Methods for storage of antimicrobial effectiveness test inoculum suspensions below freezing

Studies were conducted to determine the suitability of storage below freezing of some antimicrobial effectiveness test inoculum organisms: Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans. Several solutions used for protecting microorganisms subjected to storage below freezing were compared. Comparison of 10% dextrose with other solutions (distilled water; 0.07 M phosphate buffer, H 7.0, with 15% glycerol; and 0.07 M phosphate buffer, pH 7.0, with 7.5% dimethyl sufoxide) demonstrated that inoculum suspensions were most stable when prepared with 10% dextrose. After storage for 6 months at-50 degrees in a freezer, inoculum suspensions prepared with 10% dextrose retained viability and demonstrated suitability for use in antimicrobial effectiveness tests of dosage forms containg preservatives.