Physical Collection Efficiency Research Articles

Effects of condensational growth on culturability of airborne bacteria: Implications for sampling and control of bioaerosols

The effect of condensational growth, one of the important characteristics for both monitoring and controlling bioaerosols, on bioaerosol culturability was assessed in this study. Three species of bacteria ( Staphylococcus epidermidis, Pseudomonas fluorescens, and Escherichia coli) were used. The measured growth factors of the bioaerosols ranged from 1.07 to 1.67 under various conditions and these data coincided with calculated results using both the thermodynamic approach and the moment method. The culturable fractions obtained using an Anderson type impactor were observed to increase as a result the formation of the liquid layer around the bioaerosols via condensational growth of the particle, which correspondingly resulted in both increase of physical collection efficiency and decrease of impaction stress. The culturable fractions of sampled bioaerosols increased up to 43% (for S. epidermidis), 27% (for P. fluorescens), and 12% (for E. coli) by condensational growth, compared to no condensational growth. However, the culturable fraction decreased when the condensational growth was excessive, probably due to heat transfer to the bioaerosols during the condensational process.

New bio-aerosol collector using a micromachined virtual impactor

For collection and concentration of bioaerosols, we designed and evaluated a single stage virtual impactor, which was fabricated by micro-electro-mechanical systems (MEMS) process. The cut-off diameter of 1 μm was selected, since 1 μm is the lowest size as used in the US Government Joint Biological Point Detection System [Haglund, J. S., & McFarland, A. R. (2004). A circumferential slot virtual impactor. Aerosol Science and Technology, 38, 664–674; Moshier, T., & Buonaugurio, T. (2000). Joint Biological Point Detection System (JBPDS) requirements and design interplay. Proceedings of the First Joint Conference on Point Detection for Chemical and Biological Defense, October 23–27, 2000, Williamsburg, VA.] The design value of a 1 μm cut-off diameter required a nozzle width and thickness of 880 and 200 μm, respectively. The virtual impactor was evaluated for physical and biological collection efficiencies. For the performance evaluation of physical collection efficiency and wall loss, polystyrene latex (PSL) particles were generated from an atomizer and their size distribution was measured using an aerodynamic particle sizer (APS, TSI model 3321) and a scanning mobility particle sizer (SMPS, TSI model 3936). The measured cut-off diameter was 0.95 μm, which agreed with the calculated results (=0.94 μm) determined with a commercial computational fluid dynamics (CFD) package, FLUENT, and the measured wall loss was below 33.5%. For the performance evaluation of biological collection efficiency, Staphylococcus epidermidis bioaerosols were dispersed into air by a nebulizer. The bioaerosols were measured using APS and sampled with a bioaerosol sampler. The overall physical collection efficiency based on the number concentration was 73.8±3%, which was similar to the one based on the number of colonies (=76.7±7%). We found that most of the bioaerosols collected and concentrated by our virtual impactor were viable.

Collection efficiency and design of microbial air samplers

The variables affecting the physical collection efficiency of air samplers of the type that impact microbe-carrying particles onto agar were investigated using a simplified analytical method and computational fluid dynamics. The results from these two techniques were compared, as were the effect of jet velocity, nozzle size, and nozzle distance from the agar surface; also considered was the optimisation of these variables to obtain an efficient design of sampler. A technique is described that calculates the proportion of microbe-carrying particles that a sampler will collect from a typical size distribution of microbe-carrying particles found in an occupied room; the three air samplers studied were found to collect from about 22 % to over 99 % of the micro-organisms in the room air.

Physical Collection Efficiency of Filter Materials for Bacteria and Viruses

The purpose of this study was to determine the physical collection efficiency of commercially available filters for collecting airborne bacteria, viruses, and other particles in the 10–900 nm (nanometer) size range. Laboratory experiments with various polytetrafluoroethylene (PTFE), polycarbonate (PC) and gelatin filters in conjunction with Button™ Inhalable samplers and three-piece cassettes were undertaken. Both biological and non-biological test aerosols were used: Bacillus atrophaeus, MS2, polystyrene latex (PSL), and sodium chloride (NaCl). The B.atrophaeus endospores had an aerodynamic diameter of 900 nm, whereas MS2 virion particles ranged from 10 to 80 nm. Monodisperse 350 nm PSL particles were used as this size was believed to have the lowest filtration efficiency. NaCl solution (1% weight by volume) was used to create a polydisperse aerosol in the 10–600 nm range. The physical collection efficiency was determined by measuring particle concentrations size-selectively upstream and downstream of the filters. The PTFE and gelatin filters showed excellent collection efficiency (>93%) for all of the test particles. The PC filters showed lower collection efficiency for small particles especially <100 nm. Among the tested filters, the lowest collection efficiencies, 49 and 22%, were observed for 1 and 3-μm pore size PC filters at the particle sizes of 47 and 63 nm, respectively. The results indicate that the effect of filter material is more significant for the size range of single virions than for bacteria. The effect of filter loading was examined by exposing filters to mixtures of PSL particles, which aimed at mimicking typical indoor dust levels and size distributions. A 4-h loading did not cause significant change in the physical collection efficiency of the tested filters.

Investigation of Cut-Off Sizes and Collection Efficiencies of Portable Microbial Samplers

This research investigated the physical collection efficiencies and cut-off sizes of SMA MicroPortable, BioCulture, Microflow, Microbiological Air Sampler (MAS-100), Millipore Air Tester (MAT), SAS Super 180, and RCS High Flow portable microbial samplers when collecting Polystyrene Latex particles ranging from 0.5 to 9.8 μm in aerodynamic size. Traditional collection efficiency measurements often directly compare particle concentrations upstream and downstream of the sampler without considering the particle losses. Here, we developed a new approach which tests collection efficiencies of the sampler with and without agar collection plate loaded. This method thus allows estimating the effective collection efficiency, i.e., the fraction of incoming particles deposited onto the agar collection medium only. The experimental cut-off sizes, or d 50, of the investigated samplers ranged from 1.2 μm for the RCS High Flow, 1.7 μm for the MAS-100, 2.1 μm for SAS Super 180, to 2.3 μm for MAT; for other three samplers ...

Long-Term Sampling of Viable Airborne Viruses

A novel bioaerosol sampling technique, which utilizes the bubbling process in the collection fluid, has recently been developed and found feasible for a long-term personal sampling of airborne bacteria and fungal spores as it maintained high physical collection efficiency and high microbial recovery rate for robust and stress-sensitive microorganisms. Further tests have shown that the new technique also has potential to collect viable airborne viruses, particularly when utilized for a short-term sampling of robust strains. As the short-term sampling has a limited application for assessing personal exposure in bioaerosol-contaminated environments, the present study was undertaken to investigate the feasibility of the “bubbler” for a long-term monitoring of viable airborne viruses. Liquid droplets containing Vaccinia virions (that simulate Variola, a causative agent of smallpox) were aerosolized with a Collison nebulizer into a 400-liter test chamber, from which the droplets were collected by three identica...

The effect of filter material on bioaerosol collection of Bacillus subtilis spores used as a Bacillus anthracis simulant

The objective of this study was to determine filter materials and extraction methods that are appropriate to use for environmental sampling of B. anthracis. Four types of filters were tested: mixed cellulose ester (MCE) with a pore size of 3 microm, polytetrafluoroethylene (PTFE) with pore sizes of 1 and 3 microm, and gelatin with a pore size of 3 microm. Bacillus subtilis var. niger endospores (also known as Bacillus globigii[BG]) were used as a surrogate for B. anthracis. Endospores were collected into Button Inhalable Aerosol Samplers with sampling times of 15 minutes, 1 hour, and 4 hours. Physical collection efficiency was determined by measuring upstream and downstream B. subtilis concentrations with an optical particle counter. Vortexing with ultrasonic agitation and vortexing with shaker agitation extraction methods were evaluated. The MCE, 1 microm PTFE, and gelatin filters provided physical collection efficiencies of 94% or greater. The 3 microm PTFE filter showed inconsistent physical efficiency characteristics between filters. Epifluorescence microscopic analysis of the gelatin filter extraction fluid revealed the presence of contamination by non-culturable bacteria. Mean differences for microbial culturability were not statistically significant for filter materials and extraction methods. However, the vortexing with shaker agitation extraction method resulted in higher total microbial counts in the extraction fluids for MCE and 1 microm PTFE filters when compared to vortexing with ultrasonic agitation. In summary, the MCE and 1 microm PTFE filters in combination with vortexing and shaker extraction demonstrated the best performance for the filter collection and extraction of BG spores.

Assessment of Electrical Charge on Airborne Microorganisms by a New Bioaerosol Sampling Method

Bioaerosol sampling is necessary to monitor and control human exposure to harmful airborne microorganisms. An important parameter affecting the collection of airborne microorganisms is the electrical charge on the microorganisms. Using a new design of an electrostatic precipitator (ESP) for bioaerosol sampling, the polarity and relative strength of the electrical charges on airborne microorganisms were determined in several laboratory and field environments by measuring the overall physical collection efficiency and the biological collection efficiency at specific precipitation voltages and polarities. First, bacteria, fungal spores, and dust dispersed from soiled carpets were sampled in a walk-in test chamber. Second, a simulant of anthrax-causing Bacillus anthracis spores was dispersed and sampled in the same chamber. Third, bacteria were sampled in a small office while four adults were engaged in lively discussions. Fourth, bacteria and fungal spores released from hay and horse manure were sampled in a horse barn during cleanup operations. Fifth, bacteria in metalworking fluid droplets were sampled in a metalworking simulator. It was found that the new ESP differentiates between positively and negatively charged microorganisms, and that in most of the tested environments the airborne microorganisms had a net negative charge. This adds a signature to the sampled microorganisms that may assist in their identification or differentiation, for example, in an anti-bioterrorism network.

Assessment of Electrical Charge on Airborne Microorganisms by a New Bioaerosol Sampling Method

Bioaerosol sampling is necessary to monitor and control human exposure to harmful airborne microorganisms. An important parameter affecting the collection of airborne microorganisms is the electrical charge on the microorganisms. Using a new design of an electrostatic precipitator (ESP) for bioaerosol sampling, the polarity and relative strength of the electrical charges on airborne microorganisms were determined in several laboratory and field environments by measuring the overall physical collection efficiency and the biological collection efficiency at specific precipitation voltages and polarities. First, bacteria, fungal spores, and dust dispersed from soiled carpets were sampled in a walk-in test chamber. Second, a simulant of anthrax-causing Bacillus anthracis spores was dispersed and sampled in the same chamber. Third, bacteria were sampled in a small office while four adults were engaged in lively discussions. Fourth, bacteria and fungal spores released from hay and horse manure were sampled in a horse barn during cleanup operations. Fifth, bacteria in metalworking fluid droplets were sampled in a metalworking simulator. It was found that the new ESP differentiates between positively and negatively charged microorganisms, and that in most of the tested environments the airborne microorganisms had a net negative charge. This adds a signature to the sampled microorganisms that may assist in their identification or differentiation, for example, in an anti-bioterrorism network.

Collection of Airborne Microorganisms into Liquid by Bubbling through Porous Medium

A new method for the removal of airborne particles by air bubbling through fibrous filters immersed into a liquid has recently been developed (Agranovski et al. 1999) and shown to be very efficient for cleaning air environments with ultra-fine aerosol particles. The principal objective of the present study was to evaluate the new bubbling technique for the collection of airborne bacteria into a liquid for subsequent physical and microbiological analysis. It was found that the technique is capable of achieving a physical collection efficiency of 98.5% or higher for particles larger than 0.3 w m in aerodynamic diameter. The physical collection efficiency of the prototype bubbler remained at that high level for 8 h of continuous operation with negligible variation of the pressure drop across the device. Evaporation of the collection fluid did not exceed 20% during 8 h, and the reaerosolization effect on the physical collection efficiency of the bubbler prototype was <8%. The recovery rate of gram-negative Ps...

Development and evaluation of a new personal sampler for culturable airborne microorganisms

The objective of this study was to develop a new personal sampler for viable airborne microorganisms and to evaluate its performance under controlled laboratory conditions and in a field. In the sampler, air is bubbled through a porous medium submerged in a liquid layer, as has earlier been demonstrated to be highly efficient for air purification. The prototype had the physical collection efficiency >95% for particles >0.32 μm in aerodynamic diameter during 8 h of continuous operation. The pressure drop across the sampler was below 1700 Pa, much lower than that of most conventional bioaerosol samplers. The collection liquid losses due to evaporation and aerosolization did not exceed 18% in 8 h and the culturability of sampled microorganisms remained high: the recovery rate of stress-sensitive gram-negative P. fluorescens bacteria was 61±20%; for stress-resistant B. subtilis bacteria and A. versicolor fungal spores it was 95±9% and 97±6%, respectively. Six identical personal samplers were tested simultaneously on a simplified human manikin in an office environment. The culturable microbial concentration data obtained during 2, 4 and 8-h sampling were not affected by the sampling time. Inter-sample variation did not exceed 30%. The laboratory and field evaluations have demonstrated that the new sampler is capable of long-term personal sampling of airborne culturable microorganisms. The estimation of the detection limits has indicated that the sampler is capable of monitoring microbial exposure in the environments with the bacterial concentrations above 15 CFU/m 3 and fungal concentrations above 5 CFU/m 3 when using a sampling time of 8 h.

Survival of Airborne Microorganisms During Swirling Aerosol Collection

An earlier study evaluated the physical performance of the Swirling Aerosol Collector, also known as the ''BioSampler,'' and found it to have several advantages over the widely used AGI-30 impinger when tested with inert test particles. The present study was focused on the physical and biological performance with laboratory-aerosolized micro-organisms. The results showed that the BioSampler has better physical collection efficiency for B. subtilis and P. fluorescens than the AGI-30 when water was used as the collection fluid and the sampling flow rate ranged from 8.5 L min-1 to the standard 12.5 L min-1. Additionally, the BioSampler provided equivalent or better microbial recovery for the 2 species than the AGI-30 did. The swirling motion of the collection liquid in the BioSampler generates very few bubbles and therefore causes minimal reen trainment of already collected micro-organisms. In contrast to the AGI-30, the BioSampler can be used very effectively with nonevaporating liquids, such as mineral oil or glycerol. Mineral oil was found to be a suitable collection fluid for culture analysis, whereas glycerol can be used only for nonculture assays, such as the polymerase chain reaction. By using a more viscous, nonevaporating fluid, the BioSampler can be used for long-term sampling of low airborne bacterial concentrations, while the AGI-30 impinger, filled with a standard evaporating fluid, can only be used for up to about 30 min.

Collection of Airborne Microorganisms by Electrostatic Precipitation

The applicability of electrostatic precipitation as a method for bioaerosol collection was investigated by using a modified Electrostatic Aerosol Sampler (EAS) (Model 3100, TSI Inc., St. Paul, MN). The physical and biological efficiencies of this method were determined. The tests were performed using three bacterial species which were collected onto agar, into water, and onto filters. The physical collection efficiency was higher than 80% when using a sampling flow rate of 1 L/min. When the Bacillus subtilis var niger (BG) spores were collected on agar, about 50-60% of the collected culturable organisms formed colonies. The bioefficiency exceeded 90% when the BG spores were collected on a filter, but was only 15-22% when collected into water. The Mycobacterium bovis BCG bacteria recovered at the 0-8% level on all three collection media. The least number of colonies were formed when Pseudomonas fluorescens bacteria were collected on any of the collection media. The data show that the process of electrostatic collection is very complex for sensitive airborne bacteria and thus several effects should be considered when assessing its bioefficiency. In separate tests conducted without aerosol flow through the sampler, bacteria placed onto the collection media did not show any significant reduction in bacterial recovery while exposed to a strong electric field. It was found that evaporation from the collection media, such as agar or water, increases the humidity inside the EAS and may affect the size distribution of the particles being collected, resulting in decreased physical and biological efficiencies of the electrostatic precipitation method. For hardy microorganisms such as BG spores, the bioefficiency for electrostatic collection is high, thus encouraging further explorations of the electrostatic method for sampling bioaerosols.

Collection Efficiency and Culturability of Impingement into a Liquid for Bioaerosols of Fungal Spores and Yeast Cells

Collection efficiency of AGI-30 impingers and culturability of collected fungal bioaerosols were evaluated in a laboratory test system. A Pitt-3 generator and a Collison nebulizer aerosolized spores of Penicillium citrinum (P. citrinum) and yeast cells of Candida famata (C. famata) var. flareri , respectively. A 37-mm 3-piece cassette containing a 0.4- mu m Nuclepore filter was adapted to the outlet of the test impinger to collect the particles penetrating the sampler. The particles deposited on the inlet tube were also extracted to evaluate the wall loss effect. The physical collection efficiency was evaluated by counting the fungal particles of three portions of the impinger (inlet tube, AGI-30, and outlet) using a haemocytometer. In addition, the culturability was determined as the ratio of CFU concentration to total concentration measured by the haemocytometer. Our results demonstrated that the impinger could collect more than 90% of the particles, and less than 1% of the particles were found in the i...

Effect of Sampling Time on the Collection Efficiency of All-Glass Impingers

The changes in the physical collection efficiencies of all-glass impingers were studied experimentally with an aerodynamic particle sizer by dynamically measuring the particle concentrations upstream and downstream of AGI-4 and AGI-30 impingers. Monodisperse PSL particles of aerodynamic sizes ranging from 0.3 to 2.0 μm were used in the tests. The inner diameter of the impingement nozzle was found to be the most critical dimension affecting the collection efficiency. Significant variations were found in the performance of individual impingers due to the variations in the critical dimensions of the impingers tested. About 1% of the initial amount of impinger liquid is evaporated per minute of normal operation. On depletion to a critical minimum volume, the collection efficiency decreases drastically due to insufficient impingement into the liquid and particle bounce from the bottom surface. Particles already collected may be entrained by the air bubbles passing through the liquid and be reaerosolized, thus decreasing the overall collection efficiency. The number of reaerosolized particles increases with sampling time, but is less than 10% within the first hour of sampling with either the AGI-4 or the AGI-30. A graph has been developed for selecting the maximum sampling time for a given initial liquid volume so that the collection efficiency is approximately constant throughout the entire sampling period. The graph also indicates the limits where significant particle bounce, reaerosolization, or liquid splashing occur. The AGI-4 was found to be more efficient in collecting submicrometer particles and to be less particle-size dependent than the AGI-30.

A modular detector for flow cytometric multicolor fluorescence measurements.

A modular detector for measuring multicolor fluorescence from cells illuminated by single or multiple lasers has been developed for flow cytometers. Motion picture projector, camera, and CCTV/video lenses were evaluated for use in the detector by comparing their physical characteristics, image quality, and light collection efficiencies. A 25-mm focal length F/0.95 CCTV lens was selected, based on our criteria and test results. The detector was constructed out of square aluminum extrusion channels. A CCTV lens mounted on the outside of the first channel collected light emitted from cells and collimated it towards filters and secondary CCTV lenses located in each channel. The secondary lenses functioned as relay optics for directing and focusing light onto pinhole spatial filters for measurement by photomultipliers. The detector design allowed any number of channels to be connected together and the versatility for making simultaneous or sequential measurements. Measurements on lymphocytes labeled with four monoclonal antibodies conjugated to fluorescent dyes and measurements on viable tumor cells stained for DNA content and with three fluorescent-labeled antibodies were used to demonstrate the detector's capabilities.