Systems and methods for antimicrobial susceptibility testing (AST) are provided in which variances in anionic charge of microbes are taken into account. Cationic surfactants may be used to sensitize otherwise resistant microorganisms to polycationic antibiotics, such as polymyxins. Since microorganisms gain polycationic antibiotic resistance through mutations that decrease surface anionic charge, the susceptibility of a microorganism to a polycationic antibiotic may be indicative of its surface charge. In order to enable electrostatic interactions with the microorganism surface, a cationic surfactant may be applied to increase the anionic charge of the microorganism.