Complete reconstitution of the vancomycin-intermediateStaphylococcus aureus(VISA) phenotype of strain Mu50 was achieved by sequentially introducing mutations into six genes of vancomycin-susceptibleS. aureus(VSSA) strain N315ΔIP. The six mutated genes were detected in VISA strain Mu50 but not in N315ΔIP. Introduction of the mutation Ser329Leu intovraS, encoding the sensor histidine kinase of thevraSRtwo-component regulatory (TCR) system, and another mutation, Glu146Lys, intomsrR, belonging to the LytR-CpsA-Psr (LCP) family, increased the level of vancomycin resistance to that detected in heterogeneous vancomycin-intermediateS. aureus(hVISA) strain Mu3. Introduction of two more mutations, Asn197Ser intograRof thegraSRTCR system and His481Tyr intorpoB, encoding the β subunit of RNA polymerase, converted the hVISA strain into a VISA strain with the same level of vancomycin resistance as Mu50. Surprisingly, however, the constructed quadruple mutant strain ΔIP4 did not have a thickened cell wall, a cardinal feature of the VISA phenotype. Subsequent study showed that cell wall thickening was an inducible phenotype in the mutant strain, whereas it was a constitutive one in Mu50. Finally, introduction of the Ala297Val mutation intofdh2, which encodes a putative formate dehydrogenase, or a 67-amino-acid sequence deletion intosle1[sle1(Δ67aa)], encoding the hydrolase ofN-acetylmuramyl-l-alanine amidase in the peptidoglycan, converted inducible cell wall thickening into constitutive cell wall thickening.sle1(Δ67aa) was found to cause a drastic decrease in autolysis activity. Thus, all six mutated genes required for acquisition of the VISA phenotype were directly or indirectly involved in the regulation of cell physiology. The VISA phenotype seemed to be achieved through multiple genetic events accompanying drastic changes in cell physiology.
Spontaneous mutation accompanying decreased growth rate and cell wall thickening confers vancomycin resistance in the subpopulation of heterogeneously vancomycim-resistaut strain Mu3.
