Antibacterial Mechanisms of Reduced Iron-Containing Smectite-Illite Clay Minerals

Reduced nontronite has been demonstrated to be antibacterial through the production of hydroxyl radical ((OH)-O-center dot) from the oxidation of structural Fe(II). Herein, we investigated the antibacterial activity of more common smectite-illite (SI) clays toward Escherichia coli cells, including montmorillonite SWy-3, illite IMt-2, 50-50 S-I rectorite RAr-1, 30-70 S-I ISCz-1, and nontronite NAu-2. Under an oxic condition, reduced clays (with a prefix r before mineral names) produced reactive oxygen species (ROS), and the antibacterial activity followed the order of rRAr-1 > rSWy-3 >= rNAu-2 " rIMt-2 >= rISCz-1. The strongest antibacterial activity of rRAr-1 was contributed by a combination of (OH)-O-center dot and Fe(IV) generated from structural Fe(II)/adsorbed Fe2+ and soluble Fe2+, respectively. Higher levels of lipid and protein oxidation, intracellular ROS accumulation, and membrane disruption were consistent with this antibacterial mechanism of rRAr-1. The antibacterial activity of other S-I clays depended on layer expandability, which determined the reactivity of structural Fe(II) and the production of (OH)-O-center dot, with the expandable smectite being the most antibacterial and nonexpandable illite the least. Our results provide new insights into the antibacterial mechanisms of clay minerals.