Effect of plate preincubation on Bacillus stearothermophilus disc assay zone diameters

Effect of plate preincubation on the Bacillus stearothermophilus disc assay zone size was studied. Each of three raw milk samples were subdivided and treated with five levels of US Pharmacopeial reference standard penicillin G. Freshly prepared, 2.5- and 5-d old seeded disc assay culture plates were preincubated at 64 degrees C for 0 (control), 20, 40, 60, 80, 100, and 120 min prior to placement of discs on the medium surface. Plate age did not have a significant effect on the size of the zone diameter. Following 20 and 40 min of plate preincubation, zone diameters were in some, but not all, cases significantly different from the control. Plate preincubation of 60 min or greater always significantly decreased zone diameter. Routine preincubation of B. stearothermophilus plates is not recommended. In situations where time is a critical factor, plates should be preincubated no longer than 40 min.