The ABC-Type Multidrug Resistance Transporter LmrCD Is Responsible for an Extrusion-Based Mechanism of Bile Acid Resistance in Lactococcus lactis

Upon prolonged exposure to cholate and other toxic compounds, Lactococcus lactis develops a multidrug resistance phenotype that has been attributed to an elevated expression of the heterodimeric ABC-type multidrug transporter LmrCD. To investigate the molecular basis of bile acid resistance in L. lactis and to evaluate the contribution of efflux-based mechanisms in this process, the drug-sensitive L. lactis NZ9000 Delta lmrCD strain was challenged with cholate. A resistant strain was obtained that, compared to the parental strain, showed (i) significantly improved resistance toward several bile acids but not to drugs, (ii) morphological changes, and (iii) an altered susceptibility to antimicrobial peptides. Transcriptome and transport analyses suggest that the acquired resistance is unrelated to elevated transport activity but, instead, results from a multitude of stress responses, changes to the cell envelope, and metabolic changes. In contrast, wild-type cells induce the expression of lmrCD upon exposure to cholate, whereupon the cholate is actively extruded from the cells. Together, these data suggest a central role for an efflux-based mechanism in bile acid resistance and implicate LmrCD as the main system responsible in L. lactis.