Aims: The present study was undertaken to validate, for antibiotic discovery, a reporter gene assay based on a Bacillus subtilis strain expressing the Enterococcus faecium vanRS genes and a vanH-lacZ fusion, which produced beta-galactosidase activity in the presence of cell wall inhibitors (CWI) and lysozyme. Methods and Results: The reporter assay was miniaturized, automated and validated with antibiotics and tested against portions of chemical and microbial extract libraries. The assay is simple, fast and reproducible and can detect all CWI, sometimes at concentrations lower than those necessary to inhibit bacterial growth. However, some membrane-interfering compounds also generate comparable signals. While most CWI elicit a signal that is transcription-dependent and abolished in an osmoprotective medium, transcription is not required for beta-galactosidase activity brought about by the membrane-interfering compounds. Conclusions: At least two distinct mechanisms appear to lead to enzymatic activity in the reporter strain. Effective counterscreens can be designed to discard the undesired classes of compounds. Significance and Impact of the Study: Extensive validation is required before introducing a reporter assay in high-throughput screening. However, the ease of operation and manipulation makes the reporter assays powerful tools for antibiotic discovery.