The Salmonella phage shock protein system is required for defense against host antimicrobial peptides.

Macrophages are professional phagocytes that play a major role in engulfing and eliminating invading pathogens. Some intracellular pathogens, such as Salmonella enterica serovar Typhimurium, exploit macrophages as niches for their replication, which requires precise and dynamic modulation of bacterial gene expression in order to resist the hostile intracellular environment. Here, we present a comprehensive analysis of the global transcriptome of S. Typhimurium across four stages of infection of primary macrophages. Our results revealed a profound change in early-stage gene expression dominated by pathways linked to metabolic processes required for Salmonella adaptation to the proinflammatory conditions of the macrophage. We identified the phage shock protein (Psp) system to be highly expressed in intracellular S. Typhimurium, with sustained high expression over the course of infection. We determined that the Psp system is regulated by the virulence-associated two-component system SsrA-SsrB, which coordinates its expression with critical bacterial functions required for immune evasion and intracellular survival. Functional assays demonstrated that the Psp system mediates resistance to host antimicrobial peptides, including cathelicidin-related antimicrobial peptide (CRAMP), which we demonstrate supports bacterial persistence in host tissues and survival within macrophages. Our findings establish the Psp system as a new and critical adaptive mechanism for evading host immune defenses and highlight the utility of temporal transcriptomics in unraveling the genetic strategies employed by S. Typhimurium during macrophage infection.