Antibiotic Heteroresistance and Treatment Failure: An Emerging Clinical Threat beyond Conventional Antimicrobial Resistance

Abstract

Background: Traditional resistance categorisation may not capture the complexity of the bacterial populations and antimicrobial resistance (AMR) is one of the biggest global health crises. With antibiotic heteroresistance a large percentage of the population is susceptible while a small percentage is much less susceptible. The resistant subpopulations cannot be identified in the normal antimicrobial sensitivity testing, but are continuously selected by the use of antibiotic drugs that lead to persistent infection, treatment failure and finally to the development of stable resistance.

Objective: This review will provide a general overview of the biological and molecular mechanisms of antibiotic heteroresistance, as well as identification and characterization, distribution in clinically relevant bacterial pathogens, relationship with treatment failure and new therapeutic and diagnostic strategies.

Method: The literature on Bacterial Heteroresistance, AMS, Resistance Evolution, Clinical Treatment Outcomes, Diagnosis, PK/PD Optimization, Antimicrobial Combinations, BART and Emerging Therapeutic Technologies was critically reviewed. The microbiological, genomic, single-cell and clinical and translational data have been integrated to provide an overview of heteroresistance in the population level.

Results: Heteroresistance has been reported in various Gram-positive and Gram-negative pathogens and against a number of clinically relevant classes of antimicrobial agents, such as carbapenems, colistin, vancomycin, daptomycin and tigecycline.  AHR Can arise as a result of gene amplification, chromosomal mutation, copy number variation, regulation, increased/decreased membrane permeability, efflux activity, metabolic adaptation and/or horizontal gene transfer. Biofilm, gradients of antibiotics, nutrient limitation, stresses and host-specific factors may contribute to phenotypic heterogeneity. In addition to conventional susceptibility testing, which fails to detect low frequency resistant subpopulations, there are methods with a higher resolution like population analysis profiling, time-kill studies, single cell approaches, microfluidics and multi-omics. Therapeutically, there is a risk of subpopulation enrichment and emergence of resistant subpopulations which may be responsible for persistent infection, relapse, emergence and treatment failure. Other strategies to limit the clinical importance of heteroresistance involve rational combination therapy, dosing based on PK/PD, anti-biofilm agents and strategies, antimicrobial peptides, rapid molecular diagnostics, and AI-based predictions and host-directed therapy.

Conclusion: AHR represents a critical, underrecognized dimension of resistance that challenges standard diagnostic paradigms. By evading routine AST and serving as a reservoir for development of resistance. Future research aimed at defining and validating detection assays, at longitudinal monitoring of heteroresistance, at studying heteroresistance at the single-cell and multi-omics levels and at prospective studies, which investigate the link between heteroresistance and treatment outcomes, would be of great value. Incorporating the concept of susceptibility at a population level into the diagnostics and therapeutic process for antimicrobial treatment will be essential for enabling precision antimicrobial therapy and preventing treatment failure.