Effect of disc moistening method on Bacillus stearothermophilus disc assay zone diameters

Effect of disc moistening method on the Bacillus stearothermophilus disc assay zone size was studied. Pasteurized homogenized milk was subdivided and three levels of US Pharmacopeial reference standard penicillin G were added. Disc assay filter paper discs were moistened using the standard capillary action procedure and a method incorporating the use of a 90-microliters micropipetter. Amount of milk sample absorbed by discs prepared by the capillary action method was correlated with disc mass. Variation in amount of milk absorbed by discs was significantly greater when discs were prepared by capillary action. Zone diameter means and standard errors across all penicillin G concentrations tested were not significantly different for the two methods. The 90-microliters micropipetter method is recommended as an alternative to the capillary action method.