10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Inactivation of Aerosolized Bacillus Anthracis Spores in the Vicinity of a Flame: Simulation Study
WORRAWIT NAKPAN, Michael Yermakov, Reshmi Indugula, Tiina Reponen, Sergey A. Grinshpun, University of Cincinnati
Abstract Number: 133 Working Group: Infectious Bioaerosol
Abstract Survival of bio-warfare aerosol agents exposed to different, sometimes harsh environmental conditions, has gained substantial attention in the biodefense and biosecurity research communities. This basic simulation study aimed at assessing the survival of bacterial spores of Bacillus anthracis in the vicinity of a flame. The test conditions were intended to be relevant to a fire situation in a bio-weapon facility.
The survival of the aerosolized spores of Bacillus thuringiensis var kurstaki (Btk), a well-recognized surrogate of B. anthracis, was quantified after the spores passed a peripheral area of an air-acetylene flame in a close proximity. The flame type was chosen to represent a conservative situation since it has been demonstrated earlier that the combustion products of acetylene produce no biocidal effect on aerosolized Bacillus spores. The tests were conducted at two particle-to-flame proximity levels and seven exposure time intervals (ranging approximately from 0.1 to 6 s). The two flame positions corresponded to different weighted average temperatures in the exposure chamber, 170°C (precision=15°C) and 260°C (precision=25°C). The spores were tested at four air flow rates: 12, 18, 36, and 54 L/min. After passing the exposure chamber, the spores were collected on filters, removed into a liquid suspension, plated and subjected to a conventional culture-based analysis. Their survival was determined through comparison of the viability results obtained for exposed and non-exposed (control) samples. The aerosol concentration of viable spores in control experiments varied from one test to another ranging from ~106 to ~107 CFU/m3.
At an exposure time ranging approximately from 0.1 s to 2 s, the survival fraction exponentially decreased as the exposure time increased. This held true for both temperature (proximity) conditions. Under the lower exposure temperature (greater distance to the flame), it was observed that the survival fraction decreased from 72% to 0.5%. It more drastically dropped at the higher temperature: from 60 to 0.015%. For both temperature (spore-to-flame proximity) conditions, the slopes noticeably changed once the exposure time exceeded ~2 s: the decrease of survival spores with the exposure time became slower. For the closer proximity condition, the limit of quantification (LOQ) in measuring the spore survival was in order of 0.002% so that the survival at a prolonged exposure was about to reach the LOQ. The same is anticipated for the greater proximity (the survival could likely reach the LOQ if the time was extended beyond the current 6-s upper testing limit).
In addition to the detection limitations of the experimental methodology used in this study, there may be an alternative explanation of the plateau in survival observed at the close flame proximity. The findings may point to a sub-population of spores that may have a particularly high stress resistance to the flame-originated heat. Existence of such a sub-population can be examined in a separate investigation.
The study results show that the aerosolized Bacillus spores traveling in the vicinity of a flame peripheral area may survive in appreciable quantities. This represents a great potential risk given a high chance that the viable agents remaining airborne can be subjected to a short- or long-range atmospheric transport. The findings justify the need of developing special materials with biocidal properties that can be deployed in such scenario to inactivate bioaerosol agents and consequently mitigate the risk.
This study was supported by the Defense Threat Reduction Agency (US Department of Defense).