Moenomycin-resistance is associated with vancomycin-intermediate susceptibility in Staphylococcus aureus

We have previously isolated a vancomycin-intermediate susceptibility mutant from methicillinresistant Staphylococcus aureus(MRSA) strain COL, and demonstrated the increased glycan-chain length and the decreased moenomycin-susceptibility. To further investigate the relationship between the resistance to vancomycin and to moenomycin, we isolated moenomycin-resistant mutants (4-16 fold higher compared to the parent) from 5 MRSA and 2 methicillin-sensitive S. aureus(MSSA) strains. The MRSA mutants showed a decreased susceptibility to vancomycin (2-4 fold), teicoplanin (2-4 fold) and an increased susceptibility to methicillin (2-8 fold). MSSA strains also showed similar results with those of MRSA strains except that there was no alteration of methicillin susceptibility. Among the mutants, three mutants including two MRSA mutants and one MSSA mutant were analyzed by electron microscopy, and they showed thickened cell walls compared to those of the parents. The glycan-chain length of the peptidoglycan of the mutant was shown to be slightly longer than that of the parent, but the muropeptide profile was very similar. The expression levels of all PBPs were similar to those of the parent. Furthermore, the nucleotide sequences of sgtA, sgtB and pbp2 in the mutant were identical to those of the parent. These results indicate that the moenomycin-resistance is closely associated with vancomycin-intermediate susceptibility in S. aureus.