Cath Lab Digest

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Index: Ostomy Wound Manage. 2008;54(3):30-40.

  Silver is an effective, broad-spectrum antimicrobial agent commonly incorporated into topical creams (eg, silver sulfadiazine)^1-11 and wound dressings to manage wound infection. Despite the medical benefits of using ionic silver to manage infections, concern has been raised regarding the potential for development of bacterial resistance and an association with cross-resistance to antibiotics has been implied. Cross-resistance is of concern because plasmids encoding silver-resistant genes also may encode for antibiotic resistance.

^12,13

  Silver-resistant organisms have been reported in clinical^5,14-18 and environmental ^19-22 samples. The genetic basis for silver resistance was first reported by McHugh et al¹³ underscoring that silver resistance was plasmid-encoded. This conclusion has been confirmed by others.^16,22-25

  The physiological, biochemical, genetic, and structural studies of the silver-resistant determinant plasmid pMG101 established the molecular basis of silver resistance.^12,26-29 Plasmid pMG101 is a 182 kb, transferable plasmid²⁶ encoding resistance to silver (nine Open Reading frames [ORFs] in three transcriptional units), mercury, tellurite, ampicillin, chloramphenicol, tetracycline, streptomycin, and sulphonamide.^13,26 It confers resistance in bacteria at silver concentrations six or more times the concentration of what a sensitive Escherichia coli can tolerate.³⁰ Functions assigned to the genes are based on homologous systems encoding resistances to other metals. The silver-resistance system encodes two silver efflux pumps (one an ATPase and the other chemiosmotic) and two periplasmic Ag⁺-binding proteins.³¹ The genes associated with silver resistance provided the basis of the current investigation to determine the prevalence of silver resistance genes in clinical wound isolates.

  The goals of this study were 1) to investigate the presence and prevalence of silver resistance and silver-resistant genes in clinical wound isolates taken from diabetic foot ulcers and 2) to examine the effects of silver-containing dressings on resistant bacterial viability.

Methods

  Organism collection and identification. The micro-organisms used in this study were routinely isolated from the chronic foot wounds of patients attending the Diabetic Foot Clinic at Tameside General Hospital, UK who had diabetic foot ulcers exhibiting signs and symptoms of clinical infection, as well as ulcers clinically free from infection. All wounds were swabbed using calcium alginate swabs. All microorganisms were cultured and identified at Medi-Lab Ltd, UK using standard protocols. The isolates obtained for this study were coagulase negative Staphylococcus aureus (CNS – 17 strains), Staphylococcus aureus (24 strains), Enterobacter spp. (three strains), Enterobacter cloacae (four strains), Escherichia coli (six strains), additional coliforms (two strains), Pseudomonas spp. (six strains), Pseudomonas aeruginosa (nine strains), Enterococcus faecalis (five strains), Alcaligenes faecalis (two strains), diphtheroids (10 strains), Citrobacter spp.