A standard approach to assessing disinfection of environmental surfaces: Experiments using a mixture of surrogates for bioagents

Abstract

Background. Non-porous environmental surfaces accidentally/deliberately contaminated with bioagents might play a role in the spread of infections. The risk of such spread can be reduced with judicious use of liquid chemical disinfectants. Such chemicals are usually applied directly on the target surface as a liquid by immersion, flooding, spray, or by wiping with a disinfectant-wetted fabric. However, the assessments and label claims of microbicidal activities of such chemicals are often based on testing against individual classes of microbes without an appropriate soil load, while pathogens in the field are either unknown or may exist as mixtures in body fluids or other organic/inorganic debris. Further more, the influence of the mechanical action of wiping in decontaminating surfaces is hardly ever assessed. Wiping microbially-contaminated surfaces with an ineffective disinfectant may also spread localized contamination over a wider area, thus enhancing the risk of spread of infections. The potential impact of such spread remains virtually unknown. This study was, therefore, aimed at filling these three gaps in our knowledge. Experimental. To simulate field conditions better, the challenge was a cocktail of bacterial spores (Geobacillus stearothermophilus ), a vegetative bacterium (Acinetobacter baumannii), a mycobacterium (Mycobacterium terrae), and a non-enveloped virus (hepatitis A) in a soil load. Disks (1 cm in diameter) of brushed stainless steel were used as prototypical hard, non-porous environmental surface. Each disk received 10 muL of the microbial suspension. The inoculum was then dried prior to direct contact with the test disinfectant or assessing the effect of wiping for decontamination and transfer of contamination. In both decontamination methods, normal buffered saline (NBS) was used as the control fluid. Results. The microbial mixture successfully evaluated the efficacy claims and spectrum of activity of disinfectants concurrently, with no major differences in inactivation of microbes when challenged alone or as a mixture. Surprisingly, peracetic acid (PAA; 1,000 ppm) and accelerated hydrogen peroxide (AHP)-based formulations (40,000 to 70,000 ppm) reduced the viability of the spores to undetectable levels, but without complete inactivation of the virus at the same contact time and temperature. Chlorine dioxide (CD) and a 5.25% (w/v) solution of sodium hypochlorite, i.e., domestic bleach (DB), showed the broadest spectrum of microbicidal activity. However, their activities depended on air temperature. While CD (500 and 1,000 ppm) and DB (5,000 ppm) completely inactivated HAV and the spores at room temperature (∼22-24°C), these formulations only killed the virus at 4°C but not the spores. M. terrae and A. baumannii were relatively readily inactivated and proved to be very similar in their sensitivities to all the tested chemicals. In general, wiping with a disinfectant-wetted fabric was more effective than direct kill for decontamination. Our data suggested that applying a formulation directly on to a contaminated surface, letting it dry, and then wiping the surface with a disinfectant helped achieve greater log10 reductions while reducing substantially the risk of transfer of contaminants to clean areas. Conclusions. This study assessed the relative strengths and the spectrum of activity of the tested formulations concurrently and under conditions more akin to those expected under field conditions. Incorporation of a cocktail of microorganisms into a single test permitted a more reliable means of classifying microbicides based on their spectrum of activity. Chlorine releasing agents proved to be more effective and have a broader spectrum of activity as compared to the peroxygens. Disinfectant activity depended on air temperature, and none of them was effective against the spores at 4°C. Decontamination by wiping with disinfectants proved to be faster and more effective than direct kill. However, a higher risk of a wider spread of contamination during wiping was also evident in this study. The foundations laid by this investigation should allow more detailed studies using actual infectious bioagents. (Abstract shortened by UMI.)