Tartrolon sensing and detoxification by the Listeria monocytogenes timABR resistance operon

AO_SCPLOWBSTRACTC_SCPLOWListeria monocytogenes is a foodborne bacterium that naturally occurs in the soil. Originating from there, it contaminates crops and infects farm animals and their consumption by humans may lead to listeriosis, a systemic life-threatening infectious disease. The adaptation of L. monocytogenes to such contrastive habitats is reflected by the presence of virulence genes for host infection and other genes for survival under environmental conditions. Among the latter are ABC transporters for excretion of antibiotics produced by environmental competitors, however, most of these transporters have not been characterized. Here, we generated a collection of promoter-lacZ fusions for genes encoding ABC-type drug transporters of L. monocytogenes and screened this reporter strain collection for induction using a library of natural compounds produced by various environmental microorganisms. We found that the timABR locus (lmo1964-lmo1962) was induced by the macrodiolide antibiotic tartrolon B, which is synthesized by the soil myxobacterium Sorangium cellulosum. Tartrolon B resistance of L. monocytogenes was dependent on timAB, encoding the ATPase and the permease component of a novel ABC transporter. Moreover, transplantation of timAB was sufficient to confer tartrolon B resistance to Bacillus subtilis. Expression of the timABR locus was found to be auto-repressed by the TimR repressor, whose repressing activity was lost in the presence of tartrolon B. We also demonstrate that tartrolon sensitivity was suppressed by high external potassium concentrations, suggesting that tartrolon acts as potassium ionophore. Our results help to map the ecological interactions of an important human pathogen with its co-residing species within their joint natural reservoir. IO_SCPLOWMPORTANCEC_SCPLOWListeria monocytogenes is an environmental bacterium that can cause listeriosis in humans when ingested. The genome of this bacterium contains many genes encoding putative ATP binding cassette transporters with unknown function. Presumably, these transporters serve the excretion of antimicrobial compounds produced by co-residing species competing with L. monocytogenes for nutrients and habitats. We here fused the lacZ reporter gene to the promoters of these transporter genes and screened a natural compound library for activating substances. We discovered that tartrolon B activates expression of the L. monocytogenes timABR operon, encoding the TimAB exporter and the TimR repressor. Tartrolon B is an antibiotic produced by the soil-dwelling myxobacterium Sorangium cellulosum. We elucidate how the timABR genes mediate sensing and detoxification of this antibiotic. This represents the first known mechanism of tartrolon resistance and may help to better define the natural reservoir of L. monocytogenes.