Changes in Soil Bacterial Communities and Diversity in Response to Long-Term Silver Exposure
Citation:
Vasileiadis, S., E. Puglisi, M. Trevisan, K. Scheckel, K. Langdon, M. McLaughlin, E. Lombi, AND E. Donner. Changes in Soil Bacterial Communities and Diversity in Response to Long-Term Silver Exposure. Cindy Nakatsu (ed.), FEMS Microbiology Ecology. Oxford University Press, Cary, NC, 91(10):11 pg, (2015).
Impact/Purpose:
Soil microbial communities have been implicated as sources of antimicrobial resistance genes found in pathogenic microorganisms (Silbergeld et al. 2008; Martinez 2009). Yet due to the immense diversity in environmental microbial assemblages (Schloss and Handelsman 2006), the direct clinical impact of these suspected resistance reservoirs was difficult to conclusively establish prior to the advent of high-throughput sequencing and functional metagenomics. Here, we report new insights into the diversity and identity of silver (Ag) tolerant/resistant bacteria in soils subjected to extended Ag selective pressure. In this study, we investigated the effects of Ag selective pressure on soil bacterial diversity, and explored the traits of selected operational taxonomic units (OTUs) on the basis of their phylogenetic affiliations. The monitored community disturbance event began with a pulse application of ionic silver, during which nominal Ag treatments of (50), 100, 200, and 400 mg Ag kg-1 soil (suitable for identifying soil bacteria with Ag persistent/resistant phenotypes and a competitive advantage under Ag selective conditions) were established in triplicate for each soil. Two week (2W), nine month (9M), and matching control (CTL) treatments were sampled from each of the 5 soils, which differed in major physico-chemical characteristics such as soil texture, pH, total organic carbon (OC), total nitrogen (N) and cation exchange capacity (CEC) as shown in Table S1. Polymerase chain reaction (PCR) based counting (real-time quantitative PCR) and Illumina MiSeq partial sequencing of the 16S rRNA bacterial gene were used to determine the effects on bacterial diversity and population dynamics, and to identify silver persistent/resistant bacterial populations to the near-species level. X-ray Absorption Near Edge Structure (XANES) Spectroscopy and Diffusive Gradients in Thin-film (DGT) devices were used to assess Ag speciation and bioavailability.
Description:
Silver-induced selective pressure is becoming increasingly important due to the growing use of silver (Ag) as an antimicrobial agent in biomedical and commercial products. With demonstrated links between environmental resistomes and clinical pathogens, it is important to identify microbial profiles related to silver tolerance/resistance. We investigated the effects of ionic Ag stress on soil bacterial communities and identified resistant/persistant bacterial populations. Silver treatments of 50 - 400 mg Ag kg-1 soil were established in five soils. Chemical lability measurements using diffusive gradients in thin-film devices confirmed that significant (albeit decreasing) labile Ag concentrations were present throughout the 9-month incubation period. Synchrotron X-ray absorption near edge structure spectroscopy demonstrate that this decreasing lability was due to changes in Ag speciation to less soluble forms such as Ag0 and Ag2S. Real-time PCR and Illumina MiSeq screening of 16S rRNA bacterial genes showed β-diversity in response to Ag pressure, and immediate and significant reductions in 16S rRNA gene counts with varying degrees of recovery. These effects were more strongly influenced by exposure time than by Ag dose at these rates. Ag-selected dominant OTUs principally resided in known persister taxa (mainly Gram positive), including metal-tolerant bacteria and slow-growing Mycobacteria.
