Role of dipicolinic acid in survival of Bacillus subtilis spores exposed to artificial and solar UV radiation

Pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) constitutes approxi mately 10% of Bacillus subtilis spore dry weight and has been shown to play a significant role in the survival of B. subtilis spores exposed to wet he at and to 254-nm UV radiation in the laboratory. However, to date, no work has addressed the importance of DPA in the survival of spores exposed to en vironmentally relevant solar UV radiation. Air-dried films of spores contai ning DPA or lacking DPA due to a null mutation in the DPA synthetase operon dpaAB were assayed for their resistance to UV-C (254 nm), UV-B (290 to 320 nm), full-spectrum sunlight (290 to 400 nm), and sunlight from which the U V-B portion was filtered (325 to 400 nm). In all cases, air-dried DPA-less spares were significantly more UV sensitive than their isogenic DPA-contain ing counterparts. However, the degree of difference in UV resistance betwee n the two strains was wavelength dependent, being greatest in response to r adiation in the UV-B portion of the spectrum, In addition, the inactivation responses of DPA-containing and DPA-less spores also depended strongly upo n whether spores were exposed to UV as air-dried films or in aqueous suspen sion. Spores lacking the gerA, gerB, and gerK nutrient germination pathways , and which therefore rely on chemical triggering of germination by the cal cium chelate of DPA (Ca-DPA), were also more UV sensitive than wild-type sp ores to all wavelengths tested, suggesting that the Ca-DPA-mediated spore g ermination pathway may consist of a UV-sensitive component or components.