Vancomycin-resistant Staphylococcus aureus (VRSA)

Abstract

The mechanism of vancomycin resistance in Mu50 expressing vancomycin resistance with MIC of 8mg·ml^-1 (VRSA), and Mu3 having heterogeneous vancomycin resistance with MIC of 2mg·ml^-1 (hetero-VRSA) were investigated. Mu3 and Mu50 were found to share three features when compared with the vancomycin-susceptible control strains: 1) accelerated uptake of N-acetylglucosamine into the cell, 2) an increased cytoplasmic murein monomer precursor pool, and 3) increased production of PBP2 (and PBP2'). These phenotypes which would be recapitulated into ‘activated cell-wall synthesis’ may be a precursor of vancomycin resistance in the Mu3-Mu50 lineage of S. aureus strains.
Chromatography of untreated mutanolysin-hydrolyzed peptidoglycan components of Mu50 revealed: 1) an increased proportion of abnormal muropeptide components, and 2) decreased cross-linking as evidenced by an unusual increase in the muropeptide monomer/dimer ratio as compared to Mu3. The cell wall of VRSA strain Mu50 had a distinct structure and increased binding capacity to vancomycin as compared with hetero-VRSA. This, together with increased cell-wall thickness, may account for the difference of the level of vancomycin resistance between VRSA and hetero-VRSA.
VRSA can be detected by the current methods (MIC, E-test, paper disc), but hetero-VRSA cannot. In contrast to the known fact that combination of vancomycin and β-lactam antibiotics synergistically acts on VSSA (vancomycin-sensitive Staphylococcus aureus), it was found that the drugs antagonize in acting on hetero-VRSA (Mu3). Utilizing this antagonistic phenomenon (induction of vancomycin resistance by β-lactam antibiotics), we developed a method of detecting the Mu3 strain in which a paper disc containing β-lactam antibiotics was placed on an agar plate containing vancomycin at a concentration which does not allow the Mu3 strain to grow, forcing the Mu3 strain to grow only around the paper disc. The growth zone formed a ring around the disc when the concentration of β-lactam antibiotics contained in the paper disc was increased. From these observations we concluded that there is an optimal concentration of β-lactam antibiotics for inducing vancomycin resistance.