Estimation of Viable Spores in Bacillus atrophaeus (BG) Particles of 1 to 9 μm Size Range

Abstract

This study examined the viable organism content in a bioaerosol disseminated from a letter contaminated with Bacillus atrophaeus (BG) (anthrax simulant). In addition to using nutrient medium based slit samplers to sample the bioaerosol in the air space where the dispersion occurred, the individual who opened the contaminated letter was also used as a bioaerosol sampler by incorporating an external filter trap positioned over the inlet of the respirator filter canister worn by the individual. The reasons for this latter approach should be clear: it would provide actual quantitative information on the inhalation exposure hazard to an individual faced with this scenario. Where bioaerosol particles may be comprised of aggregates of many smaller particles, organism viability within the aggregate is important and may have an impact on the current definition of ‘infectious dose’ or ‘lethal dose’. Both terms assume single organism counts. Results from two different methods of measuring aerosols due to opening contaminated letters are presented: slit samplers and glass microfibre filter medium placed on the inlet port of a respirator cartridge. The total number of viable bioaerosol particles measured at the inhalation port of the filter canister was 40 times greater than measured by the slit samplers. Accordingly, this may be accounted for by a distribution of bioaerosol particles comprised of aggregates of single BG spores. A model has been proposed that estimates the number of individual viable spores in a distribution of aerosol particles through a relatively simple probability density function distribution for particle size, and which incorporates realistic assumptions regarding particle diameters, packing density and spore viability.

Use of Virtual Impactor (VI) Technology in Biological Aerosol Detection

“…As mentioned earlier, microbial particles are naturally sticky, so thirdly, the size of the aerosol particles should resemble threat material consisting of aggregates of individuals as reported by Duncan and Ho (2008). It is generally accepted that challenge particles should be within the range of 2-5 µm in diameter although some workers may even go beyond 10 µm (Druett and May, 1952;Thomas et al, 2008).…”

Airborne Legionella bacteria from pulp waste treatment plant: aerosol particles characterized as aggregates and their potential hazard

“…However, various chemicals may be present in air at Borregaard, a leading supplier of wood-based chemicals, which may have an impact on cell viability and reducing the aerosol infectivity. The packing density of the cells in the particle affects the number of cells present in a particle (Duncan and Ho, 2008). The packing density of rods might be lower than for cocci, exemplified by Legionella cells and Bacillus spores, respectively, and, thus, the a value of 0.6 used in this study might be an upper limit.…”

“…A 3.5-lm particle will have a spherical volume V u , of 2.3 9 10 -17 m 3 (V u = 4/3 p a 3 , a; radius). The packing density of ellipsoid cells in a spherical particle may be estimated to 0.6 (Donev et al 2004;Duncan and Ho 2008). The number of individual Legionella cells in a 3.5-lm particle is, thus, estimated, at 147 cells (V u /V s 9 0,6).…”

“…2b) (Sect. 3.1), it is possible to estimate the number of live individuals in a single airborne particle as outlined by Duncan and Ho (2008). Assuming a Legionella bacterium of 2 lm length and 0.3-0.9 lm width as an ellipsoid, the volume, V s , is estimated at 9.4 9 10 -20 m 3 (V s = 4/3 p abc).…”

“…This implies that each aerosol particle contains a single viable bacterium. However, naturally occurring biological aerosols mostly exist as aggregates that often contain multiple cells, but the cells may not but always be viable or of the same species (Duncan and Ho 2008). Environmentally, particle aggregates are not expected to exist as pure cultures and thus may contain several bacterial species.…”