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Antimicrobial Agents and Chemotherapy, May 2005, p. 1688-1694, Vol. 49, No. 5
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.5.1688-1694.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Molecular Characterization of vanB Elements in Naturally Occurring Gut Anaerobes

S. A. Ballard,1 K. K. Pertile,1 M. Lim,1 P. D. R. Johnson,1,2 and M. L. Grayson1,2,3*

Department of Infectious Diseases, Austin Health,1 Department of Medicine, University of Melbourne,2 Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Victoria, Australia3

Received 11 October 2004/ Returned for modification 9 November 2004/ Accepted 16 December 2004

Previously, we reported the isolation of 10 vancomycin-resistant gram-positive anaerobic bacilli carrying the vanB ligase gene from nine hemodialysis patients (S. A. Ballard et al., Antimicrob. Agents Chemother. 49:77-81, 2005; T. P. Stinear et al., Lancet 357:855-856, 2001). In the present study, the molecular and evolutionary relationship of the vanB resistance element within these 10 anaerobes and two vancomycin-resistant Enterococcus faecium strains were examined. PCR analysis and nucleotide sequencing demonstrated that all 12 isolates carried the vanB operon associated with an element identical to Tn1549 and Tn5382 of Enterococcus. Restriction fragment length polymorphism analysis of the vanB operon in these isolates revealed two distinct patterns, and sequencing showed that minor base differences existed. PCR amplification of the joint region of a circular intermediate was demonstrated in nine of these organisms, a finding indicative of an ability to excise and circularize, an intermediate step in transposition and conjugative transfer. Southern hybridization with a vanB-vanXB probe suggests that there is one insert of the transposon in all isolates. Sequence analysis of the integration site revealed distinct sequences: the Tn1549/5382 element within E. faecium was inserted within the host chromosome, whereas nucleotide sequences surrounding the Tn1549/5382 element in the 10 anaerobes showed no significant homology to sequences in the GenBank database. We demonstrate considerable similarity between the Tn1549/5382 element identified in 10 anaerobe isolates with that found in enterococci. The homology and potential to transpose suggest a recent horizontal transfer event may have occurred. However, the original direction of transposition and the mechanism involved remains unknown.

* Corresponding author. Mailing address: Infectious Diseases Department, Austin Hospital, Austin Health, Studley Rd., Heidelberg, Victoria 3084, Australia. Phone: (613) 9496-6676. Fax: (613) 9496-6677. E-mail: lindsay.grayson{at}austin.org.au.

Antimicrobial Agents and Chemotherapy, May 2005, p. 1688-1694, Vol. 49, No. 5
0066-4804/05/$08.00+0     doi:10.1128/AAC.49.5.1688-1694.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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