Discovery of small molecules that sensitize Salmonella to polymyxin antibiotics

Abstract

Salmonella enterica, a major bacterial cause of food‐borne illness, can survive in harsh environments due to its ability to sense and respond to toxic conditions. Salmonella senses a threatening environment, such as antibacterial conditions produced by the body’s immune system, and then activates the expression of genes necessary to survive within that environment. These genes are activated through the use of two component systems. Two component systems consist of an inner membrane‐bound histidine kinase and a response regulator protein located in the cytoplasm of the cell. The PhoP/PhoQ two component system is critical for the virulence of Salmonella and for Salmonella to resist antibiotics such as polymyxin B. Therefore, we tested commercially available small molecules capable of inhibiting the sensory histidine kinase to see if they were capable of sensitizing Salmonella to polymyxin B. Indeed, a few small molecules were successful at sensitizing Salmonella to polymyxin B. To determine how these molecules decrease polymyxin B resistance within Salmonella, we carried out several mechanistic studies. First, we tested whether these small molecules influenced Salmonella’s outer membrane permeability. This was tested by combining small molecules with vancomycin. Vancomycin is an antibiotic used effectively on gram‐positive bacteria that only contain a single membrane, but gram‐negative bacteria are intrinsically resistant to vancomycin due to their outer membrane. We found that vancomycin did not impact Salmonella’s resistance to polymyxin B. These results suggest that the small molecules do not affect outer membrane permeability. We also tested whether these molecules affect expression of PhoP‐activated genes. These molecules have been seen to decrease expression of a PhoP activated gene when transcriptional reporters were used. Using reverse transcription quantitative polymerase chain reaction (RT‐qPCR), we found that the small molecules decreased expression of some, but not all, PhoP‐activated genes. Overall, our results demonstrate that small molecules can target Salmonella’s ability to regulate gene expression and resist toxic conditions. This discovery represents a promising new way to develop antibiotic drugs to treat bacterial infections.Support or Funding InformationThis research was supported by the following funding: UWL Undergraduate Research and Creativity Grants (MKT), UWL College of Science and Health Dean’s Distinguished Fellowship (MKT), Research Corporation for Science Advancement, Cottrell Teacher Scholar Ambassadors for PUI – R1 Partnerships Award (EEC and JFM)