Erythromycin is ineffective against Listeria monocytogenes in multidrug resistant cells

Multidrug resistance of tumor cells is a well-known phenomenon in oncology. Among the substances excluded from the cells are not only antineoplastic drugs but also certain antibiotics, e.g. erythromycin. To prove the hypothesis that this might render infections with intracellular bacteria untreatable with these antibiotics we used erythromycin to treat intracellular infection of multidrug resistant (MDR) cells with Listeria monocytogenes. Erythromycin was unable to restrict the growth of L. monocytogenes in KBV-1 MDR cells in concentrations of up to 25 micrograms/ml. In contrast, 0.049 micrograms/ml of erythromycin were sufficient to restrict the growth of the bacteria in nonresistant KB 3-1 cells. When verapamil was added to the supernatant of KBV-1 cells, erythromycin regained its effectivity on L. monocytogenes multiplying in these cells. The fact that MDR cells may render intracellular bacteria inaccessible to certain antibiotics might have important implications for the persistence of these bacteria in the host and for the treatment of patients with genetically engineered MDR cells.