I’ve been asked to write a chapter providing an overview of options for antimicrobial surfaces in hospitals for a Springer book. As a result of the preliminary literature reviews for this chapter, I’ve summarized the various available options for antimicrobial surfaces in hospitals in this post.
A number of different interventions aimed at improving environmental hygiene have been evaluated. Switching from one disinfectant to a product with superior microbiological efficacy in particular has been shown to reduce transmission.1-6 However, one of the problems with available disinfectants is the lack of residual effect, meaning that recontamination occurs quickly.7,8 An attractive option is to somehow make surfaces antimicrobial to exert a continuous reduction in the level of contamination. A recent review by Prof Hilary Humphreys provides a useful overview of the various approaches to antimicrobial surfaces.9 There are several approaches to making a hospital surface ‘antimicrobial’:
- Permanently ‘manufacture in’ an agent with antimicrobial activity (e.g. copper or a chemical).
- Periodically apply an agent with antimicrobial activity (e.g. copper containing liquid agents, or chemical disinfectants with residual activity).
- Physically alter the properties of a surfaces to make it less able to support microbial contamination or easier to clean (e.g. antibiofilm surfaces).
The table below provides an overview of the various options available to make a hospital surface antimicrobial.
|Copper||Manufactured in / liquid disinfectant||Rapidly microbicidal; large evidence-base; evidence of reduced acquisition.||Sporicidal activity equivocal; cost, acceptability and durability may be questionable.|
|Silver||Manufactured in / liquid disinfectant||Broadly microbicidal.||? sporicidal; tolerance development; relies on leaching so surface loses efficacy over time.|
|Organosilane||Liquid disinfectant||Easy to apply.||Limited microbicidal activity; questionable “real-world” efficacy.|
|Light-activated (e.g. titanium dioxide or photosensitisers)||Manufactured in / liquid disinfectant||Broadly microbicidal; can be activated by natural light.||? sporicidal; requires light source for photoactivation (some require UV light); may lose activity over time.|
|Quaternary ammonium compound based||Liquid disinfectant||Easy to apply.||Limited microbicidal activity; largely untested real-world activity.|
|Triclosan||Manufactured-in / liquid disinfectant||Already adopted in some consumer markets.||Resistance / tolerance development; relies on leaching so surface loses efficacy over time.|
|Polycationic e.g. polyhexamethylene biguanide, PHMB||Liquid disinfectant||Easy to apply.||Limited microbicidal activity; questionable “real-world” efficacy.|
|Physical alteration of surface properties|
|“Liquid glass” (silicon dioxide)||Liquid application||Reduces deposition; improves ‘cleanability’.||Not microbicidal; some evidence of reduced contamination; unknown required frequency of application.|
|Sharklet pattern||Manufactured-in||Reduces deposition; reduced. biofilms.||Not microbicidal; not feasible to retrofit.|
|Advanced polymer coatings (e.g. polyethylene glycol PEG, superhydrophobic/philic, zwitterionic)||Manufactured-in||Reduces deposition; some can be ‘doped’ with copper or silver.||Not microbicidal; may be expensive; scale up to large surfaces questionable; not feasible to retrofit.|
|Diamond-like carbon (DLC) films||Manufactured-in||Reduces deposition; can be ‘doped’ with copper or silver.||Not microbicidal; likely to be expensive; feasibility of scale up to large surfaces questionable; not feasible to retrofit.|
There are some other options not listed in the table, that could be considered candidates for antimicrobial surfaces, although they are currently at an early stage of development, including:
- Negative air ionization to repel bacteria from surfaces.10
- Enzymes immobilized on surfaces.11
- Bacteriophages immobilized on surfaces.12
There is an impressive and rapidly emerging evidence-base for copper surfaces.13 The implementation of copper high-touch surfaces, which have a continuous biocidal action, results in a reduction in contamination and may reduce transmission.14-16 However, copper is expensive, difficult to retrofit and durability may be questionable.13,17 Thus, an effective disinfectant with a residual activity that does not compromise staff or patient safety or promote the development of reduced susceptibility is desirable. Several candidate disinfectants that have residual activity with a variety of active chemicals have emerged.18-22 These can be delivered through pre-existing cleaning and disinfection arrangements at little or no extra cost. However, there is very little published data on the microbiological or clinical impact of disinfectants with residual activity. A number of recent study suggest that promising in vitro activity may not translate into “real-world” impact: a recent study by Boyce et al. found that two organosilane products simply did not work as intended when applied to surfaces in a US hospital.22
During my research for this post, I came across a very useful presentation by Peter Hoffman from Public Health England, which can be downloaded here. Taking some of his ideas, plus a few of my own, the following points for discussion emerge:
- Which is the optimal deployment mode – antimicrobial agents that are manufactured in or periodically applied, or ways to make the surface physically less able to support contamination or easier to clean?
- If periodic application is selected, how frequently is a fresh application required (i.e. how durable is the antimicrobial coating)?
- Which surfaces should be made antimicrobial? It’s probably not feasible to do them all, particular for antimicrobial options that need to be manufactured in.
- Surfaces in hospitals are often dirty (obviously); it’s not clear how much the presence of organic matter would interfere with the activity of antimicrobial surfaces. Clearly, antimicrobial surfaces do not obviate the need for careful attention to hospital cleaning and disinfection. In fact, their continued effectiveness depends on it.
- The deposition of contamination and potential acquisition of contamination through contact with surfaces often occurs in quick succession, so antimicrobial surfaces with a contact time measure in minutes (rather than seconds) may be too slow to be useful.
- C. difficile spores represent a real challenge to antimicrobial surfaces. Copper seems to get closest to demonstrating inactivation, but even here data are somewhat equivocal.23 Could introducing an antimicrobial surface that is not effective against C. difficile “squeeze the balloon” and provide a selective advantage to C. difficile?
- How effective will antimicrobial surfaces that rely on an active agent leaching from surfaces be in a dry environment?
- How do we test – and compare efficacy – of antimicrobial surfaces? A standardized test has been proposed,24 but not yet adopted widely. Importantly, this methodology specifies an aerosol deposition of microbes whereas other proposed methodologies specify the deposition of microbes in a liquid suspension. Testing the ‘wet’ deposition of microbes may overestimate the antimicrobial potential of the surfaces, which would usually be challenged with dry deposition in the real world.
- Much of the literature for antimicrobial surfaces is published in materials science journals, as illustrated in this useful review by Page et al.25 I, for one, find this pretty difficult to access; as a healthcare scientist, it’s a new and daunting language to learn.
- The cost, and cost-effectiveness of implementing antimicrobial surfaces in the healthcare setting has not been rigorously assessed.
There’s a plethora of potential options and approaches to make a hospital surface ‘antimicrobial’. Copper is leading the way as a candidate, although other options are available. Making a surface less able to support contamination in the first place, and / or easier to clean is another tempting option, particularly if this can be combined with a level of antimicrobial activity. Finding and evaluating the optimal antimicrobial surface will require a multidisciplinary approach, requiring industrial partners, materials scientists, healthcare scientists and epidemiologists to refine and test the available options. More studies in the clinical setting, ultimately including those with a clinical outcome, are required.
Photo credit: Benjamin Hall.
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