In vitro activity of tigecycline (GAR-936), a novel glycylcycline, against vancomycin-resistant enterococci and staphylococci with diminished susceptibility to glycopeptides.

Abstract

Sir, Increasing antimicrobial resistance among clinical isolates of staphylococci and enterococci has complicated the treatment of infections caused by these microorganisms, and there is a need for therapeutic alternatives against them. The glycylcyclines are novel tetracycline analogues with activity against Gram-positive and Gram-negative aerobic and anaerobic bacteria, including tetracycline-resistant isolates. The glycylcycline tigecycline (GAR-936) is active against a variety of microorganisms;1–3 however, information concerning its activity against glycopeptide-resistant enterococci is very scarce, and there is none about its activity against staphylococci with diminished susceptibility to glycopeptides.2,4,5 In this study, we compared the in vitro activity of tigecycline with that of tetracycline, vancomycin and teicoplanin against 157 unrelated, non-duplicate clinical isolates obtained in our microbiology laboratory during 1998–2001. The isolates included 97 vancomycin-resistant enterococci (25 Enterococcus faecalis, 41 Enterococcus faecium, 21 Enterococcus casseliflavus, and 10 Enterococcus gallinarum), 54 coagulase-negative staphylococci and six Staphylococcus aureus with diminished susceptibility to glycopeptides. Among the enterococci, 28 isolates presented the VanA phenotype, 38 the VanB and 31 the VanC. Tigecycline (GAR-936) was provided by Wyeth-Ayerst Research Laboratories (Saint Davids, PA, USA), and vancomycin, teicoplanin and tetracycline by their respective manufacturers. Antimicrobials were tested at two-fold concentrations (tigecycline: 0.03–32 mg/L, vancomycin and teicoplanin: 0.5–512 mg/L and tetracycline: 0.5– 8 mg/L). Susceptibility studies were performed by the broth microdilution method with cation-adjusted Mueller–Hinton broth (Oxoid, Unipath Ltd, Basingstoke, UK). The recommendations of the NCCLS were followed.6 S. aureus ATCC 29213 and E. faecalis ATCC 29212 were used as control strains. Isolates were stored at –70°C, with three subcultures being made before the organisms were tested. Following inoculation (final inoculum concentration, c. 5 × 105 cfu/mL), MIC trays were incubated at 35°C in ambient air for 24 h before examination. MBCs of tigecycline were defined as a 99.9% reduction of the initial inoculum, and were determined by subculturing 0.1 mL (all volume) of every MIC test onto blood agar plates. Colonies were enumerated after 24 h of incubation at 35°C in ambient air. Tigecycline inhibited all isolates tested (157) at concentrations between ≤0.03 and 1 mg/L (Table 1). The MIC90 of tigecycline against all isolates was 0.5 mg/L, and all were inhibited at ≤1 mg/L, including tetracycline-resistant isolates. The MIC90 of tigecycline against all species of enterococci was 0.12 mg/L; it was 0.5 mg/L against coagulase-negative staphylococci, and all S. aureus tested were inhibited with 1 mg/L. Fifty-five enterococci were tetracyclineresistant (MIC > 8 mg/L), and all were inhibited with tigecycline 0.5 mg/L. Eight coagulase-negative staphylococci and four isolates of S. aureus were resistant to tetracycline, and all were inhibited with tigecycline 1 mg/L. Tigecycline was equally active against the VanA, VanB and VanC phenotypes of glycopeptide resistance of enterococci. The MICs (mg/L) of tigecycline for control organisms were as follows: for S. aureus ATCC 29213 (n = 10), 0.12 (n = 8) and 0.25 (n = 2); for E. faecalis ATCC 29212 (n = 10), 0.06 (n = 7) and ≤0.03 mg/L (n = 3). Tigecycline exhibited no bactericidal activity against any of the isolates tested. In all cases, the MBC90 was higher than 32 mg/L. Only three isolates of enterococci were killed with tigecycline 4, 8 and 16 mg/L, respectively, and one isolate of coagulasenegative staphylococci was killed with 16 mg/L. Tigecycline inhibits protein synthesis, including isolates resistant to tetracycline by either ribosomal protection or active efflux.1 In this study, tigecycline was uniformly active against clinical isolates resistant or susceptible to tetracycline, and exhibited high activity against isolates of vancomycin-resistant enterococci and methicillinresistant staphylococci with diminished susceptibility to glycopeptides. These results confirm the data of other authors concerning the activity of tigecycline against vancomycin-resistant enterococci and methicillin-resistant staphylococci,1,2,4 and the results are extended to include many isolates of coagulase-negative staphylococci with diminished susceptibility to glycopeptides. Fifty-one isolates of coagulase-negative staphylococci (94%) were also resistant to β-lactams, macrolides, quinolones and aminoglycosides, and eight were susceptible only to vancomycin 2–4 mg/L. Since many of these staphylococci (37%) were collected from blood and intravenous catheters, their emergence is a therapeutic problem. In this study, all coagulase-negative staphylococci and S. aureus were inhibited by tigecycline 1 mg/L, which suggests this antimicrobial is potentially therapeutic. The activity of tigecycline against vancomycin-resistant enterococci was independent of the phenotype of resistance, and all vancomycin-resistant isolates were inhibited at concentrations between ≤0.03–0.5 mg/L. Although tigecycline, like tetracyclines and other derivatives of tetracycline, is bacteriostatic, we determined the MBCs of this compound against all 157 isolates, and found an absence of bactericidal activity. To our knowledge, no other study has evaluated such bactericidal activity against staphylococci and enterococci. In summary, our results indicate that tigecycline is very active against glycopeptide-resistant enterococci, and staphylococci with diminished susceptibility to glycopeptides; it has potential for therapeutic application.