Aerosol and Surface Deposition Characteristics of Two Surrogates for Bacillus anthracis Spores

Abstract

Spores of an acrystalliferous derivative of Bacillus thuringiensis subsp. kurstaki, termed Btcry؊, are morphologically, aerodynamically, and structurally indistinguishable from Bacillus anthracis spores. Btcry؊ spores were dispersed in a large, openended barn together with spores of Bacillus atrophaeus subsp. globigii, a historically used surrogate for Bacillus anthracis. Spore suspensions (2 ؋ 10 12 CFU each of B. atrophaeus subsp. globigii and Btcry؊) were aerosolized in each of five spray events using a backpack misting device incorporating an air blower; a wind of 4.9 to 7.6 m s ؊1 was also flowing through the barn in the same direction. Filter air samplers were situated throughout the barn to assess the aerosol density of the spores during each release. Trays filled with a surfactant in aqueous buffer were placed on the floor near the filter samplers to assess spore deposition. Spores were also recovered from arrays of solid surfaces (concrete, aluminum, and plywood) that had been laid on the floor and set up as a wall at the end of the barn. B. atrophaeus subsp. globigii spores were found to remain airborne for significantly longer periods, and to be deposited on horizontal surfaces at lower densities, than Btcry؊ spores, particularly near the spray source. There was a 6-fold-higher deposition of Btcry؊ spores than of B. atrophaeus subsp. globigii spores on vertical surfaces relative to the surrounding airborne density. This work is relevant for selecting the best B. anthracis surrogate for the prediction of human exposure, hazard assessment, and hazard management following a malicious release of B. anthracis. IMPORTANCEThere is concern that pathogenic bacteria could be maliciously disseminated in the air to cause human infection and disruption of normal life. The threat from spore-forming organisms, such as the causative agent of anthrax, is particularly serious. In order to assess the extent of this risk, it is important to have a surrogate organism that can be used to replicate the dispersal characteristics of the threat agent accurately. This work compares the aerosol dispersal and deposition behaviors of the surrogates Btcry؊ and B. atrophaeus subsp. globigii. Btcry؊ spores remained in the air for a shorter time, and were markedly more likely to adhere to vertical surfaces, than B. atrophaeus subsp. globigii spores. Bacillus anthracis is an important bacterial agent of concern (1). It is pathogenic to humans, particularly by inhalational exposure (2). The organism's endospores manifest greater environmental persistence and higher resistance to chemical decontaminants than those of other bacteria. Understanding of the threat posed by malicious use of B. anthracis and the development of effective countermeasures require a comparable, nonpathogenic surrogate. Bacillus atrophaeus subsp. globigii has, for many decades, been a B. anthracis surrogate (3). B. atrophaeus subsp. globigii and other surrogates related to Bacillus subtilis have generated useful information regarding spore dispersal (4, 5)....

Re-aerosolization ofBacillus thuringiensisspores from concrete and turf

“…A much higher number of spores per unit area were recovered from the turf compared to the bricks. This has been observed previously (Bishop and Stapleton ) and, presumably, is due to the greater surface area and three‐dimensional structure of the turf. Previous experimentation has shown that the recovery efficiencies from turf and concrete are comparable (data not shown) and so a much higher deposition of spores does occur on turf.…”

“…This surrogate has recently been aerosolized at a large scale in a transporter aircraft (Buhr et al . ) and in an open‐ended barn (Bishop and Stapleton ). The excellent track record of human and environmental safety of this strain (Bishop and Robinson ) makes it suitable for the aerosolization of the high densities required in order to reliably detect low levels of re‐aerosolized spores.…”

An Effective Surrogate Tracer Technique for S. aureus Bioaerosols in a Mechanically Ventilated Hospital Room Replica Using Dilute Aqueous Lithium Chloride

“…A ubiquitous and phylogenetically similar Bacillus atrophaeus organism was used historically, but now it is realized that physical size and morphology alter bioaerosol behavior [32], and so this has been discarded in favor of Bacillus thuringiensis subspecies kurstaki. When viability of the species after surface disinfection is of interest, Bacillus atrophaeu proved to be more resilient than Bacillus anthracis, and although easy to culture, it is more susceptible to chlorine inactivation, so should not be used in this context [31]. Therefore, it is clear that choice of surrogate must be application specific.…”

“…Aerosol analysis techniques are abundant, ranging from real-time organism detection [33] to aerosol morphology identification [31], but hospital infection control is also interested in micron-sized particle surface deposition [25,34].…”

“…Non-pathogenic strains of S. aureus (see Table 1) have been used experimentally to evaluate the fate of methicillin-resistant staphylococcus strains [24,25,30]. In the context of bioterrorism, the use of Bacillus anthracis (anthrax), a variety of similar non-pathogenic subspecies have been trialed in experimental facilities to investigate the fate of such bioaerosols [31]. A ubiquitous and phylogenetically similar Bacillus atrophaeus organism was used historically, but now it is realized that physical size and morphology alter bioaerosol behavior [32], and so this has been discarded in favor of Bacillus thuringiensis subspecies kurstaki.…”