29 October 2013

Nanosilver, one of the most developed products of nanotechnology, is a potent and versatile antimicrobial agent that can kill microorganisms, including bacteria and viruses, but new research has unearthed that some bacteria flourish under prolonged exposure.

Nanosilver is a nanoparticle form of silver and it is included in many products that we use on a daily basis, such as hand sanitisers and wound dressings.

Incorporation of nanosilver as a core antimicrobial agent in environmental and clinical applications has significantly increased and there has been a lack of evidence on the potential development of microorganism resistance. Substantial efforts have been devoted to the development of efficient antimicrobial nanosilver, but only few have actually investigated the subsequent impact of its continuous exposure to microorganisms.

Through carefully controlled microbial experiments, researchers from the University of New South Wales (UNSW) and City University of Hong Kong (CityU) have reported for the first time a phenomenon of induced adaptation of microorganisms to antimicrobial nanosilver.

“We found an important natural ability of the widely occurring Bacillus genus—a bacteria which can move as airborne spores—to adapt resistance to antimicrobial nanosilver upon prolonged exposure,” said Dr Cindy Gunawan of the ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering at UNSW.

“In model systems, the growth of the common household and clinical pathogen Escherichia coli was suppressed in the presence of nanosilver. However, we observed the emergence, adaptation and abnormally enhanced growth of environmental Bacillus sp upon prolonged exposure."

Further work using a laboratory strain of Bacillus produced the same result and the researchers also found that the induced adaptation is stable, even following discontinuation of nanosilver exposure.

“What we can clearly see is that antimicrobial action of nanosilver is not universal— some bacteria appear to adapt quite rapidly to its presence. This has implications for the widespread use of these products which should take into consideration the potential for longer-term adverse outcomes,” Dr Gunawan said.

Director of the ARC Centre of Excellence, Scientia Professor Rose Amal (UNSW), said the discovery has placed it at the forefront of understanding the adaptive responses of microorganisms to antimicrobial nanosilver.

“The findings call for judicial use of nanosilver and the requirement of much deeper research into the antimicrobial activity of nanosilver and how microorganisms respond to these disinfecting properties and the Centre is now focussed on these research efforts.

“Whilst the application of nanosilver in consumer products is perhaps effective, the longer term impacts of these nanosilver products need more thorough investigation.”

The research findings were recently published in Small and reported that the adaptations in non-targeted bacteria species may potentially make them more potent than the original targeted strain.

Co-author of the paper, Professor Wey Yang Teoh of CityU said that for the medical use of nanosilver, this implies the potential of reduced efficacy for the applications of nanosilver and potential for development of resistant bacterial populations in clinical settings.

“There is also widening usage of nanosilver in water and air purification systems as well as in consumer products, such as antibacterial textiles, coating in toys and baby products.

“The constant presence of nanosilver in a given environment may allow resilient microorganisms to physiologically evolve and develop resistance,” Professor Teoh said.

Further, there is a possibility that this resistance trait can be transferred to other microorganisms through (horizontal) gene transfer. According to co-author Dr Christopher Marquis (UNSW): “the mechanism enables microorganisms to acquire resistance genes through gene transfer from other microorganisms, without the need of direct exposure to nanosilver”.

This research is supported by the ARC. For more information please email Dr Cindy Gunawan at the ARC Centre of Excellence for Functional Nanomaterials.


Image courtesy: ARC Centre of Excellence for Functional Nanomaterials' Dr Cindy Gunawan, Dr Christopher Marquis from UNSW and the nanosilver-adaptive Bacillus bacteria.