Flying under the radar: vancomycin heteroresistance in

Background: Antimicrobial resistance (AMR) is a global One Health problem and a growing threat to human and animal health. Antimicrobial susceptibility tests (AST) such as the broth-microdilution method (BMD) guide appropriate antimicrobial therapy but may fail to detect various forms of AMR, such as heteroresistance. We report vancomycin treatment failure in a Case Summary: A patient with cloudy PD effluent presented to his local dialysis center. Samples were sent for testing, and empirical treatment with vancomycin and gentamicin was initiated. An Conclusion: Conventional AST (VITEK 2 and BMD) misclassified the isolates as vancomycin susceptible, while population analysis profiling area under the curve confirmed heteroresistance. Whole-genome sequencing did not identify genetic mechanisms for the heteroresistance, with no mutations detected in known resistance-associated genes. This study highlights the limitations of conventional AST and genomic methods for AMR detection, emphasizing the need for further research into cryptic forms of heteroresistance to enhance diagnostic accuracy and improve patient outcomes.