Microbial extracellular polymeric substances reduce Ag+ to silver nanoparticles and antagonize bactericidal activity

Area of Science:

• Microbiology
• Environmental Science
• Materials Science

Background:

• Silver ions (Ag+) are widely used antimicrobials.
• Bacterial defense mechanisms against silver are poorly understood.
• Environmental impacts of silver release require investigation.

Purpose of the Study:

• To investigate the role of bacterial extracellular polymeric substances (EPS) in mitigating silver ion (Ag+) antimicrobial activity.
• To elucidate the mechanisms by which EPS affects silver's efficacy and bacterial survival.
• To assess the implications for environmental safety and disinfection.

Main Methods:

• Manipulation of EPS in Escherichia coli suspensions (removal/addition).
• Assessment of bacterial growth in the presence of Ag+.
• High-resolution transmission electron microscopy (HRTEM) for visualizing silver nanoparticles (AgNPs).
• X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectrometry (EDS) for elemental analysis.
• Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (NMR) spectroscopy for chemical characterization.

Main Results:

• EPS acts as a permeability barrier, hindering intracellular silver penetration.
• EPS contains reducing constituents that convert Ag+ to silver nanoparticles (AgNPs; 10-30 nm).
• AgNPs are immobilized within the EPS matrix, reducing Ag+ bioavailability.
• Bacterial growth is enhanced in the presence of Ag+ when EPS is intact.

Conclusions:

• Bacterial EPS significantly mitigates the antimicrobial efficacy of silver ions.
• EPS-mediated reduction and immobilization of Ag+ to AgNPs is a key defense mechanism.
• The quantity and composition of EPS are critical factors influencing silver's bactericidal effects and environmental fate.