Systematic investigation of germination responses of Bacillus subtilis spores in different high-salinity environments

Abstract

High-salinity environments play an increasingly important role in ecology regarding soil salinization due to human-induced processes, but also need to be considered in terms of natural soil desiccation and extreme habitats. It has been shown previously that spore germination of the ubiquitous soil bacterium Bacillus subtilis is detrimentally affected by the presence of high NaCl concentrations, but the underlying mechanisms and effects of other salts remained obscure. To address these two points, we performed a systematic analysis with 32 different salts using spectrophotometric and microscopic methods. It could be shown that inhibitory strength varies considerably among different salts. Although osmotic effects seem to play an important role, ionic composition and concentration (especially of the anion) as well as chemical properties seem to be decisive for the extent of germination inhibition. At the current state of knowledge, fluxes of ions, Ca(2+)-DPA and water are likely affected by all salts, whereas the exact inhibition mechanism of each salt might further depend on the respective properties of the involved ions. Hence, the observed inhibition likely is a result of several phenomena interacting with each other. Altogether this study highlights the complex impact of ionic environments on the life cycle of spore formers.

Keywords: ions; metals; osmotic stress; salt stress; salt-dependent germination inhibition; spore germination.