A novel method of ultra-violet disinfection and sterilization for use in Dental hospitals, practices and in the home
Aim/Principal research question
The aim of this study was to prove the efficacy, using a novel quartz beaker, of microwave generated UV-irradiation in disinfecting and sterilizing dental materials. The project investigated three main aspects: Efficacy of microwave-generated UV light in killing microorganisms, with a view to establishing the effective killing regimes for a range of oral organisms on the surfaces of the dental materials. Effects on dental materials of exposure within the beaker to UV light; as some materials (especially dentures) would be exposed repeatedly to UV-irradiation, the effects of long term repeated exposure to such irradiation on these materials were investigated,Utility of microwave-generated UV light in disinfection processes in laboratories and general practice.
Factors of interest
Current methods for disinfecting and sterilizing dental materials/instruments require the use of hazardous chemicals, considerable expenditure on autoclave equipment, and are often prohibitively time-consuming. Many materials used in dentistry are damaged by heat, making their sterilization difficult. Disinfection and sterilization by most available techniques are also time consuming, and it is not always possible to sterilize instruments between patients. We proposed that microwave-generated UV-irradiation would kill microorganisms on dental materials and instruments with greater speed, ease and economy than other commonly used methods, and that it would be applicable in the first instance to disinfection and sterilization of materials that are not adequately. or conveniently, treated by current procedures, eg. dentures, dental models coming into and leaving our laboratories, and dental instruments. It has the potential to provide significant improvements in cross-infection control and hygiene in dental practices.
Methodology
This was an experimental study, investigating killing of bacteria, yeast and viruses in suspension and dried onto materials. Killing, before and after exposure to UV within the beaker, was determined by viable counts. The effects of variables, such as increased protein concentration and time of exposure were determined. Kinetics of killing were determined by plotting dose-response curves, and D values (exposure producing 90% kill) and LD values (exposure producing >99% kill) were calculated. The effects of beaker temperature were investigated, and internal temperatures were measured using temperature strips placed at various positions within the beaker. Killing was also determined of preparations of organisms dried onto glass and the surfaces of materials used to make dentures (polymethylmethacrylate), dental impressions (silicone and alginate) and gypsum casts. Following repeated exposure of these materials to UV within the beakers, dimensional stability, impact strength and hardness were assessed as appropriate. Finally, the ease and efficiency of use of the beaker in a clinical setting was assessed by using them to disinfect silicone impressions made in clinics at the LDI, and comparing efficiency of disinfection with that attained by normal chemical disinfection procedures. Impressions taken were divided in two and one half disinfected as normal using Perform disinfecant and the other half was irradiated in the UV beaker for 60 seconds.
Sample groups
The study examined the efficacy of microwave-generated UV in reducing viable counts of the following organisms: Candida albicans (yeast - a common cause of denture stomatitis); Streptococcus mutans (Gram-positive bacterium implicated in caries); Pseudomonas aeruginosa (Gram-negative bacterium - a common environmental contaminant and one which is particularly refractive to the action of many disinfectants); Bacillus stearothermophilus (Gram-positive endosporeforming bacterium, commonly used to test sterilisation techniques); Herpes simplex virus (as a model for killing of pathogenic enveloped viruses); Polio virus (an attenuated strain as a model for killing pathogenic non-enveloped viruses). Saliva was also used as a source of contaminating organisms. Dental materials (polymethylmethacrylate, silicone, alginate and gypsum) were made in our laboratories. Silicone impressions from 12 volunteers attending a clinic at the LDI were used.
Outcome measures
For all microbiological experiments viable counts were made before and after exposure to UV in the beakers. Colony forming units (bacteria and yeast) or plaque forming units (viruses) were recorded and percentage kill attained by different exposures could be calculated. Kinetics of killing of microorganisms in suspension were determined by plotting doseresponse curves, and D values (exposure producing 90% kill) and LD values (exposure producing >99% kill) were calculated. Dimensional stability of materials was measured as a change in distance (in mm) between two points on materials cast in a standard test die. Impact strength was measured in kJ per m2, flexural strength in Mpa, and hardness in Wallace Hardness Number.
Findings
The efficacy of three beakers was compared, and most rapid killing was obtained with the plastic-encased Neutra Plasma 50(. Ultra-violet light generated within the beakers efficiently killed suspensions and surface-associated Streptococcus mutans, Pseudomonas aeruginosa, vegetative Bacillus stearothermophilus, Herpes and Polio viruses. Candida albicans and Mycobacterium phleii were less rapidly killed, and only 70% inactivation of B. stearothermophilus endospores was achieved. Irradiation for 45 s reduced viable bacterial counts in saliva by >99%. In all cases increased protein concentrations within the suspending medium protected organisms from most rapid killing. Kinetics of killing varied between organisms and conditions, reflecting that lethal mechanisms were complex. Increased temperature was important for most rapid killing, but highest temperatures resulted in decreased killing. Studies of dental materials revealed that alginate impression materials distorted during irradiation, but this was reduced by maintaining lower temperatures within the beaker. Dimensions of gypsum casts were also affected by irradiation. However, silicone was dimensionally stable under all test conditions. For denture base (polymethylmethacrylate) impact strength was measured and no change was observed after short exposure times. However, impact strength reduced with longer and repeated exposure times, as did flexural strength, and the denture base also became harder. Impressions taken from patients were divided in two and one half disinfected as normal using freshly prepared Perform disinfectant and the other half was irradiated in the UV beaker for 60 seconds. No viable bacteria were detected on any impression.
Conclusions
This novel method of generating UV light, using a cheap and widely available power source, provided a rapid, inexpensive and non-toxic method of disinfection, but sterility was difficult to attain. Mechanisms of killing were complex, probably involving interactions between UV, temperature and ozone generation. The beaker was suitable for use with silicone impressions but not alginate or gypsum casts. Repeated use to disinfect dentures would not be recommended. The process was rapid and convenient for use in clinics and was used to disinfect silicone impressions with efficiency and ease.
Implications for future research
This novel disinfection methodology has the potential to provide significant improvements in cross-infection control and hygiene in dental practices. Further applications may be found through advances in design of the beaker (for example, allowing more varied or specific sized and shaped articles to be exposed to UV). It may also be possible to reduce heat generation within the beakers, making it suitable for more heat-sensitive materials. In the long term, this technology could be an important improvement to current methodologies used in dental, medical and domestic contexts. Its development and exploitation could provide significant financial gains for all who have invested in its development and ultimate marketing, and additional significant reductions for the NHS in treatment and equipment costs..
Publications
Keech, A., Devine, D.A., Wood, D., Killington, R.A., Boyes, H., Doubleday, B. & Marsh, P.D. (2000) Ultra-violet disinfection, using a novel microwave-powered device. Journal of Dental Research 79:1201. Devine, D.A., Keech, A.P., Wood, D.J., Killington, R.A., Boyes, H., Doubleday, B. and Marsh, P.D. Ultra-violet disinfection using a novel microwave-powered device. Submitted for publication, Journal of Applied Microbiology.
Citation
Devine D (2002). A novel method of ultra-violet disinfection and sterilisation for use in Dental hospitals, practices and in the home (RDO/90/14). The Research Findings Register. Summary number 907. Retrieved 20 July 2005, from http://www.ReFeR.nhs.uk/ViewRecord.asp?ID=907