In vitro susceptibility of TB- and MDR-TB-strains against linezolid at a local center in Berlin/Germany

Background Due to migration the spectrum of Mycobacterium tuberculosis (Mtb) in patients with active tuberculosis treated at the largest TB center in Berlin, Germany, changed in the last decades, with an increasing prevalence of multidrug resistance tuberculosis (MDR-Tb). In this context, drug susceptibility testing has a high impact for treatment decisions. Aims The aim of our study was to evaluate the in vitro susceptibility of linezolid assessed by minimal inhibitory concentrations (MICs) against Tb- and MDR-Tb-strains isolated from patients treated in our center. Methods From 148 Mtb-cultures (18 MDR-Tb) from 2002 to 2012 we retrospectively evaluated the sensitivity and MIC of linezolid. Testing for MIC was performed on solid Middlebrook-7H10 and -7H11 agar plates. Results The MICs for 18 MDR-Tb-strains were in the range of 0,25 – 0,5 µg/ml and for 130 Non-MDR-Tb between 0,12 and 0,5 µg/ml (4 strains with MIC= 0,12 µg/ml, 83 with 0,25 µg/ml and 5 with 0,5 µg/ml, respectively). During the time period no change in susceptibility could be observed neither for the tested MDR- nor the Non-MDR-Tb-strains. Conclusions Our study shows a high sensitivity rate of linezolid against the tested local Mtb-strains, and with low MICs. No major difference for MICs could be shown between the strains of MDR and Non-MDR-Tb.