High-Precision Fitting Measurements of the Kinetics of Size Changes during Germination of Individual Bacillus Spores

Abstract

High-precision measurements of size changes of individual bacterial spores based on ellipse fitting to bright-field images recorded with a digital camera were employed to monitor the germination of Bacillus spores with a precision of ~5 nm. To characterize the germination of individual spores, we recorded bright-field and phase-contrast images and found that the timing of changes in their normalized intensities coincided, so the bright-field images can be used to characterize spore size and refractility changes during germination. The major conclusions from this work were as follows. (i) The sizes of germinating B. cereus spores were nearly unchanged until Trelease, the time of the completion of CaDPA (a 1:1 chelate of Ca(2+) and dipicolinic acid [DPA]) release after addition of nutrient germinants. (ii) The minor axis of germinating B. cereus spores rapidly increased by ~50 nm in a few seconds right after Trelease, while the major axis was slightly decreased or unchanged. Both the minor and major axes remained unchanged for a further 30 to 45 s and then increased by 100 to 200 nm by Tlys, the time of completion of cortex lysis. (iii) Individual spores in a population showed significant heterogeneity in the timing of germination events, such as Trelease and Tlys, but also variation in size changes during germination. (iv) Bacillus subtilis wild-type spores, B. subtilis spores lacking the cortex-lytic enzyme CwlJ, and wild-type Bacillus megaterium spores showed similar kinetics of size changes during nutrient germination. The size increases in germinating spores probably result from uptake of water and cortex lysis after completion of CaDPA release.