Abstract
Vancomycin resistance is conferred upon vancomycin-resistant enterococci (VRE) through the replacement of peptidoglycan (PG) stem terminal d-Ala-d-Ala with d-Ala-d-Lac. The d-Ala-d-Lac incorporation can affect both the fitness and virulence of VRE. Here we comprehensively investigate the changes to PG composition in vancomycin-resistant Enterococcus faecalis following the growth in presence of vancomycin using liquid chromatography-mass spectrometry. Using high-resolution mass spectrometry, 104 unique muropeptides fragments were identified and the relative abundance of each fragment was accurately quantified by integrating the ion current of a selected ion using extracted-ion chromatogram. The analysis indicates reduced PG cross-linking, increased carboxypeptidase activities, increased N-deacetylation, and increased O-acetylation in VRE when grown in the presence of vancomycin. We found that O-acetylation preferentially occurred on muropeptides fragments with reduced cross-linking with a pentapeptide stem that terminated in d-Ala-d-Lac. These findings show that O-acetylation preferentially occurred in regions of the cell wall with reduced PG cross-linking on PG units that have stems terminating in d-Ala-d-Lac, serving as markers to prevent both the PG-stem modification by carboxypeptidases and the cell wall degradation by autolysins. Accurate quantitative PG composition analysis provided compositional insights into altered cell wall biosynthesis and modification processes in VRE that contribute to lysozyme resistance and enhanced virulence for VRE grown in the presence of vancomycin.
Highlights
In this report we investigate the changes to PG composition of vancomycin-resistant Enterococcus faecalis (ATCC 51299), vancomycin-resistant enterococci (VRE) of vanB type, using liquid chromatography-mass spectrometry (LC-MS) to provide insights into altered cell wall biosynthesis and PG modifications following the induction of vancomycin resistance
This difficulty arises from multiple muropeptide species that co-elute during the chromatographic separation, obfuscating identification and quantification of muropeptide peaks solely based on the absorption integral of the elutants
Earlier studies have relied on treatment of cell walls with hydrofluoric acid to remove PG modifications prior to the LC-MS analysis
Summary
One of more striking changes to the cell wall composition that occurs in VRE following the induction of vancomycin resistance is the increase in both PG O-acetylation and N-deacetylation (Fig. 2a). Increased PG O-acetylation and N-deacetylation[12,13,14] represent significant modifications to VRE cell wall in response to vancomycin treatment.
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