Clinical concentrations of thioridazine enhance the killing of intracellular methicillin-resistant Staphylococcus aureus: an in vivo, ex vivo and electron microscopy study

Chlorpromazine (CPZ) is concentrated by human macrophages where it kills intracellular mycobacteria when the concentration outside the macrophage is sub-clinical. We have previously demonstrated that thioridazine (TZ), a much milder phenothiazine, has similar activity and kills intracellular methicillin-susceptible S. aureus at sub-clinical concentrations. We have extended this latter study to include methicillin-resistant S. aureus (MRSA) and show that TZ kills intracellular MRSA at clinically relevant concentrations. The ultrastructure of MRSA exposed to in vitro concentrations of TZ just below its MIC and that of MRSA phagocytosed by macrophages previously exposed to a clinically relevant concentration of TZ was also studied. TZ inhibits the replication of phagocytosed MRSA, affecting the structure of the cell envelope, resulting in lysis of the bacterium 6 hours post-phagocytosis. These ultrastructural changes are identical to those produced in vitro by a TZ concentration that is just below the MIC. Because macrophage intracellular MRSA is not killed by the macrophage and its intracellular location protects it from antibiotics that are unable to reach that site, recurrent infections which result may be successfully managed with the use of TZ.