Mechanisms of the Resistance of Bacillus subtilis Spores to Pulsed UV-Light

Introduction: Mechanisms involved in bacterial spore resistance to Pulsed UV-Light (PL) are poorly documented in contrast to mechanisms of UV-C resistance. Pulsed UV-Light consists in short time (< 1 ms) - high energy light flashes of broad spectrum (200 -1100 nm) and rich in UV. The UV-C wavelengths are essential for the bactericidal activity.

Purpose: Factors that play a major role in PL resistance were compared to those implicated in resistance to UV-C continuous radiation.

Methods: Ten B. subtilis strains carrying mutations of the 168 parental (Wt) strain in genes coding for proteins involved in spore resistance to UV were used to examine the relative contributions of those proteins and related mechanisms in spore resistance to PL. Spores were exposed to PL fluences comprised between 320 and 1770 mJ/cm² and continuous UV-C doses comprised between 25 and 150 mJ/cm².

Results: Spores of coat-defective strains were significantly (P <0.05) more sensitive to PL than the Wt strain, while there was no difference of sensitivity to a UV-C treatment (P > 0.1). PL was inefficient on coat-defective spores when UV-C wavelengths were eliminated. Spores lacking alpha and beta-type small acid-soluble proteins (SASP) were significantly more sensitive (P > 0.05) to PL and 254 nm UV-C exposure than Wt spores. Spores of the recA and uvrBmutant strains were more sensitive to both treatments than Wt spores.

Significance: Spore coats play a major role in spore resistance to UV applied by PL. The alpha/beta-type SASP and repair of DNA damage during spore outgrowth are some important components of spore resistance to PL, as they also are in the spore resistance to UV. Physical properties of the delivered light (energy of each light pulse, light spectrum,…) can change the effect of a UV-light source on bacterial spore inactivation.