Salmonella Hijacks Mitophagy to Evade Immunity and Persist in Host Cells

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a globally significant zoonotic pathogen responsible for substantial morbidity in both humans and animals. One hallmark of its success is the ability to survive and replicate within host cells, particularly inside specialized membrane-bound compartments called Salmonella-containing vacuoles (SCVs). Despite extensive research, the molecular mechanisms enabling this persistent intracellular lifestyle have remained incompletely understood.

This study identifies a previously uncharacterized interaction between the bacterial SPI-2 type III secretion effector SseJ and the host inner mitochondrial membrane protein PHB2 (prohibitin 2). Using co-immunoprecipitation, confocal microscopy, and bacterial infection models in macrophages and epithelial cells, the authors demonstrate that SseJ directly binds PHB2 and exploits it as a mitophagy receptor. PHB2 is known to recruit LC3 to the inner mitochondrial membrane during mitophagy, particularly after outer membrane rupture. The SseJ-PHB2 interaction was found to potently activate the canonical PINK1-PRKN (Parkin) pathway, promoting autophagosome-dependent engulfment and degradation of damaged mitochondria.

Key results showed that wild-type S. Typhimurium infection caused marked mitochondrial fragmentation, loss of membrane potential, and increased mitophagy flux, as measured by LC3-II accumulation, co-localization of mitochondria with autophagosomes, and degradation of mitochondrial markers such as COX4 and MFN2. In contrast, a SseJ-deletion mutant (S.T-ΔSseJ) induced substantially less mitophagy and replicated at significantly lower levels intracellularly. Complementation with the SseJ gene restored both mitophagy induction and bacterial replication, confirming SseJ's causal role. Knockdown of PHB2 via siRNA similarly reduced mitophagy and bacterial survival, phenocopying the SseJ deletion.

Pharmacological suppression of mitophagy using Mdivi-1 (a mitochondrial fission inhibitor), chloroquine (CQ), or bafilomycin A1 (BafA1) significantly decreased intracellular bacterial colony-forming units and alleviated infection pathology in cell culture models. These interventions reduced bacterial load without broadly disrupting autophagy, underscoring the specificity of the mitophagy axis in supporting S. Typhimurium persistence.

The study proposes a model in which S. Typhimurium strategically induces mitophagy via SseJ-PHB2 to eliminate ROS-generating, pro-inflammatory mitochondria, thereby dampening innate immune responses—including NLRP3 inflammasome activation and mtDNA-triggered cGAS-STING signaling—and preserving an intracellular niche favorable for bacterial replication. This represents a sophisticated immune evasion strategy exploiting the host's own organelle quality control machinery.

A notable caveat is that most experiments were conducted in cell culture systems; in vivo validation in animal models was limited. Additionally, the precise structural basis of SseJ-PHB2 binding and the full downstream signaling cascade remain to be elucidated. Nonetheless, this work opens a compelling avenue for developing host-directed therapies that target the SseJ-PHB2-mitophagy axis to combat persistent Salmonella infection.