Green synthesis of silver nanoparticles using Callistemon citrinus leaf extract and evaluation of its antibacterial activity

Nowadays, production and utilization of Nano materials have increased due to their unique and interesting properties. So far, different physical and chemical methods have been used to synthesize silver nanoparticles. Chemical synthesis is not compatible due to the hazardous chemicals residues on the surface of the nanoparticles (NP) as well as production of by products with high impact on the environment. Physical routes for synthesis of NPs have some drawbacks, too. These methods require high energy and space, and are expensive.  Therefore, biological methods for the synthesis of silver nanoparticles are considered emerging technologies as economic choices in the green chemistry field. Among these methods, plant-mediated synthesis of AgNPs is a rapid, simple, non-toxic and eco-friendly technique. Silver nanoparticles exhibit high bactericidal activity at their utilized concentrations with no toxic effect on human cells, and they also strongly enhance the antibacterial activity of conventional antibiotics even against multi-resistant bacteria through their synergistic effects. Callistemon citrinus belongs to the family Myrtaceae and includes more than 30 species. The plant is widespread in wet tropics, notably Australia, South America and tropical Asia, but presently can be found all over the world. Callistemon citrinus is a potential medicinal plant used to treat gastrointestinal distress, pain, and infectious diseases caused by bacteria, fungi, viruses, and parasites. In this study Callistemon citrinus aqueous extract was used to reduce silver ions in silver nitrate solution.  In the following, the antimicrobial activity of nanoparticles synthesized by various qualitative and quantitative methods on Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhi and Listeria innocua was investigated.  For the synthesis of silver nanoparticles, 25 mL of silver nitrate solution was added to 5 mL of leaf extract with a concentration of 100 mg/mL and maintained for 24 h at 20 °C.  Change the color of the solution to Red represents the production of silver nanoparticles in the solution. To stabilize the presence of silver nanoparticles, the absorption spectrum of silver nanoparticles produced by spectrophotometer was prepared. Antimicrobial activity of silver nanoparticles synthesized using Callistemon citrinus leaf aqueous extract was examined by disc diffusion agar, well diffusion agar, minimum inhibitory concentration (microdilution broth) and minimum bactericidal concentration on Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhi and Listeria innocua.  The results showed that in disc diffusion agar method, the diameter inhibition zone increased with increasing the concentration of silver nanoparticles. The maximum effect of silver nanoparticles synthesized using Callistemon citrinus leaf aqueous extract at a concentration of 150 mg / ml was observed for Pseudomonas aeruginosa. An inhibition zone was observed for all examined pathogenic microorganisms at all concentrations. The results showed that in the well diffusion agar method, nanosilver particles at a concentration of 18.75 mg/ml did not show any inhibitory effect on all the pathogenic microorganisms. The results of statistical analysis showed that there was no significant difference between all the concentrations of silver nanoparticles synthesized for Escherichia coli, Salmonella typhi and Staphylococcus aureus (P˂ 0.05(. The MIC for Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhimurium and Listeria innocua was 128, 256, 256, 256 and 512 mg/mm, respectively. The MBC for all the pathogenic strains was 512 mg/mm. The results of this study showed that the Callistemon citrinus leaf extract has a good ability to synthesize silver nanoparticles. Nanoparticles synthesized from Callistemon citrinus leaf extract had good antimicrobial activity against examined pathogenic bacteria, especially Gram-negative bacteria. Green-synthesized nanoparticles can be used as antimicrobial agent to fight infectious diseases caused by various microbial strains, although more research is needed in vitro, animal models and in vivo.