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SUMMARY: Bacillus stearothermophilus grows within the temperature range 40–70 °C in a complex medium that contains 115 μm-Ca2+ and 95 μm-Mg2+. The addition of Ca2+ to a final concentration ranging from 2·5 to 10 mm stimulated growth at suboptimal and supraoptimal temperatures, extending growth above 70°C, but had no effect on growth within the optimal temperature range. Mg2+ (2·5 mm) also stimulated growth although to a lesser extent. Furthermore, 10 mm-Mg2+ inhibited growth at temperatures higher than 65°C. This inhibitory effect was relieved by the addition of 2·5 mm-Ca2+. Sr2+ (10 mm), which often behaves as a Ca2+ analogue in biological systems, strongly inhibited growth and produced gross morphological alterations in the cells. The inhibitory effect of Sr2+ could also be relieved by addition of Ca2+.
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