Antibacterial activity of silver-modified natural clinoptilolite

The aim of the present work was to estimate the bactericidal activity and efficacy of silver pre-treated clinoptilolite-rich tuff from Marsid (Romania) in solid media (agar plates) against Gram-negative Escherichia coli ATCC 25922 and Gram-positive Staphylococcus aureus ATCC 25923. Two samples of natural clinoptilolite-rich tuff was first pre-treated with oxalic acid and sodium hydroxide solutions, respectively. The sample treated with oxalic acid was then exchanged with sodium chloride solution to obtain sodium form. Finally, both samples were exchanged with silver nitrate solution at room temperature for 24 h to obtain silver forms (P1-Ag+ and P2-Ag+) of clinoptilolite. The structure, morphology, and elemental composition of the pre-treated clinoptilolite samples were characterized by XRD, infrared (ATR-IR), SEM, and EDX analysis. The antibacterial activity was investigated by exposing E. coli and S. aureus in nutritive agar to the silver-clinoptilolite samples. Microorganisms were completely inhibited at 2 mg Ag+-clinoptilolite/mL nutritiv medium after 24 h of incubation at 37 °C. The silver-clinoptilolite sample derived from natural clinoptilolite pre-treated with oxalic acid (P1-Ag+) exhibit a stronger antibacterial effect in the presence of E. coli and the sample derived from natural clinoptilolite pre-treated with sodium hydroxide (P2-Ag+) in the presence of S. aureus.