Toxic effect of manganese on growth and sporulation of Bacillus stearothermophilus

The growth rate of B. stearothermophilus cells in a chemically defined medium was inversely proportional to the concentration of Mn²⁺ between 15 and 300 μM. In the presence of >100 μM-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 μM 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.