Beyond minimum inhibitory concentration values: clinical and diagnostic challenges of colistin heteroresistance in Acinetobacter baumannii.

In recent years, infections caused by multidrug-resistant and particularly carbapenem-resistant Gram-negative bacteria have become a major global public health concern. In this context, the progressive limitation of therapeutic options has led to the reintroduction of polymyxin antibiotics, such as colistin, into clinical practice as last-resort agents. However, the widespread use of colistin has been accompanied by an increasing incidence of resistance, among which heteroresistance has emerged as a significant challenge for both diagnosis and treatment. Heteroresistance is defined as the presence of subpopulations within a bacterial isolate that exhibit varying levels of susceptibility to a given antibiotic. In Acinetobacter baumannii, colistin heteroresistance may result in the selection and expansion of resistant subpopulations during therapy, contributing to clinical failure, persistent infections, and recurrence. Furthermore, the inability of routine antimicrobial susceptibility testing methods to reliably detect heteroresistance may lead to its underrecognition in clinical settings, potentially resulting in inappropriate therapeutic decisions. Current evidence indicates that the prevalence of colistin heteroresistance in A. baumannii varies geographically. The findings presented in this review highlight the importance of increasing awareness of the molecular basis and clinical implications of colistin heteroresistance in A. baumannii, and emphasize the need for improved diagnostic approaches and more effective treatment strategies for both microbiologists and clinicians.