How Salmonella Fights Back Against the Body's Antimicrobial Peptide Defenses Inside Immune Cells | Research

The takeaway

Salmonella uses its phage shock protein (Psp) system to resist host antimicrobial peptides like cathelicidin (CRAMP), enabling it to survive and persist inside macrophages — the very immune cells meant to kill it.

Psp system sustained throughout infection
The phage shock protein system was not transiently activated but maintained high expression across all four stages of Salmonella's life inside macrophages, indicating its continuous importance for immune evasion.

What the researchers found

Temporal transcriptomics revealed that the phage shock protein (Psp) system is highly expressed in intracellular Salmonella Typhimurium with sustained expression throughout macrophage infection. The Psp system is regulated by the virulence-associated two-component system SsrA-SsrB, coordinating its expression with other immune evasion functions.

Functional assays demonstrated that the Psp system mediates resistance specifically to host antimicrobial peptides, including cathelicidin-related antimicrobial peptide (CRAMP). This resistance supports bacterial persistence in host tissues and survival within macrophages. The findings establish the Psp system as a new adaptive mechanism for evading host antimicrobial peptide defenses.

Why it matters

Antimicrobial peptides are a critical first line of defense against bacterial infection. Understanding how pathogens like Salmonella evade these defenses reveals potential drug targets. Disabling the Psp system could make Salmonella more vulnerable to the body's natural antimicrobial peptides, potentially leading to new therapeutic strategies that enhance, rather than replace, the immune system's own weapons.

How the study worked

Comprehensive temporal transcriptome analysis of Salmonella Typhimurium across four stages of primary macrophage infection. Researchers identified highly expressed gene systems, characterized the regulation of the Psp system by the SsrA-SsrB two-component system, and performed functional assays testing Psp system mutants against host antimicrobial peptides (including CRAMP). Bacterial persistence was assessed in host tissues and within macrophages using mouse infection models.

What this study cannot tell us

The study was conducted primarily in mouse macrophages and mouse infection models, which may not fully replicate human Salmonella infection. The transcriptomic analysis captures gene expression but not necessarily protein levels or activity. The specific molecular mechanism by which the Psp system confers antimicrobial peptide resistance is not fully elucidated. The study focused on Salmonella Typhimurium; other Salmonella serovars may have different resistance mechanisms.

How to read the evidence

This is a comprehensive preclinical study combining temporal transcriptomics, genetic manipulation, functional assays, and in vivo mouse infection models. The multi-level evidence approach provides strong support for the findings within the preclinical context.

When this study was published

Published in 2025, this study uses cutting-edge temporal transcriptomics to reveal a previously unknown antimicrobial peptide resistance mechanism in a clinically important pathogen.

The bigger picture

The arms race between host antimicrobial peptides and bacterial evasion mechanisms is a fundamental aspect of infectious disease. This study reveals that Salmonella doesn't just passively endure antimicrobial peptide attack — it actively coordinates a defense system through its virulence regulatory network. Understanding these evasion mechanisms is crucial for developing therapeutics that disarm bacterial defenses and restore the effectiveness of the host's natural antimicrobial peptide arsenal.

Questions still open

Could small molecules targeting the Psp system sensitize Salmonella to host antimicrobial peptides as a therapeutic strategy?
Do other intracellular pathogens use similar Psp-mediated defense against antimicrobial peptides?
How does the Psp system specifically protect Salmonella from cathelicidin — does it modify the membrane, degrade the peptide, or use another mechanism?

Common questions

How does Salmonella survive inside the immune cells meant to kill it?

Macrophages use antimicrobial peptides like cathelicidin to kill engulfed bacteria. This study found that Salmonella activates a specialized defense system (the phage shock protein system) that makes it resistant to these peptides. This system is controlled by the same regulatory network that coordinates other virulence functions, allowing the bacteria to mount a coordinated defense against the immune system from inside its own cells.

Could this discovery lead to new treatments for Salmonella infections?

Potentially. If drugs could disable the Psp system, Salmonella would become more vulnerable to the body's natural antimicrobial peptides. This 'disarming' approach could work alongside conventional antibiotics or even reduce the need for them. It's a promising concept, but drug development targeting this system would be needed.

Read the original research

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

PLoS pathogens, 21(9), e1013132

View on PubMed ↗Publisher’s paper ↗

Citation

Massicotte, Marie-Ange; Fiebig, Aline A; Bogza, Andrei; Coombes, Brian K. (2025). The Salmonella phage shock protein system is required for defense against host antimicrobial peptides.. PLoS pathogens, 21(9), e1013132. https://doi.org/10.1371/journal.ppat.1013132