1: Antimicrob Agents Chemother  1999 Jan;43(1):41-7 

Vancomycin-dependent Enterococcus faecalis clinical isolates and revertant
mutants.

Van Bambeke F, Chauvel M, Reynolds PE, Fraimow HS, Courvalin P.

Unite des Agents Antibacteriens, Institut Pasteur, 75724 Paris Cedex 15, France.

Three vancomycin-dependent clinical isolates of Enterococcus faecalis of the
VanB type were studied by determining (i) the sequence of the ddl gene encoding
the host D-Ala:D-Ala ligase and the vanSB-vanRB genes specifying the
two-component regulatory system that activates transcription of the vanB operon,
(ii) the level of expression of resistance genes by using DD-dipeptidase
activity as a reporter, and (iii) the proportions of the peptidoglycan
precursors synthesized. Each strain had a mutation in ddl leading to an amino
acid substitution (D295 to V; T316 to I) or deletion (DAK251-253 to E) at
invariant positions in D-Ala:D-Ala, D-Ala:D-Lac, and D-Ala:D-Ser ligases. These
mutations resulted in impaired host D-Ala:D-Ala ligases since only precursors
terminating in D-Ala-D-Lac were synthesized under vancomycin-inducing
conditions. Two types of vancomycin-independent revertants of one isolate were
obtained in vitro after growth in the absence of vancomycin: (i)
vancomycin-resistant, teicoplanin-susceptible mutants had a 6-bp insertion in
the host ddl gene, causing the E251-to-EYK change that restored D-Ala:D-Ala
ligase activity, (ii) constitutive vancomycin-resistant, teicoplanin-resistant
mutants had substitutions (S232 to F or E247 to K) in the vicinity of the
autophosphorylation site of the VanSB sensor and produced exclusively precursors
ending in D-Ala-D-Lac. Vancomycin- and teicoplanin-dependent mutants obtained by
growth in the presence of teicoplanin had an 18-bp deletion in VanSB, affecting
residues 402 to 407 and overlapping the G2 ATP binding domain. The rapid
emergence of vancomycin-independent revertants in vitro suggests that
interruption of vancomycin therapy may not be sufficient to cure patients
infected with vancomycin-dependent enterococci.

PMID: 9869563 [PubMed - indexed for MEDLINE]