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.

Automated summary

The antibacterial activity of different clay minerals towards Escherichia coli cells varied based on their layer expandability, with expandable smectite showing the strongest antibacterial activity. This antibacterial effect was attributed to the production of (OH)-O-center dot from the oxidation of structural Fe(II), highlighting the potential of clay minerals as antibacterial agents.