For the technical-minded listers, you should peruse
this:
Silver cations as an antimicrobial agent: clinical
uses and bacterial resistance
Simon Silver, Jeng-Fan Lo and Amit Gupta
Department of Microbiology and Immunology, University
of Illinois, Chicago, Illinois, USA
Silver cations (Ag+) are important, often
misunderstood compounds that play a significant role
as effective and legitimate antimicrobial agents, used
particularly in the treatment of burns. Their spectrum
of uses is broad and generally unfamiliar, ranging
from beneficial clinical applications, to commercial-
and folk-practices that are of questionable value but
of little harm, to "snake oil" products and frauds
found through the internet and in health food stores.
In an effort to better understand and anticipate the
uses of these compounds, we recently studied
plasmid-mediated resistance to silver. (1) Advances in
molecular genetics have allowed us to use
epidemiological tools to establish the range and
diversity of resistance systems. Past difficulties in
measuring Ag+ resistance have been overcome. (2) The
importance of the halide concentration and initial
cell number in such measurements has been shown, and
after the sequence of the first silver resistance gene
cluster was complete, (1) we found closely homologous
genetic determinants surprisingly abundant in hospital
collections of enteric bacteria, both from
silver-exposed and not-knowingly silver-exposed
sources (A. Gupta et al., in prep.).
Clinical Uses of Silver Products
Silver cations are microcidal at low concentrations,
and are without serious side effects for humans.
Argyria (irreversible discoloration of the skin
resulting from subepithelial silver deposits) is rare
and mostly of cosmetic concern. The widest and
best-known medicinal use of silver preparations is as
preferred antimicrobial agents for the treatment of
serious burns. (3) Silver sulfadiazine cream that
contains 1% silver sulfadiazine plus 0.2%
chlorhexidine digluconate is the mostly widely used
product, marketed as Silvazine in the United States
for human and veterinary use. Flamazine is the same
product in other countries, largely in the United
Kingdom, Canada and continental Europe. Ag-coated
nylon is increasingly being used to cover burn wounds
and traumatic injuries to humans4 and large animals.
(5) Silver sulfadiazine-coated methacrylate sheet
material that provides a stable base for sustained
release of Ag+ over days is also being investigated.
(6) These silver-containing fabrics are easier to
apply and remove from large burns than is the residue
of a cream. Sometimes a low voltage DC current is
applied across a sheet to accelerate release of Ag+
from the cloth. (4,7) Additional clinical uses include
aseptic coverings for plastic surgery, traumatic
wounds, leg ulcers, skin grafts, incisions, abrasions
and minor cuts. Plastic indwelling catheters coated
with silver compounds8 are being developed to retard
the formation of biofilms and stem the incidence of
nosocomial infection. The use of Ag-coated nylon
threads in electroretinograms has allowed the
detection of tissue damage without fear of infection.
(9) Silver salts have traditionally been administered
to the eyes of newborn infants to prevent neonatal eye
infections. Dental amalgam, so-called "silver
fillings," contain about 35% Ag (0) and 50% Hg (0),
but we do not know if sufficient Ag+ is released to
have an antimicrobial effect. It is known, however,
that the release of Hg2+ from dental amalgams selects
for metal-resistant bacteria. (10)
Bacterial Resistance and Genetics
Bacterial resistance to silver sulfadiazine, with its
sometimes tragic consequences, has been periodically
reported. An Ag+-resistant Salmonella strain killed
three patients and required the closing of the burn
ward at Massachusetts General Hospital (MGH). (11)
Although silver sulfadiazine-resistant bacteria have
occasionally been observed in burn ward infections,
and while chromosomal mutations of clinical strains to
Ag+ resistance may also cause a problem in infection
(12), resistance rates have not been followed.
The plasmid-determined gene cluster for silver
resistance from the MGH Salmonella (11) contained a
total of nine genes, seven of which we have named with
the two less recognized open reading frames still
called ORFs: in order silP (ORF105) silAB (ORF96) C
silRS silE. (1) The system encodes a periplasmic
Ag+-binding protein (SilE) plus two membrane Ag+
efflux pumps (SilCBA and SilP). The central six genes
(silA through silS) produce products that are
homologous to an unstudied gene cluster on the
Escherichia coli genome (currently called ybdE, ylcD,
ylcC, ylcB, ylcA and ybcZ, in order) and less closely
to other metal resistance systems. In Southern
blotting DNA/DNA hybridization analysis of clinical
isolates with homologous DNA, the central six genes
appear to always be present together, but homologs of
the outer two genes, silP and silE, are occasionally
missing (A. Gupta et al., in prep.). The six genes,
silPORF105ABORF96silC, are co-transcribed in a very
long mRNA. (1) The regulatory gene pair silRS is
co-transcribed separately, and silE is transcribed by
itself as a third mRNA. (1)
Mechanism of Resistance
The functions of silver-resistance gene products can
be recognized by homology to other gene products that
have been studied. SilP is a membrane P-type ATPase
that pumps Ag+ from the cell1, (13,14) and is most
similar to Cu+ and Cd2+ efflux ATPases. SilCBA
(probably together with the ORF96 product) form a
second Ag+ efflux pump driven by the membrane
potential and not ATP. This pump consists of three
proteins, one in the inner membrane (SilA), another in
the outer membrane (SilC), and the third bridging the
periplasmic space (SilB). Three-protein membrane
potential-driven cation/proton exchangers were
initially recognized in our laboratory with a
bacterial Cd2+/Zn2+/Co2+ system. (14)
This silver resistance system is the first time we
have seen three different mechanisms in a single toxic
metal cation resistance determinant. It appears to be
transcriptionally controlled by the products of two
genes, SilS (a histidine-containing membrane
auto-kinase "sensor") and SilR (a cytoplasmic
DNA-binding activator "responder" that contains an
aspartate residue that is trans-phosphorylated from
SilS). SilRS is homologous in sequence to members of
the large family of two-component sensor/responder
transcriptional regulators that respond to
extracellular signals. (1,14)
SilE is a small periplasmic Ag+-binding protein that
binds Ag+ ions specifically at the cell surface,
presenting the first line of resistance against Ag+
toxicity. The SilE protein has been purified to
homogeneity and extensively studied by J-F Lo et al.
(in prep.). The SilE protein contains ten histidine
residues that bind five Ag+ cations1 (J-F Lo et al.,
in prep.). In contrast to other metal-binding
proteins, SilE has no cysteine residues. Binding of
Ag+ to the SilE protein brings about an unusually
large change in protein folding, from essentially
disordered, to a predominantly alpha helical
structure. At this early stage, we do not know whether
silE, which confers some Ag+ resistance by itself,
will ever be found alone or how the various sil gene
products interact for full resistance.
Non-Clinical Uses of Silver
Our primary concern remains Ag+ usage in the clinic
and the selection for Ag+ resistance. The wide spread,
often unchecked application of silver products as
biocides is adding to the problem. Silver-containing
products are used in hospital and hotel water
distribution systems to control infectious agents
(e.g., Legionella). Silver has been used to sterilize
recycled water aboard the MIR space station and on the
NASA space shuttle. (15) Home-water purification units
sold in the US supermarkets contain silverized
activated carbon filters and ion-exchange resins.
Silver is a health additive in traditional Chinese and
Indian Ayurvedic medicine. (16) In Mexico,
supermarkets sell Microdyn, colloidal silver in
gelatin, to disinfect salad vegetables and drinking
water. Johnson Matthey Chemicals (UK) uses an
inorganic composite with immobilized slow-release
silver as a preservative in cosmetics and toiletries.
(17) In Japan, a new compound is mixed into plastics
for lasting antimicrobial protection of telephone
receivers, calculators, toilet seats, and children's
toys. (18) Metallic silver-copper containing ceramic
disks, marketed as "Clean Power Plus," are sold as an
alternative to laundry detergents. (19) Silver
addition to fabrics (similar to clinical use of
Ag-nylon) is proposed to reduce buildup of microbial
populations and therefore offensive smells in camping
gear and clothing. While folk remedies and "snake oil"
preparations are not the same, they are coupled here
as representative of applications with suspect
benefit. (9) Over-the-counter Ag+ health food
supplements are probably not effective (20) and are
frequently mislabeled. (21) The non-clinical uses of
silver appear endless, with one possible, detrimental
side-effect being the lessening of its usefulness as
an antimicrobial agent.
What is Needed
The identification of the genes for silver resistance,
and the determination of closely related genes in
bacteria from environmental and clinical environments,
and from diverse geographical locations (A. Gupta et
al., in prep.) should eliminate recent skepticism
about the existence of silver-resistant bacteria. Now
that the means for identifying silver resistance
determinants in Enterobacteriaceae is available,
similar efforts should be made with respect to other
common pathogens, particularly those associated with
large burns (i.e., pseudomonads and staphylococci).
The wide and rather uncontrolled use of silver
products may result in increased resistance, analogous
to the emergence of antibiotic- and other
biocide-resistant bacteria. Undermining the benefits
of these compounds would be unfortunate to the
clinical and hygienic uses that depend on the
microcidal properties of silver.
References
1. Gupta A, Matsui K, Lo JF, Silver S. 1999. Nature
Medicine 5:183-188.
2. Gupta A, Maynes M, Silver S. 1998. Applied
Environmental Microbiol 64: 5042-5045.
3. Rosenkranz HS, Carr HS. 1972. Antimicrob Agents
Chemother 2: 367-372;
4. Monafo WW, West MA. 1990. Drugs 40:364-373; Fox CL
Jr, Rao TN, Azmeth R, Gandhi SS, Modak S. 1990. J Burn
Care
Rehabilitation 11:112-117.
5. Deitch EA, Marino AA, Malakanok V, Albright JA.
1987. J Trauma 27: 301-304.
6. Adams, AP, Santschi EM, Mellencamp MA. 1999.
Veterinary Surgery 28: 219-225.
7. Miller L, Hansbrough J, Slater H, Goldfarb IW ,
Kealey P, Saffle J, Kravitz M, Silverstein P. 1990. J
Burn Care Rehabilitation
11:35-41.
8. Modak S, Fox P, Stanford J, Sampath L, Fox CL Jr.
1986. J Burn Care Rehabilitation 7: 422-425.
9. Gabriel MM, Mayo MS, May LL,Simmons RB, Ahearn DG.
1996. Current Microbiol 33:1-5. The Silver Institute.
Washington,
DC, USA.
10. Lorscheider FL, Vimy MJ, Summers AO. 1995. FASEB J
9:504-508, 1499-1500.
11 McHugh SL, Moellering RC, Hopkins CC, Swartz MN.
1975. Lancet i: 235-240.
12. Li XZ, Nikaido H, Williams KE. 1997. J Bacteriol
179:6127-6132.
13. Silver S, Gupta A, Matsui K, Lo J-F. 1999.
Metal-Based Drugs 6 (in press).
14. Silver S, Phung LT. 1996 Annual Review Microbiol
50: 753-789.
15. Adachi K (editor). Colloidal Silver.
Educate-Yourself. Costa Mesa, CA, USA.
16. Reach for Life Enterprises. Fresno, CA, USA.
17. Johnson Matthey. London, England, UK.
18. Amenitop, silica gel microspheres containing a
silver-thiosulfate complex. Washington Post, February
5, 1993.
19. Mass Appeal Marketing. Torrance, CA, USA.
20. Fung MC, Weintraub M, Bowen DL. 1995. JAMA
274:1196-1197.
21. US Food and Drug Administration. 1996.
Over-the-counter drug products containing colloidal
silver ingredients or silver salts. Federal Register,
October 15, 61(200): 53685-53688; US Food and Drug
Administration. 1994. FDA Health Fraud Bulletin #19,
Colloidal Silver, October 7.
http://www.healthsci.tufts.edu/apua/Newsletter
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