2.1. Bacterial Strains, Reagents, and Antibiotic Susceptibility Tests

Rhodococcus equi 103S⁺ was kindly provided by Dr. Jesús Navas from the University of Cantabria (Spain), which was considered the wild type strain in all assays. R. equi Δmrx1Δmrx2Δmrx3 and R. equi Δmrx1Δmrx2Δmrx3 expressing mrx1-roGFP2 are derivative strains of R. equi 103S⁺ [17]. All reagents were procured from Sigma-Aldrich, unless otherwise stated.

For antibiotic susceptibility tests, exponential growth phase cultures (OD₆₀₀ = 1) of R. equi were diluted in melted (at 50 °C) trypticase soy agar (TSA) plates at 1:10 mL dilution and spread over 10 mL solid TSA plates. Disks with antibiotics were then placed onto R. equi-containing TSA plates, which were incubated at 30 °C for 24 h.