Investigating alternative materials to EPDM for automatic taps in the context of Pseudomonas aeruginosa and biofilm control
Introduction
Pseudomonas aeruginosa is an opportunistic pathogen of growing concern. In 2017, P. aeruginosa was listed as one of the highest priority organisms for discovery of new antimicrobials by the World Health Organization [1]. In the same year, reporting of P. aeruginosa bacteraemia cases to Public Health England changed from voluntary to mandatory. The challenge P. aeruginosa poses to the healthcare sector is partially due to its prevalence in water distribution systems and ability to colonize and form biofilm on and within water outlets. The colonization of water distribution systems by P. aeruginosa and the contamination of water dispensed from local outlets has been demonstrated during extensive environmental investigations following cases of infection [2,3] and under controlled laboratory conditions [4,5].
Automatic (or sensor) taps have been implicated in incidents of waterborne infection [6,7] and have been associated with higher levels of water contamination than manual taps [8,9]. One of the advantages of installing automatic taps is the ability to conserve water which saves costs [10]. Reduction in water consumption is a major consideration for the NHS, which uses approximately 40 billion litres per annum, costing ∼£47.5 million per year with a carbon footprint of ∼0.27 g/L CO2 [11,12].
Another perceived advantage of automatic taps is preventing cleaned hands from becoming re-contaminated on tap handles after handwashing. However, despite being appropriate for installation into potable water systems (i.e. complying with BS6920 [13]), materials associated with certain components, such as ethylene propylene diene monomer (EPDM) rubber within solenoid valves (SVs), have been reported to facilitate high levels of biofilm formation [14]. SVs are instrumental to the function of automatic taps, acting as the gating mechanism in lieu of a manual handle, but, if colonized with bacteria, can continually contaminate the tap water [4].
The clinical tap industry has recognised that component material and/or design can facilitate colonization and as such are producing novel designs and incorporating alternative materials. It has previously been demonstrated that contamination of tap outlet fittings can be reduced/cleared by flushing, and whilst novel designs may not prevent P. aeruginosa colonization, they may be more appropriate for P. aeruginosa control [5]. When considering SVs, nitrile-, silicone- and an ‘antimicrobial’ alternative, silver ion-impregnated silicone rubber, have all been identified as potential substitutes for EPDM by the clinical tap industry.
The aims of this study were to investigate biofilm formation on different rubber materials, the colonization of SVs incorporating different diaphragm materials and the impact on water hygiene when these were installed in an established experimental water distribution system (EWDS).
Section snippets
Preparation of Pseudomonas test suspensions
Pseudomonas aeruginosa previously isolated from the EWDS (previously described [4]), was resuscitated from -20°C cryobeads (Technical Service Consultants Ltd, Bury, UK) on to Tryptone Soya Agar (TSA; Oxoid Ltd, Basingstoke, UK) and incubated for 48 h at 37°C, then subcultured on to Cetrimide (CN) agar (Oxoid) and incubated under the same conditions. CN agar was stored at 4°C for up to two weeks. To prepare overnight liquid cultures, a single colony from the CN agar was suspended in 100 mL
Investigating biofilm on materials in vitro
Significantly higher numbers of P. aeruginosa were recovered from nitrile (median 6.2 × 105 cfu/coupon; N = 17) and silicone rubber (median 5.4 × 105 cfu/coupon; N = 17) than from EPDM rubber (median 2.9 × 105 cfu/coupon; N = 17). No other differences were significant.
Investigating biofilm on materials in situ
Immediately following installation of the new silicone and silver ion-impregnated silicone SVs (i.e. prior to artificial inoculation), water collected from seven of eight taps contained P. aeruginosa at concentrations ranging from
Discussion
In response to reports of EPDM-based SV colonization under controlled laboratory conditions [4] and in real-life settings [6,17], manufacturers are investigating the suitability of alternative materials. This study demonstrates that, when assessing biofouling, there is disparity between in vitro and in situ testing. When tested in vitro, nitrile rubber supported significantly higher levels of P. aeruginosa biofilm than EPDM, implying perhaps that EPDM (despite still being colonized at a high
Author contributions
C.F.H., G.M. and J.T.W. conceived and designed the experiments. C.F.H. conducted the experiments, G.M. assisted with flushing experiments. C.F.H. analysed the data. C.F.H. and G.M. wrote the manuscript. G.M., J.T.W. and J.W. co-supervised and were co-applicants on the studentship award to C.F.H. from the Healthcare Infection Society. All authors approved the final version of the manuscript.
Acknowledgements
The authors would like to thank the Healthcare Infection Society for funding this study. The authors would like to thank Howard Tolley for carrying out scanning electron microscopy and industrial partners for providing material surfaces and/or solenoid valves. The authors would also like to thank Isabella Romer for assistance with ICP-MS and Nanosight, David Stevenson and Simon Parks for EWDS technical assistance and the EMCOR plumbing support team.
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