Characterization of spores of Bacillus subtilis which lack dipicolinic acid

Abstract

Spores of Bacillus subtilis with a mutation in spoVF cannot synthesize dipicolinic acid (DPA) and are too unstable to be purified and studied in detail. However, the spores of a strain lacking the three major germinant receptors (termed Δger3), as well as spoVF, can be isolated, although they spontaneously germinate much more readily than Δger3 spores. The Δger3 spoVF spores lack DPA and have higher levels of core water than Δger3 spores, although sporulation with DPA restores close to normal levels of DPA and core water to Δger3 spoVF spores. The DPA-less spores have normal cortical and coat layers, as observed with an electron microscope, but their core region appears to be more hydrated than that of spores with DPA. The Δger3 spoVF spores also contain minimal levels of the processed active form (termed P41) of the germination protease, GPR, a finding consistent with the known requirement for DPA and dehydration for GPR autoprocessing. However, any P41 formed in Δger3 spoVF spores may be at least transiently active on one of this protease's small acid-soluble spore protein (SASP) substrates, SASP-γ. Analysis of the resistance of wild-type, Δger3, and Δger3 spoVF spores to various agents led to the following conclusions: (i) DPA and core water content play no role in spore resistance to dry heat, dessication, or glutaraldehyde; (ii) an elevated core water content is associated with decreased spore resistance to wet heat, hydrogen peroxide, formaldehyde, and the iodine-based disinfectant Betadine; (iii) the absence of DPA increases spore resistance to UV radiation; and (iv) wild-type spores are more resistant than Δger3 spores to Betadine and glutaraldehyde. These results are discussed in view of current models of spore resistance and spore germination.