Ag- and Zn-clinoptilolite: a comparison of their in-vitro antibacterial activity against Helicobacter pylori

Helicobacter pylori is a bacterium recognized as the most frequent cause of chronic gastritis and is classified as carcinogen by the World Health Organization. For the eradication of H. pylori, the use of silver and zinc was studied due to their antibacterial properties. The use of clinoptilolite was also considered, since ammonium is essential for the survival of the bacterium in the stomach and clinoptilolite is acid resistant and selective toward NH4+. This research compared the antibacterial activity against H. pylori of Ag- and Zn-clinoptilolite through in-vitro tests, aiming to exploit the zeolite simultaneously as a metal supplier and as an ammonium sequestrant. A powder containing approximately 90% of clinoptilolite was used to prepare, by cation exchange, two materials based on Zn-clinoptilolite (FA-Zn) and Ag-clinoptilolite (FA-Ag), containing 2.12 meq/g of Zn2+ and 2.28 meq/g of Ag+, respectively. In the agar cup test, both materials evinced a direct and linear relationship between their concentration and the width of the inhibition zones, but FA-Ag allowed the onset of the development of an inhibition halo already at a concentration of 12.5 mg/mL, whereas FA-Zn required 25 mg/mL to get the same result. The Minimum Inhibitory Concentration of FA-Ag resulted eight time lower than that of FA-Zn (0.5 and 4.0 mg/mL, respectively). Tests showed superior antimicrobial activity of FA-Ag, but it should be considered that Ag-clinoptilolite has some disadvantages compared to Zn-clinoptilolite, such as higher toxicity, higher cost, and instability to light.