Antibiotic Heteroresistance, Tolerance & Persistence: Mechanisms & Clinical Relevance

The ongoing crisis of antibiotic resistance demands faster, more accurate methods for identifying subtle shifts in bacterial vulnerability. Traditional antibiotic susceptibility testing, while foundational, often misses low-level resistance – a phenomenon where a subpopulation of bacteria exhibits reduced susceptibility, even if the overall population appears sensitive. This can lead to treatment failures and the spread of resistance. Recent research highlights a promising technique, the dilution-and-delay (DnD) susceptibility assay, offering high-resolution, high-throughput detection of these hidden resistance mechanisms.

Understanding Heteroresistance and Persistence

The challenge isn’t always about fully resistant “superbugs.” Increasingly, clinicians face situations where antibiotics fail despite lab tests indicating susceptibility. This is often due to heteroresistance – the presence of subpopulations within a bacterial population that display varying levels of antibiotic susceptibility (Coates et al., 2018). Related is bacterial persistence, where a small fraction of bacteria enter a dormant state, surviving antibiotic exposure without genetic changes (Balaban et al., 2004). DnD aims to uncover these subtle vulnerabilities that standard tests might miss.

How the Dilution-and-Delay Assay Works

Traditional methods, like minimum inhibitory concentration (MIC) determination, assess the lowest antibiotic concentration that inhibits visible growth. The DnD assay, however, focuses on the time it takes for growth to resume after brief antibiotic exposure. It operates on the principle that even a small, tolerant subpopulation will exhibit a delayed growth response compared to a fully susceptible population.

The process involves serial dilutions of the antibiotic, similar to standard MIC testing. However, instead of simply observing whether growth occurs or not, the DnD assay monitors growth resumption over time – typically using microfluidic devices or automated plate readers. The key metric is the “delay time” – the time required for the bacterial population to recover and begin growing again after antibiotic removal. A longer delay time suggests the presence of a tolerant or heteroresistant subpopulation.

Standardized methods for determining MICs are outlined by organizations like the Clinical and Laboratory Standards Institute (CLSI) (CLSI, 2025) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) (EUCAST, 2003). DnD builds upon these foundations but adds a temporal dimension to the analysis.

Implications for Clinical Practice and Antibiotic Stewardship

The ability to detect low-level resistance has significant implications. It could support explain cases of treatment failure where standard tests show susceptibility, prompting clinicians to reconsider antibiotic choices or dosages. DnD could be a valuable tool for antibiotic stewardship programs, helping to identify emerging resistance trends before they turn into widespread.

The Centers for Disease Control and Prevention (CDC) emphasizes the importance of antibiotic stewardship to optimize patient safety and combat antimicrobial resistance (CDC, 2026). Improved antibiotic use is a key strategy, and more sensitive detection methods like DnD can contribute to this effort. The CDC’s 2026 update to the Core Elements of Outpatient Antibiotic Stewardship highlights the role of health system leadership in supporting these programs.

Evidence and Limitations of the DnD Assay

Research demonstrates the potential of DnD to reveal resistance mechanisms missed by conventional methods. For example, studies have shown that DnD can detect colistin heteroresistance in carbapenem-resistant Enterobacterales, a critical threat to public health (Band et al., 2021). It can also identify subpopulations with increased tolerance due to gene amplification (Nicoloff et al., 2019).

However, the DnD assay is not without limitations. It requires specialized equipment and expertise, potentially limiting its widespread adoption. The interpretation of delay times can also be complex, as factors other than resistance (e.g., metabolic state, growth phase) can influence growth resumption. Establishing standardized protocols and breakpoints for DnD remains an ongoing challenge. The dynamic nature of heteroresistance, where resistant subpopulations can fluctuate, also complicates detection (Pereira et al., 2021).

What Comes Next: Refining and Implementing DnD

The future of DnD lies in further refinement, and standardization. Researchers are working to develop more automated and user-friendly platforms for performing the assay. Establishing clear criteria for interpreting delay times and correlating them with clinical outcomes is crucial. Integration with existing laboratory workflows and electronic health records will also be essential for widespread implementation. Continued research into the mechanisms underlying heteroresistance and persistence will further inform the development and application of DnD and other advanced susceptibility testing methods. The United Nations General Assembly’s recent focus on reducing antimicrobial resistance mortality (The BMJ, 2024) underscores the urgency of these efforts.