breeding sites. Two insecticidal bacteria have been used as larvicides to control larvae of nuisance and vector mosquitoes in many countries, Bascillus thuringienesis ssp. Israelensis and Bascillus sphaericus [4]. Field studies have shown that both are effective, but serious resistance, as high as 50,000 fold, has evolved where B. sphaericus is used against Culex mosquitoes. Unfortunately, the development of resistance against the larvicide in various mosquito populations has also been reported.
Therefore, biological control can thus provide and effective and environmental friendly approach, which can be used as an alternative to minimize the mosquito population. Fungi and fungus derived products are highly toxic to mosquitoes, yet have low toxicity to non-target organisms [5]. The secondary metabolites of entomopathogenic fungi Chrysosporium [6], Fusarium [7], Aspergillus [8], and Verticillium [9] have been screened successfully as a potential larvicide.
Fungi are also been used in nanotechnology for producing nanoparticles. Therefore, present green synthesis has shown that the environmentally benign and renewable source of fungi used as an effective reducing agent for the synthesis of silver nanoparticles. Biosynthesis of silver nanoparticles (AgNPs) by using a fungus Trichoderma [10,11], Aspergillus [12,13], and Fusarium [14,15] have been reported.
The larvicidal activities of mycosynthesized silver nanoparticles against vectors: Ae. aegypti and An. stephensi responsible for diseases of public health importance have been evaluated [16]. The silver and gold nanoparticles synthesized with C. tropicum have been tested as a larvicide against the mosquito larvae [17,18]. The silver nanoparticles synthesized by using the fungi have also been tested against adult mosquito [19].
The present communication describes the larvicidal and pupicidal effect of extracellular synthesized silver nanoparticles by using the soil fungi A. niger 2587 against the An. stephensi, Ae. aegypti and Cx. quinquefasciatus mosquitoes. A. niger is filamentous keratinophilic fungi with compact white or yellow basal felt covered by a dense layer of dark-brown to black conidial heads. This fungus secret some reducing agents which convert silver nitrate into silver nanoparticles. Therefore, it can be a useful green exercise to invent and discover new fungal nanolarvicide for respective ecology and environmental management system.
2. Experimental
2.1. Fungal Strain, Preparation of Broth and
Culture of A. niger
The fungal strain of A. niger (MTCC 2587) was obtained from the Microbial Type Culture Collection and Gene Bank, Institute of Microbial Technology, Chandigarh, India, and was routinely maintained in the laboratory on Czapek-Dox Agar (CDA) at 25˚C.
The broth was prepared for culture of A. niger following the method [20]. Five 250 ml conical flasks, each containing 100 ml of Czapek-Dox Broth (sucrose 30 g, sodium nitrate 3 g, dipotassium phosphate 1 g, magnesium sulphate 0.05 g, potassium chloride 0.05 g, ferrous sulphate 0.01 g, and deionized water 1000 mL), were autoclaved at 20 psi for 20 minutes. The broth was supplemented with chloramphenicol (50 µg/mL) as a bacteriostatic agent. A. niger colonies grown on CDA plates were transferred to each flask by inoculation needle. The conical flasks inoculated with A. niger were incubated at 25˚C for 15 days.
2.2. Synthesis and Characterization of AgNPs
The fungal colonies of A. niger was grown on CDA. After 7 days incubation of fungal colonies on CDA plates were further transferred CDB containing conical flask by inoculation needle. The conical flasks inoculated with A. niger were incubated at 25˚C for 15 days. After 15 days incubation the fungal biomass was separated from the medium by filtration through whatman-1 filter paper. The biomass was washed thrice in sterile distilled water to remove any nutrient media that might interact with silver ions. Approximately 10 g of fungal wet biomass of fungus was transferred to 250 ml conical flask containing 100 ml of distilled water and incubated for 72 h at 25˚C. After then the aqueous solution component was separated by filtration using whatman-1 filter paper. To this solution (aqueous solution component of A. niger), AgNO3 (103 M) solution was added and kept for 72 h at 25˚C in BOD incubator. Simultaneously, the control without adding AgNO3 was maintained under the same conditions, separately. The protein, enzyme and other compound present in the fungal liquid work as reducing agents and are responsible for conversion of silver nitrate to silver nanoparticles. The reaction may be written as A. niger (fungal liquid) +
Silver nitrate solution 
Silver nanoparticles. Periodically, aliquot of the reaction solution was removed and their absorption was measured in UV-vis spectrophotometer. The micrograph of AgNPs was obtained by Philips CM-10 Transmission Electron Microscope.
2.3. Bioassays and Statistical Analysis
The larvicidal and pupicidal activity of synthesized AgNPs against the Cx. quinquefasciatus, Ae. aegypti and An. stephensi was assessed by using the standard method [21]. Bioassays were conducted separately at six different test concentrations (2, 4, 6, 8, 10, and 12 ppm) of aqueous AgNPs. To test the larvicidal and pupicidal activity of synthesized AgNPs, 20 larvae and pupae of Cx. quinquefasciatus, Ae. aegypti and An. stephensi were separately exposed to 100 ml of test concentrations. Similarly, the control (without AgNPs) was run to test the natural mortality. Thereafter, we could further examine the mortality which was determined after different hours of treatment, the experiment time. No food was offered to the larvae during the experiment. Experiments were replicated thrice to validate the results. The data on the efficacy was subjected to probit analysis [22]. The control mortality was corrected by Abbott’s formula [23].
3. Results
3.1. UV-Vis Spectrophotometer and TEM Analysis of Synthesized AgNPs
By mixing the fungal liquid component of A. niger with the aqueous solution of Ag ions, the colour of fungal liquid changed from white to dark brown colour after 72 h of incubation. The change in colour is a signal for the formation of AgNPs. Figure 1(a) shows the UV-vis spectra of AgNPs synthesized by using the A. niger recorded from the reaction medium before (1) and after immersion of AgNO3 (2) after 72 h. Absorption spectra of AgNPs formed in the reaction medium has a broad absorption band centred at ca. 480 nm. The presence of broad resonance indicated an aggregated structure of the AgNPs in the solution.
Figure 1(b) shows the TEM micrograph of A. niger synthesized AgNPs. The 20 - 70 nm sized and spherical shaped AgNPs were observed.