Phenotype–Genotype Discordance in Antimicrobial Resistance of Acinetobacter baumannii: Implications for Diagnostics and Surveillance

Acinetobacter baumannii has emerged as one of the most challenging opportunistic pathogens in modern healthcare due to its remarkable ability to acquire and disseminate antimicrobial resistance determinants. Carbapenem-resistant A. baumannii (CRAB) is now recognized by the World Health Organization as a critical priority pathogen, highlighting the urgent need for improved diagnostic, surveillance, and therapeutic strategies. This review synthesizes current evidence on the relationship between phenotypic antimicrobial susceptibility patterns and underlying genetic determinants of resistance in A. baumannii. A structured literature search was conducted across major biomedical databases (PubMed/MEDLINE, Scopus, Web of Science, and the Cochrane Library), supplemented by citation tracking and relevant institutional sources, focusing on studies published between 2016 and 2026. The analysis integrates findings from studies examining phenotypic antimicrobial susceptibility testing (AST) with molecular and genomic investigations of resistance mechanisms, including carbapenemases, aminoglycoside-modifying enzymes, efflux pumps, and resistance-associated genomic islands. Particular attention is given to the complex and sometimes discordant relationship between genotype and phenotype, where the presence of resistance genes does not always translate directly into phenotypic resistance due to regulatory mechanisms, gene expression variability, and genomic context. The review further discusses methodological differences in AST standards and genomic prediction approaches that may contribute to variability across studies. Collectively, the evidence supports a multidimensional interpretation of antimicrobial resistance in A. baumannii, emphasizing the necessity of integrating phenotypic and genotypic data for accurate diagnostics, surveillance, and clinical decision-making. This integrated perspective may contribute to improved understanding of resistance evolution and support the development of more effective strategies for managing multidrug-resistant A. baumannii infections.