Hydrophobic Agent Permeability Assay (HAPA) for Rapid Antibiotic Susceptibility Testing in Gram-Negative Bacteria

Timely antibiotic susceptibility testing (AST) is essential for effective infection management and resistance control. Standard methods such as broth microdilution require 16-24 hours, delaying appropriate therapy. Existing rapid AST approaches are often limited by high cost, low throughput, or reduced accuracy in heterogeneous bacterial populations. We developed the Hydrophobic Agent Permeability Assay (HAPA), a phenotypic AST method that combines morphology profiling and hydrophobic dye uptake to detect antibiotic-induced membrane changes. HAPA classifies Gram-negative bacterial susceptibility within 4 hours. In susceptible strains, antibiotic-triggered reactive oxygen species (ROS) remodel lipopolysaccharide-mediated membrane permeability, enhancing dye entry. This shift coincides with MIC-level inhibition, enabling accurate breakpoint determination. HAPA demonstrated 97.1% categorical agreement with BMD across 75 clinical isolates, including E. coli , K. pneumoniae , A. baumannii , and P. aeruginosa , with error rates below regulatory thresholds. Compatible with automation and point-of-care imaging, HAPA offers a cost-effective, scalable solution for rapid diagnostics and improved antimicrobial stewardship.