The in-vitro activity of linezolid (U-100766) and tentative breakpoints

The in-vitro activity of linezolid, a novel oxazolidinone, was investigated in comparison with those of amoxycillin, cefuroxime, quinupristin/dalfopristin, trovafloxacin and vancomycin against 420 recent Gram-positive and anaerobic clinical isolates. Linezolid was equally active (MIC90 1 mg/L) against methicillin-susceptible and -resistant Staphylococcus aureus. It demonstrated uniform activity against streptococci and enterococci and no cross-resistance with other agents. The time-kill kinetic data demonstrated that the in-vitro activity of linezolid was predominantly bacteriostatic; slow bactericidal activity was only observed at the higher concentration with streptococci. An increase in inoculum from 10(4) to 10(6) cfu on selected strains had little effect on the MICs (MIC90 within one dilution step) of linezolid and an increase in inoculum from 10(5) to 10(7) cfu/mL had no notable effect on the in-vitro bactericidal activity. A tentative linezolid breakpoint of 2 mg/L was chosen after analysis of distribution of susceptibilities.