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Journal of Clinical Microbiology, June 2008, p. 1961-1964, Vol. 46, No. 6
0095-1137/08/$08.00+0     doi:10.1128/JCM.01778-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Improved Detection of vanB2-Containing Enterococcus faecium with Vancomycin Susceptibility by Etest Using Oxgall Supplementation{triangledown}

E. A. Grabsch,1* K. Chua,1,2 S. Xie,1 J. Byrne,1 S. A. Ballard,2 P. B. Ward,1 and M. L. Grayson2,3,4

Microbiology,1 Infectious Diseases Departments, Austin Health, Austin Hospital, Heidelberg, Australia,2 Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Australia,3 Department of Medicine, University of Melbourne, Melbourne, Australia4

Received 6 September 2007/ Returned for modification 1 November 2007/ Accepted 11 April 2008

We have isolated a number of vanB-containing Enterococcus faecium isolates on bile esculin screening agar containing 6 mg/liter vancomycin, which on subsequent susceptibility testing using Etest have repeatedly demonstrated vancomycin MICs of ≤4 mg/liter. To investigate this genotype-phenotype incongruence of "low-MIC vancomycin-resistant enterococci" (LM-VRE), we examined the molecular characteristics of these isolates, including the presence of the vanB operon, using PCR amplification and DNA sequencing. All LM-VRE isolates contained vanB associated with the transposon Tn1549 and were polyclonal. Sequencing of the vanB ligase gene showed no differences from the prototype vanB2. In addition, we examined supplemented media to improve phenotypic detection of these isolates. Etest detection of LM-VRE improved when Mueller-Hinton agar (MHA) and brain heart infusion agar (BHIA) were supplemented with 10 g/liter oxgall (MHA-Oxg and BHIA-Oxg, respectively). We assessed the sensitivity and specificity of these media to detect vancomycin resistance using vancomycin-resistant vanB-containing E. faecium (n = 11), vancomycin-susceptible (van negative) E. faecium (n = 11), vancomycin-susceptible (van negative) E. faecalis (n = 11), and our LM-VRE (n = 23) isolates. After 48 h of incubation, both MHA-Oxg and BHIA-Oxg were 100% (34/34) sensitive and 100% (22/22) specific in the identification of vancomycin resistance. These findings suggest that supplementation of MHA or BHIA with 10 g/liter oxgall should be considered in laboratories where VRE detection protocols rely primarily on strain phenotype rather than early vanB gene detection by PCR.

* Corresponding author. Mailing address: Microbiology Department, Austin Hospital, Studley Rd., Heidelberg, Victoria, Australia 3084. Phone: (613) 9496-5137. Fax: (613) 9457-2590. E-mail: Elizabeth.Grabsch{at}austin.org.au

{triangledown} Published ahead of print on 23 April 2008.

Journal of Clinical Microbiology, June 2008, p. 1961-1964, Vol. 46, No. 6
0095-1137/08/$08.00+0     doi:10.1128/JCM.01778-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Grabsch, E. A., Ghaly-Derias, S., Gao, W., Howden, B. P. (2008). Comparative Study of Selective Chromogenic (chromID VRE) and Bile Esculin Agars for Isolation and Identification of vanB-Containing Vancomycin-Resistant Enterococci from Feces and Rectal Swabs. J. Clin. Microbiol. 46: 4034-4036 [Abstract] [Full Text]  


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