FIG. 1.

Typical time course of yheI biomarker induction by linezolid between 1 and 4 h and concentration-induction patterns with different translation inhibitors. (A) Linezolid induces the biosensor cells at the earliest after a 3-h exposure time, leading to a maximum signal of 1,300%. (B) The concentration-induction pattern reveals that linezolid, doxycycline, and lead to an approximately 10-fold signal induction, while fusidic acid doubles the signal after the optimal assay incubation time of 4 h. Using 200% signal induction as a significance cutoff, low concentrations of antibiotics (0.02 μM doxycycline, 0.1 μM linezolid, 0.6 μM fusidic acid, and 0.3 μM chloramphenicol) can be detected. These concentrations are 1 to 2 orders of magnitude below the corresponding MICs (MICs, 3.13 μM for linezolid, fusidic acid, and doxycycline; 12.4 μM for chloramphenicol). The fourfold-increased MIC of chloramphenicol is reflected by the maximal signal-inducing concentration of 5 μM in comparison to the other reference antibiotics, which reach the induction maximum around 1 μM. The growth-inhibitory effects of the antibiotics at and above their MICs lead to reduced numbers of viable cells and therefore to a reduction of the signal intensities.