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Related Experiment Videos

Improved 18-hour methyl red test.

A L Barry, K L Bernsohn, A P Adams

    Applied Microbiology
    |December 1, 1970
    PubMed
    Summary
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    Standardizing the methyl red (MR) test volume and inoculum is crucial for accurate clinical laboratory results. A refined MR test protocol provides reliable identification of Enterobacteriaceae within 24 hours.

    Area of Science:

    • Clinical Microbiology
    • Bacteriology

    Background:

    • Standard methyl red (MR) test methods are impractical for routine clinical use due to long incubation times.
    • Overnight incubation of the standard MR test often yields equivocal or false-positive results.

    Purpose of the Study:

    • To demonstrate the importance of standardizing broth volume, vessel size, and inoculum density for the MR test.
    • To develop a practical and reliable MR test protocol for routine clinical identification of Enterobacteriaceae.

    Main Methods:

    • Inoculation of a single colony into 0.5 ml of MR-Voges Proskauer (VP) broth in a 13 x 100-mm tube.
    • Incubation at 37°C for 18–24 hours, followed by the addition of one drop of MR reagent.
    • Comparison with standard MR test protocols using larger volumes and varying incubation periods.

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    Main Results:

    • The proposed technique yielded definite red (positive) or yellow (negative) colors, unlike the ambiguous orange shades from larger volumes.
    • 18-hour MR tests in 0.5 ml broth were comparable to standard 5-day incubation results and superior to 48-hour tests in 6.0 ml broth.
    • Standardizing culture conditions significantly improved MR test reliability and reduced false positives.

    Conclusions:

    • The proposed standardized MR test technique is practical for routine clinical laboratories.
    • This method allows for reliable identification of Enterobacteriaceae within 18–24 hours.
    • Optimized MR test protocols enhance diagnostic accuracy and efficiency in clinical microbiology settings.