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Superbug Resistance to Antimicrobial Nanosilver

By Cindy Gunawan & Christopher P. Marquis

A group of widely-occurring bacteria has been able to overcome the antimicrobial activity of nanosilver upon prolonged exposure.

We are and always have been at constant war against pathogens, not only in extreme situations such as epidemics but also on a daily basis. Along the way we have encountered a vast number of antimicrobial agents, each with unique target microorganisms and potency. These antimicrobials range from natural forms of disinfectant, such as moulds and plant extracts, to more sophisticated engineered chemicals including antiviral drugs.

Antimicrobial nanosilver is one of the earliest and most developed products of nanotechnology. Nanosilver is a versatile antimicrobial agent with proven efficacy against a broad range of microorganisms, including bacteria, yeast, fungi, algae and even viruses.

The disinfecting properties of silver have been known and employed since ancient times, most commonly to disinfect water and food in storage. We now see the incorporation of nanosilver as the core antimicrobial ingredient in consumer products ranging from wound dressings and antibacterial textiles to water and air purification systems, coatings in food packaging and even in baby products.

Using nanotechnology, the antimicrobial properties of silver are harnessed by manipulating the physicochemical characteristics. Silver in its nano form (less than 100 nm – or ten-thousandths of a millimetre) is many times more potent than the much larger “bulk” silver. It is more reactive because its surface area is much bigger given its mass. The antimicrobial activity of nanosilver originates from the released soluble silver and non-soluble nanosilver solids. Exposure to nanosilver stimulates the generation of reactive oxygen species inside cells, resulting in cell death.

With the increasing commercialisation of nanosilver have come growing concerns about the development of nanosilver-resistant microorganisms. These concerns are prudent, considering the problems worldwide caused by antibiotic-resistant microorganisms such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus, just to name a few, due to the widespread use of antibiotics.

According to the World Health Organization, the death rate of patients infected by antibiotic-resistant bacteria is about double that in patients infected by non-resistant bacteria. The spread of antibiotic-resistant pathogens has been one of the reasons for the exploitation of nanosilver as an alternative antimicrobial. Until recently there has been no evidence of microorganism resistance to nanosilver.

In a current study, our research group has found that the antimicrobial activity of nanosilver is not universal – some bacteria have been shown to adapt quite rapidly to its presence. Our research group used model systems to simulate antimicrobial nanosilver exposure in an environmental or clinical setting and found that while the presence of nanosilver effectively suppressed the growth of the common household and clinical pathogen Escherichia coli, there was unexpected emergence and ultimate domination of nanosilver-resistant environmental Bacillus. The environmental Bacillus were not only resistant to the otherwise lethal dosage of nanosilver, but also were rapidly multiplying under the toxic conditions (Fig. 1). The near-ubiquitous Bacillus genus of bacteria range from benign to potentially harmful pathogenic species.

Fig 1

We then investigated whether the nanosilver-resistance trait is a form of an induced adaptive response of the Bacillus to the antimicrobial activity. We exposed a laboratory strain of Bacillus subtilis to the prolonged presence of nanosilver, and found that it developed resistance in the forms of nanosilver tolerance and abnormally enhanced extent of growth. The resistant bacteria were able to proliferate even when exposed to highly toxic nanosilver conditions, under which the growth of control bacteria (that were not pre-exposed to nanosilver) was suppressed. The findings are in agreement with observations from the dominating environmental Bacillus species. Upon prolonged exposure, the adaptive response was induced in the Bacillus regardless of the levels of nanosilver-stimulated cellular reactive oxygen species generated and, therefore, the extent of toxicity. Further, we found that the adaptive response was stable: the effects of nanosilver tolerance and enhanced growth were still present even after prolonged nanosilver exposure was discontinued.

Nanosilver is one of the fastest-growing antimicrobial products in the market. Much like the bacterial response to the widespread use of antibiotics, antimicrobial resistance to nanosilver appears to be developing. Our work is the first unambiguous evidence of the induced adaptive responses of microorganisms to nanosilver.

Bacillus bacteria are present almost everywhere as air-borne spores, so adverse implications of nanosilver exposure could increase. There is also a possibility that the resistance trait could be transferred to other microorganisms through “horizontal gene transfer” – microorganisms can acquire resistance genes from other microorganisms without direct exposure to nanosilver.

Antimicrobial resistance in microorganisms is a growing worldwide problem. Silver and nanosilver have been seen as a partial solution because their mode of action is very different to that of conventional antibiotics. Unfortunately, the constant presence of nanosilver in a given microbial population may allow resilient microorganisms to evolve and develop resistance.

For the use of nanosilver in medical and environmental settings, our discovery of microbial adaptation implies the need for higher nanosilver doses to achieve desirable antimicrobial effects, and therefore reduced efficacy of nanosilver treatment.

Our pioneering findings signal the need for caution in the use of nanosilver and the need for deeper research into antimicrobial mechanisms that lead to the adaptive responses. While the application of nanosilver in consumer products is perhaps effective, the long-term impact and disposal of nanosilver products demand more thorough investigation.

Cindy Gunawan, School of Chemical Engineering, University of New South Wales; Australian Research Council Centre of Excellence for Functional Nanomaterials

Christopher P. Marquis, School of Biotechnology and Biomolecular Sciences, University of New South Wales