β-Lactam Combinations with Vancomycin Show Synergistic Activity against Vancomycin-Susceptible Staphylococcus aureus, Vancomycin-Intermediate S. aureus (VISA), and Heterogeneous VISA

Abstract

Increasing utilization of vancomycin due to the high prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infections has led to the emergence of vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA) strains. In vitro data suggest the potential for potent synergy between several beta-lactams and vancomycin. The objective of this study is to evaluate the synergy between beta-lactams and vancomycin against MRSA that is vancomycin susceptible, vancomycin-susceptible Staphylococcus aureus (VSSA), hVISA, and VISA. Fifty randomly selected clinical MRSA strains with various susceptibility levels to vancomycin were evaluated for vancomycin alone and vancomycin in combination with various concentrations of cefazolin (CFZ), cefepime (FEP), ceftaroline (CPT), and nafcillin (NAF). The potential for synergy was assessed by 24-h time-kill studies. Beta-lactams reduced vancomycin MIC values against all strains (4- to 16-fold reduction). In time-kill studies against MRSA, CFZ, FEP, CPT, and NAF all demonstrated similar degrees of killing at 24 h, and all showed synergistic activity with vancomycin against VSSA, hVISA, and VISA. Each of these combinations was also superior to any single agent against isolates of all three phenotypes, and each was bactericidal (P < 0.001 for all comparisons). All single-agent exposures demonstrated no activity at 24 h. The combination of vancomycin and beta-lactams significantly improved antibacterial activity against VSSA, hVISA, and VISA strains compared to the activity of any agent alone, supporting the potential use of vancomycin-beta-lactam combination therapy in infections caused by MRSA. Further clinical research is warranted to investigate the synergy of vancomycin against these Staphylococcus strains.

Keywords: MRSA; Staphylococcus aureus; cefazolin; cefepime; ceftaroline; combination therapy; in vitro; infection; nafcillin; time kill; vancomycin.

FIG 1

(a to c) Vancomycin (VAN) MIC values in the presence of several beta-lactam agents against VSSA strains R7534, R5371, and R6033. No combo, vancomycin MIC values in Mueller-Hinton broth without the presence of beta-lactams. (d to f) Twenty-four-hour time-kill curves against strains R7534, R5371, and R6033. CPT, ceftaroline; FEP, cefepime; CFZ, cefazolin; NAF, nafcillin. GC, drug-free growth control. Vancomycin was used at 0.5x the MIC in each experiment. Concentrations of CFZ, CPT, FEP, and NAF were performed at 0.5x the MIC or the biological free peak concentration, whichever was lower. Color identification: maroon solid line, growth control; black solid line, VAN; pink solid line, CFZ; pink dashed line, VAN+CFZ; red solid line, CPT; red dashed line, VAN+CPT; blue solid line, FEP; blue dashed line, VAN+FEP; green solid line, NAF; green dashed line, VAN+NAF.

FIG 2

(a to c) Vancomycin (VAN) MIC values in the presence of several beta-lactam agents against hVISA strains R5450, R5483, and R6911. No combo, vancomycin MIC values in Mueller-Hinton broth without the presence of beta-lactams. (d to f) Twenty-four-hour time-kill curves against strains R5450, R5483, and R6911. CPT, ceftaroline; FEP, cefepime; CFZ, cefazolin; NAF, nafcillin. GC, drug-free growth control. Vancomycin was used at 0.5x the MIC in each experiment. Concentrations of CFZ, CPT, FEP, and NAF were performed at 0.5x the MIC or the biologic free peak, whichever was lower. Color identification: maroon solid line, growth control; black solid line, VAN; pink solid line, CFZ; pink dashed line, VAN+CFZ; red solid line, CPT; red dashed line, VAN+CPT; blue solid line, FEP; blue dashed line, VAN+FEP; green solid line, NAF; green dashed line, VAN+NAF.

FIG 3

(a to c) Vancomycin (VAN) MIC values in the presence of several beta-lactam agents against VISA strains R6496, R4404, and R6296. No combo, vancomycin MIC values in Mueller-Hinton broth without the presence of beta-lactams. (d to f) Twenty-four-hour time-kill curves against strains R6496, R4404, and R6296. Broken lines, single agents; continuous lines, combination regimens. CPT, ceftaroline; FEP, cefepime; CFZ, cefazolin; NAF, nafcillin. GC, drug-free growth control. Vancomycin was used at 0.5x the MIC in each experiment. Concentrations of CFZ, CPT, FEP, and NAF were performed at 0.5x the MIC or the biologic free peak, which ever was lower. Color identification: maroon solid line, growth control; black solid line, VAN; pink solid line, CFZ; pink dashed line, VAN+CFZ; red solid line, CPT; red dashed line, VAN+CPT; blue solid line, FEP; blue dashed line, VAN+FEP; green solid line, NAF; green dashed line, VAN+NAF.