Bacillus subtilis spores inactivated by short pulses of soft X-rays in the water window

Bacterial endospores are highly resistant to environmental stressors. Their resistance complicates sterilization efforts, leading to exploration of novel inactivation techniques. Radiation in the water window spectral range (284-543 eV), typically studied using synchrotron sources, offers high contrast for bio-imaging and potential for efficient spore inactivation due to its high absorption and shallow penetration.

A compact pulsed laser-plasma soft X-ray source was used to irradiate Bacillus subtilis spores in the water window spectral range. A combination of STEM-EDS and Geant4 Monte Carlo simulations provided data on spore geometry, elemental composition, and dose deposition. The spores were deposited on glass coverslips, dried, irradiated in vacuum, and resuspended for viability testing. Spore survival was assessed via colony-forming units, and the survival curve was fitted with a modified biphasic model.

The decimation dose (D₁₀) was reached in less than one second at a 10 Hz repetition rate, indicating high inactivation efficiency. Effective photoabsorption coefficients were calculated from simulations and validated against tabulated values. Average dose rates in the spore core reached 2.4 kGy·s^-1, with peak values exceeding 10¹¹ Gy·s^-1. The survival curve exhibited biphasic kinetics with saturation behavior due to spore clustering.

The study confirms that soft X-rays in the water window range are highly effective in rapidly inactivating bacterial spores. The efficiency is attributed to the spectral region's short attenuation lengths and high absorption cross-sections, aligning with data from conventional ionizing sources.