This is another strategy of VISA. PG now works as a shield for vancomycin penetration, like the outer membrane of Gram-negative bacteria. Evidence is accumulating for the view that vancomycin resistance in VISA is caused by altered cell wall structure and metabolism. Since S. aureus cell wall synthesis is regulated by multiple regulator genes, it is reasonable that hVISA/VISA clinical strains carry various mutations in the regulator genes associated with cell wall biosynthesis [12].

Whole-genome sequencing and microarray analysis of Mu50 identified the vraSR two-component regulatory system (TCRS) whose transcriptional LEE011 ic50 upregulation is responsible for the raised vancomycin resistance of

Mu50 [18], [19] and [20]. In Mu50 (and also in Mu3), the sensor kinase gene vraS was constitutively activated by the incorporated mutation vraS(I5N). The mutated VraS in turn activated the cognate response regulator VraR and raised expression of the genes encoding www.selleckchem.com/products/KU-55933.html several key enzymes of cell wall biogenesis such as murZ, pbp2, sgtB, tarA, fmtA and lcpC (SA2103) [20] and [21]. Experimental introduction of the mutation vraS(I5N) as well as of another experimentally obtained mutation vraS(S329L) into VSSA strain N315ΔIP conferred an hVISA phenotype on the strain [21]. The mutation vraS(I5N) is carried by many VISA strains isolated from various districts of Japan [2], [22], [23] and [24], indicating clonal spread of Mu3 throughout Japan. Recently, vraSR is regarded as part of a four-membered operon, vraU–vraT (or yvqF)–vraS–vraR. vraT is reported to be essential for vraSR function as the upregulator of cell wall synthesis [25] and [26]. The vraT(Y220C) 3-mercaptopyruvate sulfurtransferase mutation was shown to activate vraSR and raise both methicillin and vancomycin resistance [26]. Another mutation [vraT(T125I)]

is also shown to raise vancomycin and imipenem MICs [23]. This cross-resistance between vancomycin and β-lactams through activation of the vraSR TCRS has a historical implication in the emergence of VISA from MRSA in Japan [22]. Acquisition of homogeneously high β-lactam resistance by hetero-MRSA (see below) appears to have prepared the way for Japanese MRSA to conquer vancomycin as well. The vraUTSR operon is frequently mutated in clinical VISA and hVISA strains ( Table 1). Of 33 VISA strains, 14 (42.4%) possessed mutations in either one of the three genes vraT, vraS and vraR. In Japan, however, the vraTSR mutation frequency among 86 S. aureus clinical strains with reduced teicoplanin susceptibility (teicoplanin MIC ≥ 2 mg/L; equivalent to hVISA or ‘pre-hVISA’ described below) was as high as 67.4% (58 strains) [23]. We found that the development of hVISA clinical strains among MRSA strains in Japan occurred before the introduction of vancomycin into clinical use [22].