Chemical genetics and immunomodulation in Salmonella enterica serovar typhimurium infection

Abstract

Salmonella sp. are versatile bacteria that survive and replicate in a plethora of environmental niches and hosts, including humans. The illnesses caused by various serovars in humans range from mild gastroenteritis to severe systemic disease such as typhoid fever that still cause an important number of casualties especially in developing countries. Salmonella enterica serovar Typhimurium is the research model for typhoid fever and has provided many valuable insights into the molecular pathogenesis of salmonellosis. Its ability to cause disease relies on expression of specific virulence factors, often coded for by distinct genomic entities termed “Salmonella pathogenicity islands” (SPIs). The invasion of the intestinal lining is mediated by SPI1, while SPI2 enables the bacteria to survive, replicate and disseminate in the host inside phagocytes. Both SPI1 and SPI2 code for two distinct type three secretion systems (T3SSs), which translocate into the host cell bacterial effectors that hijack host cells functions.

Resistance to first line antibiotics has been reported in attempts to treat salmonellosis, even extended spectrum β–lactamase resistance. Hence, there is a need for new treatment alternatives. We report in Paper I a new antimicrobial property of the proton pump inhibitor omeprazole, which interfered with SPI2-mediated intracellular replication of S. Typhimurium in RAW264.7 murine macrophages. Its effect was bacteriostatic and manifested if the compound was applied at early stages of infection. Along with interference with bacterial virulence, omeprazole also posed a strong anti-inflammatory effect on macrophages. In Paper II we further characterized the immunomodulatory effect of omeprazole, alone or in combination with INP0010, another small molecular virulence inhibitor previously shown to inhibit the activity of SPI1 and SPI2 T3SSs. The two compounds had either antagonistic or synergistic effects on bacterial virulence and on host cell responses such as expression of inducible nitric oxide synthase (iNOS), nitric oxide (NO), pro-inflammatory cytokines and reactive oxygen species (ROS), depending on the dose and the model of infection used. Notably, in contrast to omeprazole, INP0010 enhanced the inflammatory responses of macrophages including NO production and hence part of its inhibitory effect on bacterial intracellular replication might be due to up-regulation of NO in infected cells.

Paper III reveals that bacterial thioredoxin 1, a reductase important for maintaining protein redox homeostasis, contributes to virulence of S. Typhimurium by participating in the activity of SPI2 T3SS. SPI2 and thioredoxin 1 had a convergent contribution to intracellular replication of S. Typhimurium both in vitro and in vivo. Catalytic as well as non-catalytic functions of thioredoxin 1 appeared important contributors to S. Typhimurium virulence, depending on the infection model used.

The outcome of the infection at individual cell level is dictated by the ever oscillating balance between host cell antimicrobial responses and the ability of bacteria to overcome them. In Paper IV we show that phagocytes infected with S. Typhimurium have a heterogeneous pattern of iNOS expression, which is not dependent on active virulence proficient bacteria and that the majority of infected cells are iNOS irresponsive, both in vitro and in vivo. Our study underlines the need of shifting from bulk to individual cell analysis when studying host-pathogen interactions.