GREEN SYNTHESIS OF SILVER NANOPARTICLES APPLIED AS AN ANTIMICROBIAL AGAINST Escherichia coli

The main objective of this work was to prepare and apply silver nanoparticles (AgNPs), obtained from green synthesis (using natural extract as reducing agent), against Escherichia coli, which is an important foodborne pathogen and indicator of fecal contamination in food and water. AgNPs were synthetized from silver nitrate (AgNO3, concentrations of 10, 30, 50 and 70mg/L) and extract from red guava (Psidium cattleianum, concentrations of 0.1g/mL to 0.001g/mL). Briefly, natural extract was slowly dripped over the AgNO3 solution under constant stirring at 25ºC ± 1°C, and the pH adjusted for 6 to 7.5. After 1 hour, an aliquot was collected, and the particle formation analyzed in an UV-Vis spectrophotometer at 400-420nm. Antimicrobial activity of the AgNPs against Escherichia coli was assessed by inoculating of it strain in Tryptic Soy Broth (TSB media) at 1:30 h/35 ºC, resulting in a final concentration of 105 UFC/mL. Subsequently, the AgNPs were added to the tubes at 1.0, 0.5, 0.1 and 0.05mg/mL and incubated. Sodium hypochlorite (a traditional sanitizer used in the food industry) 1% was used as a positive control and a tube without any antimicrobial treatment was used as negative control. Minimum bactericidal concentration (MBC) was determined at 0, 24 and 48 h of incubation, in duplicates, through plate counts of surface and characterized as the absence of growing after 48 hours of incubation on the dilution 10-1. Results proved the formation of AgNPs (maximum peak of 415nm and brown color) and no antimicrobial activity was observed when the concentrations of 0.05 and 0.1 mg/mL were used. Concentration of 0.5 mg/mL shows a 2 log-reduction after 24 hours, keeping this reduction in 48 hours, while no growth was observed at the concentration of 1.0 mg/mL along all 48 h, being this concentration considered the MBC. Results showed feasibility of systems carried out from green synthesis to reduce bacterial charges of Escherichia coli and consumption of organic solvents as well as, demonstrated a potential possibility to replace (fully or partially) the traditional sanitizers.