Silver nanoparticle enhanced silver ion stress response in Escherichia coli K12

Abstract

This study investigated the dissolution-based toxicity mechanism for silver nanoparticles to Escherichia coli K12. The silver nanoparticles, synthesised in the vapour phase, are effective anti-bacterial agents against the Gram-negative bacterium, E. coli K12. The nanoparticles associate with the bacterial cell wall, appearing to interact with the outer and inner membranes, and then dissolve to release Ag⁺ into the cell and affect a transcriptional response. The dissolution of these nanoparticles in a modified LB medium was measured by inductively coupled plasma mass spectrometry (ICP-MS) and has been shown to follow a simple first-order dissolution process proportional to the decreasing surface area of the nanoparticles. However, the resulting solution phase concentration of Ag⁺, demonstrated by the ICP-MS data, is not sufficient to cause the observed effects, including inhibition of bacterial growth and the differential expression of Cu⁺ stress response genes. These data indicate that dissolution at the cell membrane is the primary mechanism of action of silver nanoparticles, and the Ag⁺ concentration released into the bulk solution phase has only limited anti-bacterial efficacy.

Keywords::

• Silver nanoparticles
• Escherichia coli
• real-time PCR
• toxicity
• adaptive stress response

Acknowledgements

J McQuillan would like to thank the BBSRC for a CASE award with ENBL Ltd. We would like to thank Dr Sara Burton and Prof. Hilary Lappin-Scott for their support during the early parts of this work. We would like to thank Dr Paul Reip for the supply of the silver nanoparticle powder. H. Groenaga Infante and E. Stokes acknowledge the UK National Measurement Office for secondment funding.