Figure 3 Germination of B. licheniformis with casein hydrolysate. Germination is followed as a change in initial absorbance at 600 nm (A600) of phase bright spores in Tris HCl buffer pH 7.4 at 30 °C after addition of 1% (w/v) casein hydrolysate. Complete germination (>99% phase dark spores as observed by phase contrast microscopy) was

observed at ~40% of initial A600. The results shown are representative of experiments performed in duplicate on two individual spore batches repeated at least twice. D-alanine is a well-known inhibitor of L-alanine germination of B. subtilis and B. licheniformis [64, 65, 46, 15, 66]. D-alanine has also been shown BVD-523 mouse to reduce L-valine induced germination of B. subtilis [15, 66], but we are not aware of studies reporting the effect of D-alanine on L-valine induced germination of B. licheniformis. In order to abolish germination by L-alanine present in the casein hydrolysate, we added D-alanine in learn more some of the above experiments. In these experiments, the germination response of both MW3 and

NVH-1311 was hardly measurable (results not shown), indicating that L-alanine through its triggering of the gerA receptor is an important germinant of B. licheniformis. The contribution to germination of the remaining amino acids in the casein hydrolysate when D-alanine was present, appear to be minimal. Although one can not rule out that D-alanine also inhibits the effect of other amino acids present in casein hydrolysate (e.g. L-valine), all the findings support the view that gerA and

L-alanine constitute one of the main germination pathways of B. licheniformis. Germination of B. licheniformis with Ca2+-DPA In order to by-pass the spore germination receptor apparatus, experiments using exogenous Ca2+-DPA to trigger Thalidomide germination of spores of B. licheniformis MW3 and the mutant strain NVH-1307 were performed. In B. subtilis spores, Ca2+-DPA induced germination is believed to act through activation of the cortex lytic enzyme CwlJ, without any requirement of functional germinant receptors [10, 67]. Bioinformatic analysis of complete genomes of different spore formers has shown that also B. licheniformis contains a B. subtilis homologous cwlJ gene [43]. If the germination apparatus of B. licheniformis spores is similar to that of its close relative B. subtilis, the wild type and disruption mutant of B. licheniformis should exhibit a similar germination response as B. subtilis to exogenous Ca2+-DPA. The DPA concentration needed to trigger germination in B. subtilis is ~ 20 – 60 mM, supplemented together with equal (or excess) amounts of Ca2+ (allowing formation of a 1:1 chelate of calcium and dipicolinic acid) [10]. Also spores of B. cereus and B. megaterium germinate when exposed to Ca2+-DPA [68, 69]. For B. cereus it has been shown that a final level of 60 mM Ca2+-DPA is sufficient to ensure germination [69].