High salinity alters the germination behavior of Bacillus subtilis spores with nutrient and non-nutrient germinants

Abstract

The effect of high NaCl concentrations on nutrient and non-nutrient germination of Bacillus subtilis spores was systematically investigated. Under all conditions, increasing NaCl concentrations caused increasing albeit reversible inhibition of germination. High salinity delayed and increased the heterogeneity of germination initiation, slowed the germination kinetics of individual spores and the whole spore population, and decreased the overall germination efficiency, as observed by variety of different analytical techniques. Germination triggered by nutrients which interact with different germinant receptors (GRs) was affected differently by NaCl, suggesting that GRs are possible targets of NaCl inhibition. However, NaCl also inhibited GR-independent germination, suggesting that there is at least one additional target for NaCl inhibition. Strikingly, a portion of the spore population could initiate germination with L-alanine even at NaCl concentrations near saturation (∼ 5.4 M), suggesting that spores lack a salt-sensing system preventing them from germinating in a hostile high-salinity environment. Spores that initiated germination at very high NaCl concentrations excreted their large depot of Ca²⁺-DPA and lost their heat resistance, but remained in a phase-grey state in the phase-contrast microscope, suggesting that there was incomplete germination. However, some metabolic activity could be detected at up to 4.8 M NaCl. Overall, high salinity seems to exert complex effects on spore germination and outgrowth, whose detailed elucidation in future investigations could give valuable insights on these processes in general.