L.A. Fletcher

Bactericidal action of positive and negative ions in air

BackgroundIn recent years there has been renewed interest in the use of air ionisers to control of the spread of airborne infection. One characteristic of air ions which has been widely reported is their apparent biocidal action. However, whilst the body of evidence suggests a biocidal effect in the presence of air ions the physical and biological mechanisms involved remain unclear. In particular, it is not clear which of several possible mechanisms of electrical origin (i.e. the action of the ions, the production of ozone, or the action of the electric field) are responsible for cell death. A study was therefore undertaken to clarify this issue and to determine the physical mechanisms associated with microbial cell death.ResultsIn the study seven bacterial species (Staphylococcus aureus, Mycobacterium parafortuitum, Pseudomonas aeruginosa, Acinetobacter baumanii, Burkholderia cenocepacia, Bacillus subtilis and Serratia marcescens) were exposed to both positive and negative ions in the presence of air. In order to distinguish between effects arising from: (i) the action of the air ions; (ii) the action of the electric field, and (iii) the action of ozone, two interventions were made. The first intervention involved placing a thin mica sheet between the ionisation source and the bacteria, directly over the agar plates. This intervention, while leaving the electric field unaltered, prevented the air ions from reaching the microbial samples. In addition, the mica plate prevented ozone produced from reaching the bacteria. The second intervention involved placing an earthed wire mesh directly above the agar plates. This prevented both the electric field and the air ions from impacting on the bacteria, while allowing any ozone present to reach the agar plate. With the exception of Mycobacterium parafortuitum, the principal cause of cell death amongst the bacteria studied was exposure to ozone, with electroporation playing a secondary role. However in the case of Mycobacterium parafortuitum, electroporation resulting from exposure to the electric field appears to have been the principal cause of cell inactivation.ConclusionThe results of the study suggest that the bactericidal action attributed to negative air ions by previous researchers may have been overestimated.

Bioaerosol Production on a Respiratory Ward

Although much hospital acquired infection is associated with person-to-person contact, there is increasing evidence that some nosocomial infections may be transmitted via the airborne route. However, the knowledge base concerning airborne microflora in hospitals is poor. In particular, there is a need for good quality data relating bioaerosol production to clinical activity in hospital wards. A short aerobiological survey was therefore undertaken by the authors on a respiratory ward at St James’s University Hospital in Leeds in order to gain an understanding of the relationship between activity and bioaerosol production. This survey involved regular microbiological and particulate (0.3–5 m) sampling of the ward air, together with an observational study of ward activity. Two identical four-bed ward bays were surveyed, one containing high dependency patients who regularly used noninvasive ventilators (NIVs), and the other containing patients who did not require mechanical ventilation. The survey found a correlation between activity and aerosol production.

CFD simulation of airborne pathogen transport due to human activities

Abstract Computational Fluid Dynamics (CFD) is an increasingly popular tool for studying the impact of design interventions on the transport of infectious microorganisms. While much of the focus is on respiratory infections, there is substantial evidence that certain pathogens, such as those which colonise the skin, can be released into, and transported through the air through routine activities. In these situations the bacteria is released over a volume of space, with different intensities and locations varying in time rather than being released at a single point. This paper considers the application of CFD modelling to the evaluation of risk from this type of bioaerosol generation. An experimental validation study provides a direct comparison between CFD simulations and bioaerosol distribution, showing that passive scalar and particle tracking approaches are both appropriate for small particle bioaerosols. The study introduces a zonal source, which aims to represent the time averaged release of bacteria from an activity within a zone around the entire location the release takes place. This approach is shown to perform well when validated numerically though comparison with the time averaged dispersion patterns from a transient source. However, the ability of a point source to represent such dispersion is dependent on airflow regime. The applicability of the model is demonstrated using a simulation of an isolation room representing the release of bacteria from bedmaking.

Use of CFD modelling to optimise the design of upper-room UVGI disinfection systems for ventilated rooms

The installation of upper-room ultraviolet germicidal irradiation (UVGI) devices in ventilated rooms has the potential to reduce transmission of infections by an airborne route. However, the performance of such devices is dependant on several factors including the location of the lamp and the ventilation airflow in the room. This study uses a CFD model to evaluate the performance of UVGI devices by considering the cumulative UV-C dose received by the bulk room air in a ventilated room. By evaluating the UV dose rather than the resulting micro-organism inactivation the methodology can be used to optimise UVGI systems at the design stage, particularly when the source location of bioaerosol contaminants is not known. The study investigates the relationships between the lamp location, lamp power, ventilation system and room heating in a small, ventilated room. The results show that with ventilation air supplied at low level and extracted at high level the UVGI system performs better than with the air supplied at high level and extracted close to the floor. In addition the results show the presence of a heater in the room is unlikely to have a detrimental effect on performance and may promote mixing to increase the extent of disinfection.

A laminar flow model of aerosol survival of epidemic and non-epidemic strains of Pseudomonas aeruginosa isolated from people with cystic fibrosis

BackgroundCystic fibrosis (CF) is an inherited multi-system disorder characterised by chronic airway infection with pathogens such as Pseudomonas aeruginosa.Acquisition of P. aeruginosa by patients with CF is usually from the environment, but recent studies have demonstrated patient to patient transmission of certain epidemic strains, possibly via an airborne route. This study was designed to examine the survival of P. aeruginosa within artificially generated aerosols.ResultsSurvival was effected by the solution used for aerosol generation. Within the aerosols it was adversely affected by an increase in air temperature. Both epidemic and non-epidemic strains of P. aeruginosa were able to survive within the aerosols, but strains expressing a mucoid phenotype had a survival advantage.ConclusionThis would suggest that segregating individuals free of P. aeruginosa from those with chronic P. aeruginosa infection who are more likely to be infected with mucoid strains may help reduce the risk of cross-infection. Environmental factors also appear to influence bacterial survival. Warming and drying the air within clinical areas and avoidance of humidification devices may also be beneficial in reducing the risk of cross-infection.

An aerobiological model of aerosol survival of different strains of Pseudomonas aeruginosa isolated from people with cystic fibrosis

Pseudomonas aeruginosa is a common and important pathogen in people with cystic fibrosis (CF). Recently epidemic strains of P. aeruginosa associated with increased morbidity, have been identified. The method of transmission is not clear, but there is evidence of a potential airborne route. The aim of this study was to determine whether different strains of P. aeruginosa isolated from people with CF were able to survive within artificially generated aerosols in an aerobiological chamber. Viable P. aeruginosa could still be detected up to 45min after halting generation of the aerosols. All of the strains of P. aeruginosa expressing a non-mucoid phenotype isolated from people with CF had a reduced ability to survive within aerosols compared to an environmental strain. Expression of a mucoid phenotype by the strains of P. aeruginosa isolated from people with CF promoted survival in the aerosol model compared to strains expressing a non-mucoid phenotype.

Air Ion Behavior in Ventilated Rooms

Air ionizers have seen increasing use as devices for improving indoor air quality, including applications designed to reduce the transmission of infection in healthcare environments. However, little attention has been given to understanding and quantifying the physical behavior of ions in indoor air. This study presents experimental data and a theoretical model to examine the factors that influence the concentration of ions in a ventilated room. The results demonstrate how, with an ionizer in operation, the ion concentration is governed by ion—ion interactions and electrical deposition at the walls, with the ventilation rate having a minimal influence. The results also demonstrate that an ion concentration > 1010 ions · m ×3 is necessary for these electrical effects to be significant, which has implications for the suitability of an ionizer for a particular location.

Survival of Mycobacterium abscessus isolated from people with cystic fibrosis in artificially generated aerosols

Non-tuberculous mycobacterium (NTM) are increasingly found in the sputum of people with cystic fibrosis (CF), both in Europe and North America [1]. Specifically, Mycobacterium abscessus has emerged as a potentially important pathogen, with evidence of accelerated lung function decline [2]. Studies from two CF centres have found evidence of cross-infection between individuals with CF [3, 4], whereas studies from other centres have not replicated this finding [5–7]. M. abscessus has been isolated from household water and has been previously isolated from shower aerosols of people with pulmonary NTM disease [8, 9]. M. abscessus survives aerosolisation suggesting airborne transmission between people with CF may occur http://ow.ly/K2x6302JpAW

The role of nursing activities on the bioaerosol production in hospital wards

Transport of infectious particles through the air has the potential to contaminate the indoor environment creating reservoirs of infectious material on surfaces. There is evidence that typical nursing activities can release large quantities of bacteria including MRSA into the hospital air, which may lead to surface contamination thereby increasing opportunities for further spread. Air sampling studies were conducted over a period of 5 days on a four-bed bay in a respiratory ward. Results showed that sampled bioaerosols are more likely to be carried on large particles >5 µm in diameter, and that the relationship between bioaerosols and particle size varies when respiratory interventions are in use. Increased activity in the hospital bay was shown to correlate to increased concentrations of bioaerosols whereas sedentary visitors did not. In particular, the occurrence of patient washing that occurred behind closed curtains correlated to large values of bioaerosol release. Floor cleaning generated large number of particles, but with no significant increase in sampled bioaerosols. This provides valuable information for understanding when and where bioaerosols are released on a hospital ward which may inform future research into physical segregation of patients and the definition of bioaerosol sources in computer simulations.

101 Survival of Mycobacterium abscessus in artificially generated aerosols

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