Optimized dipicolinate activation for viable counting of Bacillus stearothermophilus spores

Abstract

Spores of Bacillus stearothermophilus were exposed to calcium and sodium salts of dipicolinic acid (DPA) in phosphate and Tris acid maleate buffers over the range pH 4.5–10.0. The exposed spores were enumerated using a standard plate counting technique from which the kinetics of colony formation were determined and maximum colony counts were obtained for each condition examined. Exposure of the spores to calcium‐DPA (50‐40 mmol/l) in Tris acid maleate buffer pH 9.0 maintained at 10°C was found to produce an optimal response. Following this method the total viable population of a spore suspension was enumerated. This was demonstrated statistically using the Wilcoxon rank‐sum test for significance. Calcium‐DPA was found to produce activation in spores but further germinants and nutrients were required for colony formation. The Ca‐DPA treatment was found to be effective in enumerating both naturally dormant spores and heat injured spores.