A non-canonical role of Caspase-1 in regulating bacterial antimicrobial resistance

Summary Antimicrobial resistance (AMR) is a prominent healthcare threat with an estimated 4.95 million cases associated with bacterial AMR in 2019, and poses a particularly difficult challenge with regard to intracellular pathogens that have evolved to hijack host defenses for their own benefit. A deep view of host-pathogen interactions including

a greater understanding of how pathogens adapt and change in response to host cues is critical to gain insights into the factors responsible for AMR. While the response of hosts to bacterial infection has been extensively studied, the converse i.e. how pathogen gene expression and in turn pathogen physiology is modulated in

response to host cues has gathered little attention. Lack of robust technologies to sequence minute amounts of bacterial RNA from infected cells has been one of the limiting factors. Using a recently developed technology - Path-Seq, we reliably sequenced macrophage-resident Salmonella to discover a role for host Caspase-1 in

dampening AMR of intracellular Salmonella. This was mediated through inhibition of the bacterial two-component signal transduction system – PhoPQ, a major contributor responsible for Salmonella's ability to resist host cationic antimicrobial peptides (CAMPs) and the drug Polymyxin B, which is a last resort antibiotic against Gram-

negative pathogens. Interestingly, Caspase-1 which is conventionally recognized as a protease important for inflammasome activation and pyroptosis, dampens CAMP resistance of Salmonella in a manner independent of its catalytic/protease activity. In this proposal we will thoroughly investigate the mechanism by which host

Caspase-1 inhibits PhoPQ activation and CAMP resistance of intracellular Salmonella independent of its activity. In Aim 1 we will conduct a molecular dissection of the bacterial pathway and the pathogen effectors downstream of PhoPQ activation that are targeted by Caspase-1. In Aim 2 we will investigate how host Caspase-1 inhibits

CAMP resistance of intracellular Salmonella, either by directly acting on the bacterium or indirectly by regulating host processes that in turn control PhoPQ activation and CAMP resistance. Targeting this non-canonical, pyroptosis independent arm of Caspase-1 may be particularly useful in curbing AMR of pathogens such as

Salmonella that evade inflammasome activation and replicate intracellularly. Our findings will reveal a novel activity-independent role for Caspase-1 in controlling bacterial signaling and AMR, and because PhoPQ activation dictates multiple aspects of pathogen physiology such as intracellular replication and virulence, will

also form a framework for exploring the impact of host Caspase-1 on processes beyond AMR in intracellular Gram-negative pathogens.