The multiple paths to heteroresistance and intermediate resistance to vancomycin in Staphylococcus aureus.

A comprehensive understanding of the mechanisms by which Staphylococcus aureus achieves intermediate levels of resistance to vancomycin could foster the development of improved diagnostic tests and therapeutic agents, with potentially improved clinical outcomes. Vidaillac et al, in this issue of the Journal [1], used well-characterized successive S. aureus clinical isolates that had acquired serial mutations together with reduced susceptibility during vancomycin treatment in order to compare the in vivo evolution of resistance in these isolates to that seen in vitro [2]. They reasoned that the patient’s isolates, all recovered from blood, may not have been fully representative of organisms present at tissue site(s) of infection. Using an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model with simulated vegetations, theywere able to observephenotypic changes, including reduced viability, loss of pigmentation, and changes in colony size and hemolytic activity, prior to the identification of the detection of resistant mutants. With the appearance of the latter, daptomycin minimal inhibitory concentrations (MICs) rose, along with those of vancomycin, and this was accompanied by thickened cell walls and reduced autolysis, decreased surface anionic charge, and susceptibility to cationic peptides. Whole-genome sequencing demonstrated that the vancomycin-intermediate S. aureus (VISA) phenotype was acquired via at least 3 different genetic pathways and that, in one of the 2 cases, the pathway differed from that seen in vivo. High-level resistance of S. aureus to vancomycin, resulting from horizontal transfer of transposon Tn 1546 carrying the vanA operon from Enterococcus species, is well understood but rare [3]. In contrast, intermediate resistance is more common, more complex, and less well understood. VISA and vancomycinheteroresistant S. aureus (hVISA; heteroresistance to vancomycin is considered a transitional state to intermediate resistance) exhibit a number of typical phenotypic characteristics, including cell wall and septal thickening, abnormal cell division, reduced cell wall turnover and autolysis, reduced hemolytic activity, and reduced virulence in animal models [4]. The vancomycin heteroresistance and intermediate resistance phenotypes are believed to result from altered cell envelope charge and diminished penetration of the drug through the thickened cell wall. The excess D-ala-D-ala targets in the cell wall act as a molecular sink, impairing vancomycin from accessing its pentapeptide target [5, 6]. Mutations associated with the intermediate resistance phenotype have been identified in VraSR (vancomycin resistance–associated sensor/regulator), GraSR (glycopeptide resistance–associated sensor/regulator), WalKR (also known as “YycGF” and “VicKR”), as well as rpoB and others. VraSR, GraSR, and WalKR are each 2-component regulatory systems (TCS). TCS are ubiquitous among bacteria, allowing them to sense and respond to their environment [7]. They possess a cell membrane histidine kinase acting as a sensor/transducer that responds to its external signal by autophosphorylation in the presence of adenosine triphosphate. The phosphoryl group is then transferred to a response regulator, generally a DNA-binding protein that regulates transcription. S. aureus encodes 16 such signal transduction systems in its chromosome, and SCCmec encodes an additional one [7, 8]. VraSR had previously been found to be upregulated in hVISA strain Mu3 and VISA strain Mu50 [8, 9]. It acts under the regulation of yvqF as an on-off switch of Received and accepted 1 March 2013; electronically published 28 March 2013. Correspondence: Stan Deresinski, MD, Department of Medicine, Stanford University, 2900 Whipple Ave, Ste 115, Redwood City, CA 94062 [[email protected]]. The Journal of Infectious Diseases 2013;208:7–9 © The Author 2013. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. [email protected]. DOI: 10.1093/infdis/jit136