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Antimicrobial Agents and Chemotherapy, November 2000, p. 3055-3060, Vol. 44, No. 11
Groupe de Recherche sur les Antimicrobiens et
les Micro-organismes (GRAM, EA 2656), 76031 Rouen, France
Received 2 March 2000/Returned for modification 1 July
2000/Accepted 24 August 2000
Several studies have previously reported synergistic effects
between vancomycin and a given
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
In Vitro Synergistic Effects of Double and Triple
Combinations of
-Lactams, Vancomycin, and Netilmicin against
Methicillin-Resistant Staphylococcus aureus
Strains
-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 cefazolin-vancomycin 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.
*
Corresponding author. Mailing address: Laboratoire de
Bactériologie, Centre Hospitalier Universitaire Charles Nicolle,
76031 Rouen Cedex, France. Phone: 33 2 32 88 80 52. Fax: 33 2 32 88 80 24. E-mail: Martine.Pestel-Caron{at}chu-rouen.fr.
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