The emergence of daptomycin-resistant (DAP-R) Staphylococcus aureus strains has become a global problem. Point mutations in mprF are the main cause of daptomycin (DAP) treatment failure. However, the impact of these specific point mutations in methicillin-resistant S. aureus (MRSA) strains associated with DAP resistance and the "seesaw effect" of distinct beta-lactams remains unclear. In this study, we used three series of clinical MRSA strains with three distinct mutated mprF alleles from clone complexes (CC) 5 and 59 to explore the seesaw effect and the combined effect of DAP plus beta-lactams. Through construction of mprF deletion and complementation strains of SA268, we determined that mprF-S295A, mprF-S337L, and one novel mutation of mprF-I348del within the bifunctional domain lead to DAP resistance. Compared with wild-type mprF cloned from a DAP-susceptible (DAP-S) strain, these three mprF mutations conferred the seesaw effect to distinct beta-lactams in the SA268ΔmprF strains, and mutated mprF (I348del and S337L) did not alter the cell surface positive charge (P > 0.05). The susceptibility to beta-lactams increased significantly in DAP-R CC59 strains, and the seesaw effect was found to be associated with distinct mutated mprF alleles and the category of beta-lactams. The synergistic activity of DAP plus oxacillin was detected in all DAP-R MRSA strains. Continued progress in understanding the mechanism of restoring susceptibility to beta-lactam antibiotics mediated by the mprF mutation and its impact on beta-lactam combination therapy will provide fundamental insights into treatment of MRSA infections.
Keywords: MRSA; beta-lactams; daptomycin; methicillin-resistant Staphylococcus aureus; mprF; seesaw effect; treatment.