Use of Methyl-alpha-D-glucopyranoside Test for Species Identification of Vancomycin Resistant Enterococci

Kim, Mi Na; Sung, Heung Sub; Park, Jun Seok; Pai, Chik Hyun

Abstract

BACKGROUND: The precise identification of Enterococcus gallinarum and E. casseliflavus has assumed additional importance in clinical microbiology due to the intrinsic low-level resistance to vancomycin and the difficulty in differentiating them from E. faecium or E. faecalis, which are frequently found to be clinically significant vancomycin resistant enterococci(VRE). We evaluated the usefulness of Methyl-alpha-D-glucopyranoside(MDG) test for accurate species identification among them.
METHODS
A total of 23 enterococci isolates including 18 clinical isolates of VRE from Nov 1997 to Aug 1998 and 5 VRE strains which had previously been reported as E. faecalis (2), E. faecium(2), E. avium(1) carrying vanC were tested for acidification of MDG. MDG test was done using 1% MDG in phenol red broth base and yellow coloration was interpreted as positive after 1 and 2 days of incubation at 35 degrees C. MDG results were compared with species identification by MicroScan Pos Combo type 6 (Dade, US A), motility test, pigment production, and PCR results of vanA, vanB, vanC1, vanC2/C3.
RESULTS
Vancomycin resistance of 23 strains were genotyped as 7 strains of vanA, 12 strains of vanC1, 4 strains of vanC2/C3. MicroScan identified 7 vanA VRE as E. faecalis(1) and E. faecium(6), 12 VRE carrying vanC1 as E. faecalis(3), E. faecium(8) and E. avium(1), and 4 VRE carrying vanC2/C3 as E faecalis(3) and E. avium(1). Sixteen vanC VRE strains were all positive for MDG test and only 8(50%) of the 16 strains were motile. Yellow pigment were detected in all 4 vanC2/C3 VRE but only after a careful examination with a prolonged incubation. Seven vanA VRE were all negative in MDG tests, motility test and pigment production.
CONCLUSIONS
MicroScan system plus motility and pigment production test was not able to differentiate reliably E. gallinarum and E. casseliflavus from E. faecalis and E. faecium. The MDG test was shown to be superior to motility test in differentiating those from E. faecalis and E. faecium. We conclude that the MDG test should be included for identifcation of VRE.