Accuracy of Six Antimicrobial Susceptibility Methods for Testing Linezolid against Staphylococci and Enterococci

Abstract

A challenge panel of enterococci (n ‫؍‬ 50) and staphylococci (n ‫؍‬ 50), including 17 and 15 isolates that were nonsusceptible to linezolid, respectively, were tested with the Clinical and Laboratory Standards Institute broth microdilution and disk diffusion reference methods. In addition, all 100 isolates were tested in parallel by Etest (AB Biodisk, Solna, Sweden), MicroScan WalkAway (Dade, West Sacramento, CA), BD Phoenix (BD Diagnostic Systems, Sparks, MD), VITEK (bioMérieux, Durham, NC), and VITEK 2 (bioMérieux) by using the manufacturers' protocols. Compared to the results of the broth microdilution method for detecting linezolidnonsusceptible staphylococci and enterococci, MicroScan results showed the highest category agreement (96.0%). The overall categorical agreement levels for VITEK 2, Etest, Phoenix, disk diffusion, and VITEK were 93.0%, 90.0%, 89.6%, 88.0%, and 85.9%, respectively. The essential agreement levels (results within ؎1 doubling dilution of the MIC determined by the reference method) for MicroScan, Phoenix, VITEK 2, Etest, and VITEK were 99.0%, 95.8%, 92.0%, 92.0%, and 85.9%, respectively. The very major error rates for staphylococci were the highest for VITEK (35.7%), Etest (40.0%), and disk diffusion (53.3%), although the total number of resistant isolates tested was small. The very major error rate for enterococci with VITEK was 20.0%. Three systems (MicroScan, VITEK, and VITEK 2) provided no interpretations of nonsusceptible results for staphylococci. These data, from a challenge panel of isolates, illustrate that the recent emergence of linezolidnonsusceptible staphylococci and enterococci is providing a challenge for many susceptibility testing systems.

Investigation of Linezolid Resistance in Staphylococci and Enterococci

“…They compared seven methods, the Vitek, Vitek2, Microscan WalkAway, BD Phoenix (BD Diagnostic Systems, Sparks, MD), Etest, disk diffusion, and broth microdilution (16). As in our study, they found that there was poor categorical agreement between methods.…”

“…Linezolid is a bacteriostatic antibiotic and as such can generate susceptibility patterns that can be challenging to interpret (16,17). Etest, for example, requires that 90% inhibition be considered when determining the MIC, while disk diffusion requires the use of transmitted light.…”

Limited detectability of linezolid-resistant Staphylococcus aureus by the Etest method and its improvement using enriched media

“…Although hazy colonies or microcolonies were not observed on MH agar by the agar dilution method, these appeared on BHI agar in some strains. Previous studies have reported the following: (i) an MRSA strain with a cfr mutation with a linezolid MIC of 8 mg l 21 obtained by the agar dilution method showed a faint halo of growth with the Etest method and the MIC was difficult to read (Arias et al, 2008); (ii) the MICs of linezolid by Etest in linezolid-susceptible clinical isolates appeared to be one to two dilution values lower than those obtained by the microdilution method (Tubau et al, 2001); and (iii) the MIC reading by the disc diffusion and Etest methods in staphylococci needs to be improved (Tenover et al, 2007). The results reported in these studies are consistent with the present results, although a solution for these difficulties remains to be determined.…”

Challenges in predicting Staphylococcus spp. -lactamic resistance in pet animals

“…This was also true for the antibiotics with restricted use due to high cost, such as imipenen (2%) and linezolide (7%) ( Table 2). Tenover et al (2007) comparing the most commonly used susceptibility testing methods challenged with linezolid-non susceptible staphylococci, concluded that generally the problem was much greater in the non-detection of resistance rather than a possible overcalling of resistance. For this study, the criterium adopted to report linezolide resistance followed CLSI standards that consider any discernible growth within the zone of inhibition as indicative of resistance to linezolide.…”