Evaluation of Teicoplanin Resistance Detected by Automated System in Coagulase Negative Staphylococci: A Comparison with Gradient Test and Broth Microdilution Methods

Abstract

Upon the observation of an increase in teicoplanin resistance rates in coagulase negative staphylococci (CoNS) isolates determined by the automated system, we aimed to compare the automated system and gradient test methods with the gold standard broth microdilution method. In addition, the effect of standard antimicrobial susceptibility guidelines on teicoplanin susceptibility test results in CoNS was investigated. A total of 81 CoNS isolates, 52 resistant and 29 susceptible to teicoplanin determined by automated system (Phoenix, Becton Dickinson, USA), were tested. The minimum inhibitory concentration (MIC) values were determined by gradient test (M.I.C. Evaluators, OXOID, UK) and broth microdilution methods. Susceptibility categories were determined according to EUCAST and CLSI criteria and the results were compared. Among 29 isolates found to be susceptible by automated system, one isolate was found resistant by gradient and broth microdilution tests. Of the 52 resistant isolates determined by automated system, 12 (23%) were found to be resistant by gradient test and 22 (42.3%) were resistant by broth microdilution. According to CLSI criteria, no resistant isolates were detected by broth microdilution and six isolates were intermediately susceptible while, two isolates were detected to be resistant and five isolates were found to be intermediately susceptible by the gradient test. In conclusion, compared to microdilution, teicoplanin resistance was detected at a higher rate in CoNS isolates by the automated system used. On the other hand, the gradient test method which is frequently used for confirmation was not reliable in MIC values close to the EUCAST breakpoint values (4 μg/mL). In addition, lower resistance rates were observed when the CLSI breakpoints were used in gradient test and broth microdilution methods.