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Appl Environ Microbiol. 1970 December; 20(6): 866-870
Copyright © 1970 American Society for Microbiology. All Rights Reserved.

Improved 18-Hour Methyl Red Test

Hospital Infection Research Laboratory, Departments of Medical Microbiology and of Medicine, College of Medicine, University of California, Irvine, California 92650
Los Angeles County General Hospital, Unit II, Los Angeles, California 90033
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

Standard methods for the methyl red (MR) test are not practical for routine use in clinical laboratories because of the necessarily prolonged incubation period. When read after overnight incubation, the usual MR test is often equivocal or falsely positive. The present study demonstrates the importance of standardizing the total volume of broth, the size of the vessel in which the cultures are incubated, and the density of the inoculum. In very small volumes of broth, cultures are better exposed to atmospheric oxygen, and thus MR-negative organisms tend to revert the initial acidic pH much more quickly than in deeper, large-volume broth cultures. In the proposed technique, a single colony was inoculated into a 0.5-ml amount of MR-Voges Proskauer (VP) broth (13- by 100-mm tube), and, after 18 to 24 hr at 37 C, one drop of MR was added. With this technique, the broth cultures produced either a definite red (positive) or yellow (negative) color, whereas various shades of orange were frequently observed when larger volumes of broth were tested after only 1 to 2 days of incubation. With 6.0-ml broth cultures, 18-hr MR tests were totally unreliable, but 18-hr tests in 0.5 ml of broth were comparable to the standard MR test performed after 5 days of incubation and superior to those performed after 48 hr in 6.0-ml broth cultures. With the proposed technique, the MR test can be incorporated readily into the routine scheme for identification of Enterobacteriaceae.