American Society for Microbiology, Antimicrobial Agents and Chemotherapy, 5(53), p. 1974-1982, 2009
DOI: 10.1128/aac.00034-09
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ABSTRACT Three Enterococcus faecium strains isolated successively from the same patient, vancomycin-resistant strain BM4659, vancomycin-dependent strain BM4660, and vancomycin-revertant strain BM4661, were indistinguishable by pulsed-field gel electrophoresis and harbored plasmid pIP846, which confers VanB-type resistance. The vancomycin dependence of strain BM4660 was due to mutation P 175 L, which suppressed the activity of the host Ddl d -Ala: d -Ala ligase. Reversion to resistance in strain BM4661 was due to a G-to-C transversion in the transcription terminator of the vanRS B operon that lowered the free energy of pairing from −13.08 to −6.65 kcal/mol, leading to low-level constitutive expression of the resistance genes from the P RB promoter, as indicated by analysis of peptidoglycan precursors and of VanX B d , d -dipeptidase activity. Transcription of the resistance genes, studied by Northern hybridization and reverse transcription, initiated from the P YB resistance promoter, was inducible in strains BM4659 and BM4660, whereas it started from the P RB regulatory promoter in strain BM4661, where it was superinducible. Strain BM4661 provides the first example of reversion to vancomycin resistance of a VanB-type dependent strain not due to a compensatory mutation in the ddl or vanS B gene. Instead, a mutation in the transcription terminator of the regulatory genes resulted in transcriptional readthrough of the resistance genes from the P RB promoter in the absence of vancomycin.