Drug resistant pathogens are on the rise, and new treatments are needed for bacterial infections. Efforts toward antimicrobial discovery typically identify compounds that prevent bacterial growth in microbiological media. However, the microenvironments to which pathogens are exposed during infection differ from rich media and alter the biology of the pathogen. We and others have therefore developed screening platforms that identify compounds that disrupt pathogen growth within cultured mammalian cells. Our platform focuses on Gram-negative bacterial pathogens, which are of particular clinical concern. We screened a panel of 707 drugs to identify those with efficacy against Salmonella enterica Typhimurium growth within macrophages. One of the drugs identified, clofazimine (CFZ), is an antibiotic used to treat mycobacterial infections that is not recognized for potency against Gram-negative bacteria. We demonstrated that in macrophages CFZ enabled the killing of S. Typhimurium at single digit micromolar concentrations, and in mice, CFZ reduced tissue colonization. We confirmed that CFZ does not inhibit the growth of S. Typhimurium and E. coli in standard microbiological media. However, CFZ prevents bacterial replication under conditions consistent with the microenvironment of macrophage phagosomes, in which S. Typhimurium resides during infection: low pH, low magnesium and phosphate, and the presence of certain cationic antimicrobial peptides. These observations suggest that in macrophages and mice the efficacy of CFZ against S. Typhimurium is facilitated by multiple aspects of soluble innate immunity. Thus, systematic screens of existing drugs for infection-based potency are likely to identify unexpected opportunities for repurposing drugs to treat difficult pathogens.
Keywords: Gram-negative bacteria; Salmonella enterica serovar Typhimurium; antibiotics; clofazimine (CFZ); macrophage.