Alteration of such a diverse metabolic pathway genes seems to change the flow of nutrients and metabolites towards the enhanced production of cell-wall peptidoglycan (PG) and/or reduction in autolysis [42]. Besides supporting the cell to tolerate the cytokilling activity of vancomycin, R428 chemical structure reduced autolysis is considered to contribute to the maintenance of thick cell-wall PG

layers by decreasing the rate of cell-wall turnover. In fact, considerable number of mutations affecting the above 20 genes are speculated to contribute to the enhanced cell-wall synthesis [33] and [42]. PG contains many D-alanyl-d-alanine residues to which vancomycin binds. Therefore, thickened PG layers trap more vancomycin molecules than the PG layers of normal thickness [43], [44] and [45]. Moreover, the PG mesh structure is clogged

5-Fluoracil by the entrapped vancomycin, and serves as an obstacle for further penetration of vancomycin to the cytoplasmic membrane where the real targets of vancomycin exist [46]. 2) VRSA: cross-genus transmission of resistance gene. Vancomycin MIC of VISA is 4–8 mg/L, which ‘was not’ considered resistant according to the CLSI criteria of the time. Therefore, the word VISA was coined for Mu50 indicating its ‘intermediate’ level of vancomycin susceptibility. Five years later, in 2002, a VRSA clinical strain with MIC ≥ 16 mg/L was isolated [47]. It turned out to have acquired a vanA-transposon from vancomycin-resistant Enterococcus (VRE). The transposon carried vanA-gene complex containing vanA, vanH, vanX, and vanY. If the four genes function in concert, all the D-Alanyl-D-Alanine residues of the substrate for PG synthesis are replaced by D-Alanyl-D-lactate to which vancomycin cannot bind. This amazing mechanism of resistance is described elsewhere in Venetoclax order detail [48]. In spite of the acquisition of this ingenious system, however, so far only a dozen of VRSA clinical strains have been reported in the world after more than a decade of its first isolation. The

fitness cost of the carriage of vanA plasmid was suspected although growth retardation of the vanA plasmid-carrying strain is reported to be minimum [49]. In fact, the vanA-mediated vancomycin resistance is an inducible type, and does not cause much fitness cost during the growth in the absence of vancomycin [70]. As an explanation for the unpopularity of the resistance, we initially speculated that the level of methicillin resistance might be much lowered due to the loss of D-Alanyl-D-Alanine residues from the cell wall to which PBP2’ is supposed to bind. However, we found that a VRSA clinical strain VRS1 simultaneously expressed high-level resistance to both vancomycin and oxacillin [70].