Siderophore-Guided Strategies to Overcome Gram-Negative Barriers: β-Lactamase-Triggered Periplasmic Activation, Efflux Evasion, and Cytosolic Self-Assembly

Multidrug-resistant (MDR) Gram-negative pathogens pose a critical threat within the broader Antimicrobial Resistance (AMR) crisis, which caused an estimated 1.27 million direct deaths globally in 2019. Their restrictive outer membranes and broad resistance–nodulation–cell division (RND) family efflux systems severely limit antibiotic access. While siderophore–antibiotic conjugates (SACs) utilize TonB-dependent receptors (TBDRs) for outer-membrane entry, success in reaching the cytosol remains limited. Here, we outline a conceptual framework that advances testable hypotheses across three main strategies—(i) β-lactamase–triggered periplasmic activation to release a small, eNTRy-compatible cytosolic payload; (ii) efflux evasion by design to favor inner-membrane transit and residence; and (iii) siderophore-mediated intracellular self-assembly, whereby two periplasm-released precursors re-assemble an active ribosomal agent in the cytosol via bio-orthogonal chemistry—treating siderophore-guided entry as the unifying backbone across all approaches. We also consider a dual-pressure variant combining periplasmic and cytosolic action, and a minimal exporter-decoy element. Owing to technical and financial constraints, no experiments were conducted; our aim is to propose testable hypotheses.