Another One Bites the Dust: Evaluating the Proteomics Response of Methicillin-Susceptible vs. Resistant Staphylococcus aureus (USA-300 MSSA/MRSA) to a Novel Antimicrobial EPC-373K vs. Beta-Lactam Antibiotics

Introduction: Staphylococcus aureus is a highly virulent, antimicrobial-resistant bacterial pathogen responsible for nosocomial and community-acquired infections with a high prevalence in skin and wound infections. The Centers for Disease Control identify methicillin-resistant S. aureus as a serious threat to public health; increasing pressure to develop non-traditional antimicrobials which not only addressing resistance but mitigating risk for acquired resistance.

Method: MIC90 and time kill curves for two pediatric USA300 community of methicillin-resistant S. aureus (MRSA, TCH1516) strain and methicillin-susceptible S. aureus (MSSA, TCH959) were determined for EPC-373K (K3Ag [IO4(OH)2]2(H2O)4) and oxacillin sodium salt. Label free quantitative proteomic response was conducted following two hours of treatment with each antimicrobial agent at 2x MIC90 versus untreated control. Harvested cells were lysed, proteins precipitated, and digestion followed by purification on C18 STAGE-tips. Peptides were detected by MS/MS and MaxQuant and Perseus software were used for data analysis and bioinformatics.

Results: In the present work, the susceptibility of the S. aureus strains to oxacillin, was significantly greater (p < 0.001) for MSSA than MRSA, with MIC90 values of 2.3 ± 0.0 μM and 300.2 ± 0.0 μM respectively. The susceptibility of MRSA to EPC-373K, MIC90 = 19.5 ± 0.0 μM, was greater (p < 0.001) than MSSA,MIC90 of 39.1 ± 0.0 μM. MSSA was more susceptible to the conventional antibiotic oxacillin where MRSA was more susceptible to EPC-373K (p < 0.001) versus the beta-lactam antibiotic. At a timepoint of 2-hours following 2xMIC90 treatment, a significantly differentiated response in protein abundance was observed between untreated MRSA and MSSA and the individual treatment groups against either MRSA or MSSA (FDR ≤ 0.05, fold change ≥ 2.0). A total of 1160 and 965 differentially abundant proteins were observed for EPC-373K versus untreated control in MSSA and MRSA respectively. In comparison to 246 and 372 differentially abundant proteins observed for oxacillin versus untreated in MSSA and MRSA respectively.

Discussion: Methicillin-susceptible and resistant strains of USA300 community isolates of S. aureus are susceptible to the novel antimicrobial EPC-373K. The proteomic response of MRSA & MSSA to EPC-373K are significantly different than the conventional beta-lactam antibiotic compound evaluated.