Silver resistance in Gram-negative bacteria: a dissection of endogenous and exogenous mechanisms

Abstract

ObjectivesTo gain a more detailed understanding of endogenous (mutational) and exogenous (horizontally acquired) resistance to silver in Gram-negative pathogens, with an emphasis on clarifying the genetic bases for resistance.MethodsA suite of microbiological and molecular genetic techniques was employed to select and characterize endogenous and exogenous silver resistance in several Gram-negative species.ResultsIn Escherichia coli, endogenous resistance arose after 6 days of exposure to silver, a consequence of two point mutations that were both necessary and sufficient for the phenotype. These mutations, in ompR and cusS, respectively conferred loss of the OmpC/F porins and derepression of the CusCFBA efflux transporter, both phenotypic changes previously linked to reduced intracellular accumulation of silver. Exogenous resistance involved derepression of the SilCFBA efflux transporter as a consequence of mutation in silS, but was additionally contingent on expression of the periplasmic silver-sequestration protein SilE. Silver resistance could be selected at high frequency (>10−9) from Enterobacteriaceae lacking OmpC/F porins or harbouring the sil operon and both endogenous and exogenous resistance were associated with modest fitness costs in vitro.ConclusionsBoth endogenous and exogenous silver resistance are dependent on the derepressed expression of closely related efflux transporters and are therefore mechanistically similar phenotypes. The ease with which silver resistance can become selected in some bacterial pathogens in vitro suggests that there would be benefit in improved surveillance for silver-resistant isolates in the clinic, along with greater control over use of silver-containing products, in order to best preserve the clinical utility of silver.

Preventing Implant-Associated Infections by Silver Coating

“…In contrast, the Gram-negative Enterobacter cloacae strain (ATCC 13047) with a genomically encoded silver resistance cassette became resistant after 5 cycles, whereas another randomly chosen Enterobacter cloacae strain (ATCC 23355) did not acquire resistance. It has recently been shown in Gram-negative bacteria that silver resistance requires an efflux transport system and either a loss of outer membrane porin or an additional periplasmatic silver sequestration protein (26). Our study strengthens the fact that this concerted action against intracellular silver is so far neither known to be inherent nor inducible for Gram-positive bacteria, which makes silver coatings interesting for clinical use.…”

“…Despite this broad clinical use, little is known about the stability of silvercoated alloys, their efficacy on biofilm-forming bacteria-especially in combination with antibiotics-and the kinetics of release. Moreover, silver has a broad range of bacterial targets, including the respiratory chain (21)(22)(23)(24), and has been shown to induce resistance in Gram-negative bacteria and toxicity in eukaryotic cells (25,26).…”

Survival in amoeba—a major selection pressure on the presence of bacterial copper and zinc resistance determinants? Identification of a “copper pathogenicity island”

“…Rather, resistance to silver is a by-product of copper resistance and cadmium is a by-product of zinc resistance. In fact, resistance studies with S. aureus were unable to produce silver-resistant strains even after 42 days of continuous passage in the presence of AgNO 3 (Randall et al 2013) Similar results were observed for some Gram-negative organisms, whereas in E. coli strains, silver resistance arises as a result of mutations in both ompR and cusS or mediated through the sil system (Randall et al 2015).…”

“…One, the pco determinant was first isolated from plasmid pRJ1004 from an Australian pig E. coli isolate (Brown et al 1995) and confers copper resistance. The other, the sil determinant originally located on Salmonella Typhimurium plasmid pMG101, but later shown to have transferred into the chromosome of the host E. coli K-12 J53 strain (Randall et al 2015), is associated with silver resistance (McHugh et al 1975;Gupta et al 1999). Later sequencing of pRJ1004 (NCBI accession no.…”

Unprecedented Silver Resistance in Clinically Isolated Enterobacteriaceae: Major Implications for Burn and Wound Management

“…Like the Sil system, the cus operon includes genes for a sensor kinase (Sil/CusS), a periplasmic efflux transporter (Sil/CusBCA), and a periplasmic Ag ϩ binding protein (Sil/ CusF). The two systems share Ͼ80% amino acid identity, and both are thought to function primarily via silver efflux (24,25).…”