MGP test: Introduction, principle, procedure, result and interpretation

MGP test

Introduction of MGP test

MGP stands for methyl-α-D-glucopyranoside. MGP test is effective for species identification of Enterococcus. Enterococcus gallinarum and Enterococcus casseliflavus are MGP test positive while Enterococcus faecalis and Enterococcus  faecium negative. Vancomycin-resistant Enterococcus faecium (VREF) is a serious nosocomial problem which is difficult to differentiate from  Enterococcus gallinarum and Enterococcus casseliflavus, while not important for purposes of control of nosocomial infections, show intrinsic resistance to vancomycin due to the vanC gene. Another side, E. gallinarum and E. casseliflavus are difficult to differentiatefrom VREF by conventional biochemical tests, since they are phenotypically closely related. Therefore,MGP test  is useful in preventing the misidentification of vancomycin-resistant E. gallinarum as VREF for laboratories that cannot perform the rapid tube motility test. It is an alternative to the motility test. Typically, E. gallinarum and E. casseliflavus can be differentiated from E. faecium based on motility: E. gallinarum and E. casseliflavus are motile, while E. faecium is non motile.

Principle of MGP test

MGP broth is useful to differentiate enterococci based on the ability to acidify the carbohydrate MGP. Phenol red indicator present in medium that helps to identify the organisms on the basis color development i.e. development of yellow color,  MGP test positive where as red or orange negative.

Requirements for test

  • Organisms should be tested having one of the following features- 
  •  Test any GPC that is catalase negative and PYR positive and grows on plates with 6 µg of vancomycin per ml or demonstrates resistance to vancomycin by antimicrobial susceptibility testing but shows susceptibility to ampicillin.Test any isolate that is identified as Enterococcus  faecium by a commercial kit system but is ampicillin susceptible.
  • Test any nonmotile enterococcus that demonstrates resistance to vancomycin with an MIC less than 32 µg/ml but is ampicillin susceptible.
  • MGP test can be performed from any blood agar plate or any plate medium with enterococcal growth, even  including media with bile-esculin or azide.
  • MGP brothComposition of MGP brothPancreatic digest of casein :10.0 gSodium chloride :5.0 gMGP :10.0 g

    Phenol red :18.0 mg

    Distilled or deionized water: 1000 ml

  • Inoculating needle
  • Incubator
  • Control organisms 
  • Positive Control(PC) E. gallinarum ATCC 49573
  •  Negative Control (NC)  E. faecalis ATCC 29212

Procedure of MGP test

  1. With the help of inoculating needle, Inoculate the MGP broth by lightly touching a single, isolated colony.
  2. Do not use a heavy inoculum.
  3. Incubate aerobically at 35°C for 24 hours.
  4. Observe for development of  yellow color change in the medium.

Result interpretation

 MGP test Positive: Development of a yellow color  acidification of MGP

MGP test Negative: Red or orange color

E. gallinarum ATCC 49573: MGP test Positive

E. faecalis ATCC 29212: MGP Negative

Limitations

  • A heavy inoculum may give false-positive result.
  • Longer incubation times may result in false-positive reactions.
  • The test has reported sensitivity of  98-100% and a specificity of 95%
  • Enterococcus vagococcus gives a positive result.

Bibliography

  • Cowan and Steel’s ,manual for the identification of medical bacteria
  • Lynne S. Garcia ,Clinical Microbiology Procedures Handbook
  • https://catalog.hardydiagnostics.com/cp_prod/product/z225-rapid-mgp-media-for-enterococcus-10ml-20-tests-per-package-by-hardy-diagnostics-rapid-id-test-kits
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