Extension of Bacillus endospore gas dynamic heating studies to multiple species and test conditions

Shock wave-induced damage to a variety of Bacillus endospore species is studied for a wide range of postshock temperatures and test times in oxidative and non-oxidative gas environments.

Bacillus atrophaeus and Bacillus subtilis endospores are nebulized into an aqueous aerosol, loaded into the Stanford aerosol shock tube (SAST) and subjected to shock waves of controlled strength. Endospores experience uniform test temperatures between 500 and 1000 K and pressures ranging from 2 to 7 atm, for either a short test time (∼2·5 ms) or a relatively long test time (∼45 ms). During this process, the bioaerosol is observed using in situ laser absorption and scattering diagnostics. Additionally, shock-treated samples are extracted for ex situ analysis including viability plating and flow cytometry. For short test times, results are consistent with previous studies; all endospore species begin to lose the ability to form colonies when shock-heated to temperatures above 500 K, while significant breakdown in morphology is observed for postshock temperatures above 700 K. Oxidative bath gases did not affect viability losses or morphological breakdown rates. Experiments with extended postshock test time showed increased viability loss with minimal morphological damage for shocks between 600 and 700 K.

Genetic differences between B. subtilis and B. atrophaeus endospores do not confer noticeable gains in resistance to shock heating. Oxidative environments do not exacerbate shock-induced damage to endospores. Extended test time experiments reinforce our hypothesis that a temperature/time-dependent inactivation mechanism that does not involve morphological breakdown exists at low-to-moderate postshock temperatures.

The methodology and experiments described in this paper extend the study of the interactions of endospores with shock/blast waves to new species and environmental conditions.