Effects of silver selective pressure on soil microbial communities revealed by
high throughput sequencing diversity screening
Sotirios Vasileiadis1,2, Edoardo Puglisi3, Marco Trevisan2, Kirk G Scheckel4, Kate Langdon5,
Mike McLaughlin5, Enzo Lombi1, Erica Donner1
1
entre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, South Australia, 5095;
C
sotirios.vasileiadis@unisa.edu.au; erica.donner@unisa.edu.au
2
Istituto di Chimica Agraria e Ambientale, Universitá Cattolica del Sacro Cuore, Piacenza, Italy
3
Istituto di Microbiologia, Universitá Cattolica del Sacro Cuore, Piacenza, Italy
4
. S. Environmental Protection Agency, National Risk Management Research Laboratory, Land Remediation and Pollution Control
U
Division, Cincinnati OH, USA
5
CSIRO Land and Water, Glen Osmond SA 5064, Australia
Silver-induced selective pressure on bacterial communities is increasingly relevant due to the growing
commercial and biomedical use of silver nanoparticles. Genetic silver resistance is carried by intermediate
to broad host range IncH group conjugative plasmids and has been observed in both clinical pathogens and
environmental bacteria. Horizontal gene transfer mechanisms may facilitate the spread of silver resistance,
but the role of soil as a reservoir of silver resistant genotypes remains virtually unexplored.
Five Australian soils with diverse physico-chemical characteristics were exposed to silver for 2 weeks (2W)
and 9 months (9M) at concentrations ranging from 50-400 mg/kg. Control samples were also maintained.
XANES and DGT were used to track changes in silver speciation and bioavailability. Total DNA extracts
were used for real-time PCR-based counting and 16S rRNA partial sequencing (Illumina MiSeq).
A rapid drop in 16S rRNA gene counts occurred in the silver treated soils, and persisted after 9M incubation
in some soils. Analysis of 1,446,210 sequences revealed an inverse relationship between the α-diversity
and 16S rRNA gene counts. Minimal community convergence attributable to silver selective pressure was
observed. When analysed collectively, the samples grouped according to soil type. However, when soils were
analysed separately, distance based redundancy analysis attributed 55-78 % of the observed variance to silver
exposure time and dose. Differential abundance tests for dominant (>1%) OTUs revealed three major trends.
OTUs showing potential adaptation included genotypes usually associated with metal resistance and halotolerance, and persistent sequences in one soil were classified as slow growing mycobacteria.