Mycobacterium leprae In Vitro Persistence: A Unified Mechanistic Model of Lipid, Iron, and Thiolic Constraints

The inability to cultivate Mycobacterium leprae continuously in cell-free media has resisted solution for 150 years, despite multiple cultivation attempts reporting transient growth. This paper proposes that cultivation failure reflects three biomechanically coupled constraints, each driven by genomic decay and each necessary for sustained growth: (1) loss of mbtB renders ferric iron unavailable except through chaperoned delivery; (2) residual host lipid droplets deplete within 12–24 weeks, requiring replacement via structured amphiphilic geometry (not micellar detergents) and lipid-based central carbon metabolism; (3) katG pseudogenization eliminates catalase-peroxidase, creating absolute dependence on mycothiol-mediated defenses and microaerophilic oxygen tension. The integration of these constraints through genomic decay, lipid biochemistry, transcriptomics, and membrane biophysics provides a unified mechanistic framework explaining why historical successes (Osawa 1997: 10³-fold growth, 18 serial passages) were not replicated, and identifying specific constraint combinations necessary for sustained axenic replication. The framework's contribution is synthetic rather than mechanistic—each constraint is substantially characterized—but the three-feature integration with explicit falsification criteria provides an actionable research roadmap. Importantly, acid-fastness loss is reinterpreted as an envelope stress phenotype (lipid starvation signature), not contamination or cell death, potentially recovering viable stressed cultures in historical cultivation attempts. The framework operates through systematic constraint testing rather than media trial-and-error, providing specific benchmarks for evaluating whether proposed formulations satisfy all three constraints simultaneously.