Multi-laboratory assessment of the linezolid spectrum of activity using the Kirby-Bauer disk diffusion method: Report of the Zyvox (R) Antimicrobial Potency Study (ZAPS) in the United States

The in vitro activity of linezolid against common Gram-positive pathogens w as compared to that of penicillin or ampicillin or oxacillin (depending upo n genus), cefazolin, erythromycin, clindamycin, quinupristin/dalfopristin, levofloxacin, nitrofurantoin and vancomycin by disk diffusion methods. One hundred and six centers (31 states in US) tested recent clinical isolates o f Staphylococcus aureus coagulase-negative staphylococci, Enterococcus faec ium, E. faecalis, Streptococcus pneumoniae, and other streptococci. Testing was conducted using the standardized disk diffusion method and concurrent quality control testing was performed. Strains with linezolid zone diameter s of less than or equal to 20 mm were requested for referral to the microbi ology monitor for confirmation. A total of 3,100 isolates (97% compliance) were tested. Susceptibility (zone diameters, greater than or equal to 21 mm ) of staphylococci and streptococci to linezolid was reported in 100% and 9 9.4% of staphylococci and streptococci, respectively. Susceptibility (zone diameters, greater than or equal to 23 mm) of enterococci to linezolid was 96.0% with only three isolates (0.4%) reported as resistant (zone diameters , less than or equal to 20 mm; unconfirmed). Among a total of nine isolates (0.3%) reported to have zone diameters 20 mm, six were not submitted for f urther testing, two were contaminated with Gram-negative bacilli and one wa s determined to be linezolid-susceptible. There were no differences in line zolid susceptibility in the vancomycin- or oxacillin- or penicillin-resista nt subsets of strains. This susceptibility pattern for US medical centers i s indicative of the excellent and nearly complete in vitro activity against the key Gram-positive pathogens for which linezolid has received US Food a nd Drug Administration indications for clinical use. (C) 2001 Elsevier Scie nce Inc. All rights reserved.