In Vitro Synergistic Effects of Double and Triple Combinations of β-Lactams, Vancomycin, and Netilmicin against Methicillin-Resistant Staphylococcus aureus Strains

Abstract

Several studies have previously reported synergistic effects between vancomycin and a given ␤-lactam or a given aminoglycoside against methicillin-resistant Staphylococcus aureus (MRSA) strains. The aim of our study was to exhaustively compare the effects of different combinations of a ␤-lactam, vancomycin, and/or an aminoglycoside against 32 clinical MRSA strains with different aminoglycoside susceptibility patterns. The effects of 26 different ␤-lactam-vancomycin and 8 different aminoglycoside-vancomycin combinations were first studied using a disk diffusion screening method. The best interactions with vancomycin were observed with either imipenem, cefazolin, or netilmicin. By checkerboard studies, imipenem-vancomycin and cefazolinvancomycin each provided a synergistic bacteriostatic effect against 22 strains; the mean fractional inhibitory concentration (FIC) indexes were 0.35 and 0.46 for imipenem-vancomycin and cefazolin-vancomycin, respectively. The vancomycin-netilmicin combination provided an indifferent effect against all of the 32 strains tested; the mean of FIC index was 1.096. The mean concentrations of imipenem, cefazolin, netilmicin, and vancomycin at which FIC indexes were calculated were clinically achievable. Killing experiments were then performed using imipenem, cefazolin, netilmicin, and vancomycin at one-half of the MIC, alone and in different combinations, against 10 strains. The vancomycin-netilmicin regimen was rarely bactericidal, even against strains susceptible to netilmicin. The imipenem-vancomycin and cefazolin-vancomycin combinations were strongly bactericidal against six and five strains, respectively. The addition of netilmicin markedly enhanced the killing activity of the combination of cefazolin or imipenem plus vancomycin, but only for the MRSA strains against which the ␤-lactam-vancomycin combinations had no bactericidal effect. It is noteworthy that the latter strains were both susceptible to netilmicin and heterogeneously resistant to methicillin.

Evaluation of the synergistic potential of vancomycin combined with other antimicrobial agents against methicillin-resistant Staphylococcus aureus and coagulase-negative Staphylococcus spp strains

“…In both trials, the combination of 1/2 MIC of VAN + 1/2 MIC of IPM was shown to be synergistic, with an average reduction of 5 log CFU/mL for all four selected strains in relation to VAN alone after 24 h of incubation. These results are in agreement with other studies which showed the bactericidal activity of this combination after 24 h of incubation with MRSA strains, using the time-kill curve method (Totsuka et al 1999, Rochon-Edouard et al 2000, Jacqueline et al 2005. There was no significant decrease in log CFU after antimicrobial reapplication, indicating a non-concentration-dependent activity previously described as a pharmacological propriety of β-lactam drugs (Brunton et al 2006).…”

“…Due to a lack of therapeutic options, some studies have focused on the combination of two or more antibiotics as an alternative treatment (Totsuka et al 1999, Rochon-Edouard et al 2000, Kobayashi 2005, Yamaoka 2007, Nguyen & Graber 2010. Promising results in vitro and in vivo could lead to effective therapy along with a reduction of therapeutic doses, adverse effects and treatment duration.…”

“…In our study, MRSA strains with MIC ≥ 32 μg/mL for IPM when combined with 1/2 MIC of VAN became susceptible to IPM with MIC values ≤ 0.4 μg/mL (Table II). Similarly, Rochon-Edouard et al (2000) observed synergistic bactericidal effects of the VAN + IPM combination in sub-inhibitory concentrations using the checkerboard technique in 69% of the MRSA strains they tested.…”

Synergy of β-Lactams with Vancomycin against Methicillin-Resistant Staphylococcus aureus: Correlation of Disk Diffusion and Checkerboard Methods

“…This concept is based on several in vitro and animal studies that demonstrated a synergistic effect of vancomycin-␤-lactam combinations against MRSA (6)(7)(8)(9)(10)(11).…”

Antibiotics Against Endocarditis – Past, Present and Future (Experimental Data)

“…Penicillin-binding protein 2A (PBP2A) is the protein responsible for the methicillin resistance mechanism in S. aureus. Methicillin resistance confers resistance to all the beta-lactams, including cephalosporins and carbapenems; however, many studies have reported a potent efficacy of imipenem against S. aureus when used in combination with other antimicrobial agents, including fosfomycin (Nakazawa et al, 2003), vancomycin (Totsuka K et al, 1999;Benquan et al, 2002;Rochon-Edouard et al, 2000), and cephalosporins (Uete & Matsuo et al, 1995).…”