Association Between Increased DNA Mutational Frequency and Thermal Inactivation of AerosolizedBacillusSpores Exposed to Dry Heat

Abstract

Inactivation of viable bioaerosol particles, especially stress-resistant microorganisms, has important implications for biodefense and air quality control. It has earlier been shown that the loss of viability of bacterial endospores due to exposure to dry heat is associated with mutational damage. Previous studies, however, used non-aerosolized spores, long exposure times, and moderately elevated temperatures. This study was designed to investigate the mechanism of inactivation of aerosolized Bacillus endospores exposed to high temperatures for sub-second time periods. Bioaerosol was tested in a continuous air flow chamber under two flow rates, 18 L/min and 36 L/min. The chamber had a cylindrical electric heating element installed along its axis. The estimated characteristic exposure temperature (Texposure) ranged from 164°C to 277°C (with an uncertainty of 21–26°C). To quantify mutational frequency, spores were cultivated after dry heat exposure on tryptic-soy agar and on antibiotic nalidixic acid media. Increases in the exposure temperature caused viability loss and increase in mutational frequency of the spore DNA. Significant association was found between the inactivation factor and the mutational frequency ratio (heat exposed versus non-exposed) with R2 of 0.985 for both flow rates combined. The results suggest that mutational damage is involved in the causal chain of events leading to inactivation of aerosolized endospores exposed to heat for sub-second time periods.

Nanoscale Structural and Mechanical Analysis of Bacillus anthracis Spores Inactivated with Rapid Dry Heating

“…This is reasonable since at very short timescales (less than 1 s), the timescales of processes such as heat and mass transfer in a spore become comparable to heating times and may contribute significantly to spore killing in addition to that from heat-promoted biochemical reactions. By far, only a very limited number of studies have addressed this issue, so the inactivation mechanism under short-time heating conditions is just being unraveled (19,20,53). One recent finding is that mutational damage is involved in the causal chain of events leading to inactivation of aerosolized endospores exposed to heat for subsecond time periods (19).…”

“…By far, only a very limited number of studies have addressed this issue, so the inactivation mechanism under short-time heating conditions is just being unraveled (19,20,53). One recent finding is that mutational damage is involved in the causal chain of events leading to inactivation of aerosolized endospores exposed to heat for subsecond time periods (19). In this study, we attempted to provide insights in spore inactivation from the perspective of structural damages by using atomic force microscopy and other techniques.…”

“…While various methods, including heating, chemical treatment, radiation, and UV treatment, have been used to inactivate spores (4,6,7), thermal inactivation is often the method of choice for many applications (8). Thermal inactivation of Bacillus spores in laboratory studies is most often achieved by wet heat in which spores are fully hydrated during heating (9)(10)(11) or dry heat in which dry spores are heated on a solid substrate, in an ampoule heated by an oil bath, in a hot air plume, or by infrared heating (5,8,(12)(13)(14)(15)(16)(17)(18)(19)(20). It has long been observed that spores are much less resistant to heat in a well-hydrated environment than in a dry state (4).…”

Inactivation of bacterial spores subjected to sub-second thermal stress

“…However, the exact mechanism of spore inactivation during rapid heating, which is closer to conditions seen under combustion, is poorly understood. Recent studies of bacterial spores heated in hot air show that high-temperature gas induced severe damage to the spore core [50], and the extent of inactivation was attributed to the DNA damage [22].…”

“…Thus, stand-off neutralization methods which can sustain higher temperatures (>200°C) for short periods of time (sub-seconds) are being considered, and these new temperaturetime-kill histories of spore inactivation require evaluation. Aerosol methods have recently been employed to evaluate thermal and chemical kills of spores at extreme heating conditions [17][18][19][20][21][22][23][24][25]. These approaches have the advantage of isotropic and rapid heat transfer to individual spores, as well as controllable shorter heating times (milliseconds to seconds) and higher heating temperatures (>200°C) [17][18][19][20][21][22][23][24][25].…”