Contribution of a thickened cell wall and its glutamine nonamidated component to the vancomycin resistance expressed by Staphylococcus aureus Mu50

Staphylococcus aureus Mu50, which has reduced susceptibility to vancomycin, has a remarkably thickened cell wall with an increased proportion of gluta mine nonamidated muropeptides. In addition, Mu50 had enhanced glutamine syn thetase and L-glutamine D-fructose-6-phosphate aminotransferase activities, which are involved in the cell-wall peptidoglycan synthesis pathway. Furth ermore, significantly increased levels of incorporation of C-14-labeled D-g lucose into the cell wall was observed in Mu50. Unlike a femC mutant S, aur eus strain, increased levels of production of nonamidated muropeptides in M u50 was not caused by lower levels of glutamine synthetase activity but was considered to be due to the glutamine depletion caused by increased glucos e utilization by the cell to biosynthesize increased amounts of peptidoglyc an. After the cells were allowed to synthesize cell wall in the absence or presence of glucose and glutamine, cells with different cell-wall thickness es and with cell walls with different levels of cross-linking were prepared , and susceptibility testing of these cells demonstrated a strong correlati on between the cell-wall thickness and the degree of vancomycin resistance. Affinity trapping of vancomycin molecules by the cell wall and clogging of the outer layers of peptidoglycan by bound vancomycin molecules were consi dered to be the mechanism of vancomycin resistance of Mu50, The reduced cro ss-linking and the increased affinity of binding to vancomycin of the Mu50 cell mall presumably caused by the increased proportion of nonamidated muro peptides may also contribute to the resistance to some extent.