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The growth rate of Bacillus stearothermophilus cells in a chemically defined medium was inversely proportional to the concentration of Mn2+ between 15 and 300 mu;m. In the presence of >100 mu;m-Mn2+, cells grew with doubling times of >60 min. Cessation of exponential growth due to a high concentration of Mn2+ was most pronounced in cultures low in sulphur or carbon. This was due to the toxic effect of high Mn2+ concentrations since dilution of cultures to a final Mn2+ concentration of 15 mu;m or less restored the growth rate to maximum. Sporulation depended upon the nature of the growth-limiting nutrient. The manganese effect on sporulation of sulphur-limited cells depended upon the concentration of glucose and the aeration rate in a qualitatively similar manner. The highest spore yield at optimal aeration rates was obtained when the initial Mn2+ concentration in the medium was 10–30 μm. Although the growth rate of bacteria in low-sulphate media was determined by the manganese concentration, its effect on sporulation frequency was independent of the growth rate.
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