title: Unprecedented Silver Resistance in Clinically Isolated Enterobacteriaceae: Major Implications for Burn and Wound Management.
abstract: Increased utilization of inorganic silver as an adjunctive to many medical devices has raised concerns of emergent silver resistance in clinical bacteria. Although the molecular basis for silver resistance has been previously characterized, to date, significant phenotypic expression of these genes in clinical settings is yet to be observed. Here, we identified the first strains of clinical bacteria expressing silver resistance at a level that could significantly impact wound care and the use of silver-based dressings. Screening of 859 clinical isolates confirmed 31 harbored at least 1 silver resistance gene. Despite the presence of these genes, MIC testing revealed most of the bacteria displayed little or no increase in resistance to ionic silver (200 to 300 μM Ag(+)). However, 2 isolates (Klebsiella pneumonia and Enterobacter cloacae) were capable of robust growth at exceedingly high silver concentrations, with MIC values reaching 5,500 μM Ag(+). DNA sequencing of these two strains revealed the presence of genes homologous to known genetic determinants of heavy metal resistance. Darkening of the bacteria's pigment was observed after exposure to high silver concentrations. Scanning electron microscopy images showed the presence of silver nanoparticles embedded in the extracellular polymeric substance of both isolates. This finding suggested that the isolates may neutralize ionic silver via reduction to elemental silver. Antimicrobial testing revealed both organisms to be completely resistant to many commercially available silver-impregnated burn and wound dressings. Taken together, these findings provide the first evidence of clinical bacteria capable of expressing silver resistance at levels that could significantly impact wound management.
authors.affiliation: Mercy Hospital, Springfield Division of Trauma and Burn Research, Springfield, Missouri, USA phillip.finley@mercy.net.
authors.forename: Phillip J
authors.initials: PJ
authors.lastname: Finley
authors.affiliation: Jordan Valley Innovation Center, Center for Biomedical and Life Sciences, Springfield, Missouri, USA.
authors.forename: Rhy
authors.initials: R
authors.lastname: Norton
authors.affiliation: Mercy Hospital, Springfield Division of Trauma and Burn Research, Springfield, Missouri, USA.
authors.forename: Cindy
authors.initials: C
authors.lastname: Austin
authors.affiliation: Mercy Hospital, Springfield Division of Trauma and Burn Research, Springfield, Missouri, USA.
authors.forename: Amber
authors.initials: A
authors.lastname: Mitchell
authors.affiliation: Mercy Hospital, Springfield Division of Trauma and Burn Research, Springfield, Missouri, USA.
authors.forename: Sara
authors.initials: S
authors.lastname: Zank
authors.affiliation: Jordan Valley Innovation Center, Center for Biomedical and Life Sciences, Springfield, Missouri, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Durham
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001458:Bandages; D002056:Burns / Q000188:drug therapy* / Q000382:microbiology; D024881:Drug Resistance, Bacterial; D004755:Enterobacteriaceae / Q000187:drug effects*; D004756:Enterobacteriaceae Infections / Q000188:drug therapy*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D012834:Silver / Q000494:pharmacology*; D014946:Wound Infection / Q000188:drug therapy* / Q000382:microbiology
chemical_list: D000900:Anti-Bacterial Agents; D012834:Silver