Dielectric properties of native and decoated spores of Bacillus megaterium

Abstract

A general model for use in interpreting dielectric data obtained with bacterial endospores is developed and applied to past results for Bacillus cereus spores and new results for Bacillus megaterium spores. The latter were also subjected to a decoating treatment to yield dormant cells with damaged outer membranes that could be germinated with lysozyme. For both spore types, core ions appeared to be completely immobilized, and decoating of B. megaterium spores did not affect this extreme state of electrostasis in the core. The cortex of B. megaterium appeared to contain a high level of mobile ions, in the cortex of B. cereus. The outer membrane-coat complex of B. megaterium acted dielectrically as an insulating layer around the cortex, so that native dormant spores showed a Maxwell-Wagner dispersion over the frequency range from about 1 to 20 MHz. The decoating treatment resulted in a shift in the dispersion to frequencies below the range of observation. Increases in cell conductivity in response to increases in environmental ionic strength indicated that the coats. of B. megaterium could be penetrated by environmental ions and that they had an inherent fixed charge concentration of about 10 to 20 milliequivalents per liter. In contrast, the dispersion for B. cereus spores was very sensitive to changes in environmental ion concentration, and it appeared that some 40% of the spore volume could be penetrated by environmental ions and that these ions traversed a dielectrically effective layer, either the exosporium or the outer membrane. It appears that dormancy is associated with extreme electrostasis of core ions but not necessarily of ions in enveloping structures and that the coat-outer membrane complex is dielectrically effective but not required for maintenance of extreme electrostasis in the core.