In vitro activity of quinupristin/dalfopristin and other antibiotics against ampicillin-resistant enterococcus faecium.

BACKGROUND

Enterococcus faecium constitutes approximately 10% of clinical isolates of enterococci and is noted for its antimicrobial resistance. In particular, E faecium is commonly resistant to ampicillin. The optimal treatment for severe infections caused by these multi-resistant organisms has yet to be determined.

METHODS

Enterococci tested were isolated from blood, pleural fluid and cerebrospinal fluid. Ampicillin-resistant Enterococcus faecium (AREF) was identified using the API Rapid Strep Kit system. A total of 58 isolates of AREF were enrolled in this study. Ten different antibiotics were tested, including Synercid (quinupristin/dalfopristin), teicoplanin, vancomycin, ampicillin, trimethoprim/sulfamethoxazole (TMP/SMX), ciprofloxacin, gentamicin, chloramphenicol, rifampicin and tetracycline. The agar dilution method described by the National Committee for Clinical Laboratory Standards was used to determine the minimum inhibitory concentrations (MICs) of the antibiotics tested.

RESULTS

Teicoplanin showed the best in vitro activity. Its MIC ranged from 0.25 to 2 micrograms/ml with an MIC90 of 1 microgram/ml. The MIC of vancomycin was 0.5-128 micrograms/ml with an MIC90 of 2 micrograms/ml. Three strains were vancomycin resistant, and they were the VanB phenotype. The MIC of quinupristin/dalfopristin was 0.5 to 8 micrograms/ml with an MIC90 of 2 micrograms/ml. Chloramphenicol and tetracycline showed moderate susceptibility. AREF showed high resistance to other antibiotics tested, including ciprofloxacin, gentamicin, TMP/SMX and rifampicin. High-level gentamicin resistance (MIC > 1,000 micrograms/ml) was found in 78% of AREF tested.

CONCLUSIONS

Teicoplanin showed the best in vitro activity against AREF. Clinical studies are necessary to confirm the efficacy of quinupristin/dalfopristin in vivo.