Staphylococcus aureus VRSA-11B is a constitutive vancomycin-resistant mutant of vancomycin-dependent VRSA-11A

Abstract

Vancomycin-resistant Staphylococcus aureus VRSA-10 was isolated in 2009, whereas VRSA-11A and VRSA-11B were isolated from the same patient in 2010. Growth curves and determination of the nature of the peptidoglycan precursors and of the VanX D,D-dipeptidase activity in the absence and in the presence of vancomycin indicated that vancomycin resistance was inducible in VRSA-10, that VRSA-11A was partially dependent on glycopeptide for growth, and that VRSA-11B was constitutively resistant. Both VRSA-11A and -11B harbored an insertion sequence, ISEf1, at the same locus in the vanX-vanY intergenic region of Tn1546 and an S183A mutation in the chromosomal D-alanyl:D-alanine ligase (Ddl). This substitution has been shown to be responsible for a drastic diminution of the affinity of the enzyme for D-Ala at subsite 1 in Escherichia coli DdlB. VRSA-11B exhibited an additional mutation, P216T, in the transcriptional regulator VanR, most probably associated with constitutive expression of vancomycin resistance. It is thus likely that VRSA-11B is a constitutive derivative of VRSA-11A selected during prolonged vancomycin therapy. Synthesis of peptidoglycan precursors ending in D-Ala-D-lactate was responsible for oxacillin susceptibility of VRSA-11A and VRSA-11B despite the presence of a wild-type mecA gene in both strains. Copyright © 2012, American Society for Microbiology. All Rights Reserved.