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Appl Environ Microbiol. 1972 April; 23(4): 710-713
Copyright © 1972 American Society for Microbiology. All Rights Reserved.

Rapid Ornithine Decarboxylase Test for the Identification of Enterobacteriaceae

Section of Infectious and Immunologic Diseases, Department of Internal Medicine, School of Medicine, University of California, Davis, California 95616
Clinical Microbiology Laboratory, Sacramento Medical Center, Sacramento, California 95817

ABSTRACT

Conventional methods for detecting ornithine decarboxylase activity require an extended period of incubation. However, with a few simple modifications, accurate results were obtained within a few hours rather than several days. The broth medium was modified, primarily by omitting glucose and by decreasing the pH to 5.5. A 1-ml amount of this broth was inoculated with one colony and then overlaid with sterile mineral oil. Within 2 to 4 hr, the pH increased if ornithine was decarboxylated, thus changing the color of the internal pH indicator to a dark purple. If the amino acid was not decarboxylated, the pH decreased to pH 5.0 to 5.2, enough to give a definite yellow color. With 347 selected clinical isolates, the rapid test gave results identical to those obtained in 1 to 4 days with Moeller's decarboxylase medium. Less reliable results were obtained with Difco's decarboxylase medium with 0.3% agar which was stabinoculated and read after 18 to 24 hr without a mineral oil seal. The rapid ornithine decarboxylase test represents a simple, accurate technique which is well suited for the clinical microbiology laboratory.