Recovery of heat-stressed (121.1°C) spores of Bacillus stearothermophilus (ATCC 12980) produced on solid sporulation agar media (Cook and Brown) and recovered on antibiotic assay medium + 0.1% soluble starch agar media appeared to be dependent on the concentration of two trace metals, calcium (Ca) and magnesium (Mg), in the agar component of the nutrient growth medium. Results from the present investigation indicated that heat-stressed (121.1°C, 0–20 min) spores of B. stearothermophilus were not recovered on agar media containing commercial agar products from BBL, Bitek (Difco), and Oxoid; however, under similar experimental conditions, heat-stressed spores were recovered on solid agar media containing Bacto (Difco) or Acumedia agar products. Chemical analysis of the trace minerals content of all five agar products indicated that the Ca content (% dry weight) of BBL, Bitek, and Oxoid products was ≤0.01% of the dry weight; the Ca content of Bacto and Acumedia agars was 15- to 20-fold greater. Similarly, the Mg content varied considerably between agars. Bacto and Acumedia agar products contained 0.073 and 0.043% Mg, respectively; the Mg content of BBL, Bitek, and Oxoid products was 0.003, 0.006, and 0.0007%, respectively. The recovery potential of mineral-deficient agar media (Ca and Mg ≤0.01% of dry weight) was restored by increasing the Ca and Mg levels to those found in mineral sufficient agar media (≥0.2 and 0.06% Ca and Mg, respectively).

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