Enterococcus faecium: In vitro Activity of Antimicrobial Drugs, Singly and Combined, with and without Defibrinated Human Blood, against Multiple-Antibiotic-Resistant Strains

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Abstract
The minimal inhibitory (MICs) and bactericidal concentrations of 14 antimicrobial drugs were determined against 17 clinical isolates of Enterococcus faecium, including 4 glycopep-tide-resistant strains. Both teicoplanin and vancomycin lacked bactericidal activity against all 13 susceptible isolates. Time-kill experiments served to test various antibiotic combinations chiefly against glycopeptide-resistant strains in Mueller-Hin-ton broth (MHB) and in MHB supplemented with 65% (v/v) fresh defibrinated human blood. Co-trimoxazole, fusidic acid, and novobiocin yielded bacteriostatic effects. Rifampin was bactericidally active against rifampin-susceptible strains (MICs = 0.125 μg/ml), but less so against low-level-rifampin-resistant (MICs = 2-8 μg/ml) strains in MHB. However, in the presence of human blood, rifampin (2 μg/ml) combined with co-trimoxazole (0.25/4.75 μg/ml) killed rifampin-susceptible and low-level-rifampin-resistant, but not moderate-level-ri-fampin-resistant (MICs = 16-32 ug/ml) strains of E. faecium. Of two topical drugs examined, mupirocin merely inhibited strains of E. faecium; conversely, taurolidine at 2,000 μg/ml was efficacious against all strains examined, although the kinetics of bactericidal activity were retarded somewhat in the presence of 65 vol% human blood.

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