Reduced Iron-Containing Clay Minerals as Antibacterial Agents

Area of Science:

• Environmental Science
• Microbiology
• Materials Science

Background:

• Previous studies identified hydroxyl radical (•OH) production from soluble Fe(II) as a key antibacterial mechanism of iron-rich clays.
• The specific role of structural iron within clay minerals at near-neutral pH and the precise antibacterial targets remained less understood.

Purpose of the Study:

• To investigate the contribution of structural Fe(II) in nontronite NAu-2 to hydroxyl radical (•OH) production at near-neutral pH.
• To identify specific bacterial components targeted by the clay's antibacterial action.
• To refine the model of antibacterial activity for iron-containing clays.

Main Methods:

• Structural Fe(III) in nontronite NAu-2 was reduced to Fe(II) (rNAu-2).
• Escherichia coli (E. coli) were exposed to rNAu-2 in oxic suspensions across a range of pH values.
• Hydroxyl radical scavenging and anaerobic conditions were employed to assess the role of •OH.
• In-situ imaging was used to observe cellular damage.

Main Results:

• Antibacterial activity was pH-dependent, with complete E. coli killing at pH 6 but survival at pH 7 and 8.
• Reduced survival in the presence of •OH scavengers or anaerobic conditions confirmed •OH generation as the primary cause of cell death.
• In-situ imaging revealed damage to cardiolipin, a membrane lipid in the polar region of E. coli.
• Structural Fe(II) in rNAu-2 was identified as the main source of •OH, leading to cardiolipin damage and subsequent Fe(II) influx and cell death.

Conclusions:

• Structural Fe(II) in reduced nontronite NAu-2 is the primary source of hydroxyl radicals (•OH) at near-neutral pH.
• Hydroxyl radicals damage the membrane lipid cardiolipin, initiating a cascade of events leading to bacterial cell death.
• This study advances the understanding of iron-clay antibacterial mechanisms, highlighting structural iron's critical role and cardiolipin as a key target.