Long-term storage of microorganisms used in antimicrobial effectiveness tests.

Liquid nitrogen storage of organisms designated in the U.S. Pharmacopeia (XIX) for the assessment of antimicrobial effectiveness is described. By use of simple apparatus and procedures, 85-95% viabilities of pre-frozen cells are attained immediately upon freezing. Viability levels have remained constant over a 3-year period and the use of such inocula has considerably improved the reliability of the test. The superiority of liquid nitrogen storage for microbial test inocular over other types of storage is discussed.