Abstract
Overall, 141 centers in North America enrolled in this international surveillance study designed to evaluate the in vitro antimicrobial activity and spectrum of linezolid, a new oxazolidinone. Each participant tested the susceptibility of clinical isolates of staphylococcal species ( n = 85) against 12 drugs, and enterococcal species ( n = 40) against 6 drugs using reference broth microdilution trays; and of streptococcal species ( n = 25) against 6 drugs using Etests (AB BIODISK, Solna, Sweden). Quality control testing was conducted using recommended strains, and verification of resistance to linezolid and select other agents was performed by a regional monitor. Of the 20,161 isolates collected from sites across the United States (US; n = 132) and Canada ( n = 9), 18,307 were included in this analysis. Oxacillin resistance occurred in 38.7 and 70.6% of Staphylococcus aureus and coagulase-negative staphylococcal (CoNS) isolates, respectively. Vancomycin resistance was reported in 65.9 and 2.6% of Enterococcus faecium and E. faecalis, respectively. Penicillin resistance occurred in 37.2% of Streptococcus pneumoniae, 17.5% constituting high-level resistance (MIC, ≥2 μg/ml). The MIC 90 for linezolid was 1 μg/ml for streptococci, 2 μg/ml for enterococci and CoNS isolates, and 4 μg/ml for S. aureus. Using the US FDA-recommended susceptible breakpoints for linezolid, there were no confirmed reports of linezolid resistance (i.e., MIC ≥8 μg/ml). The occurrence of linezolid MICs was unimodal and generally varied between, 1–4 μg/ml for staphylococci (94% of recorded results), 1–2 μg/ml for enterococci (93%), and 0.5–1 μg/ml for streptococci (85%). Susceptibility to linezolid was not influenced by susceptibility to other antiicrobials such as vancomycin, β-lactams or macrolides. Only linezolid was universally active against essentially all tested Gram-positive specimens. The unimodal susceptibility pattern is indicative of excellent and near complete activity against key Gram-positive pathogens including multiply resistant strains, but surveillance for emerging resistances (rare) and the performance of routine susceptibility tests to guide patient therapy seems prudent.
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