Biocide Resistance, Biofilms, and AMR: Why Scientific Testing is Critical for Long-Term Efficacy

While concerns over antimicrobial resistance (AMR) have traditionally centred on antibiotic use, recent scientific reviews including a 2025 article[1] in Nature’s npj Antimicrobial Resistance underscore another emerging potential threat: reduced microbial susceptibility to biocides.

The Expanding Role and Risk of Biocides

Biocides are indispensable to modern public health and industrial hygiene. Unlike antibiotics, they act rapidly and target a broad spectrum of organisms including bacteria, fungi, algae, and viruses, by disrupting multiple cellular pathways and structures. This multi-target action has made biocides a mainstay in hospitals, laboratories, food production, and consumer products. They are also widely applied in agriculture, chemical manufacturing, energy infrastructure, and other sectors to control microbial contamination and biofouling.

However, the very ubiquity of biocides has introduced new challenges. Frequent and sometimes sub-optimal use across diverse settings has led to increasing environmental exposure to biocide residues. As a result, microorganisms are more regularly encountering sub-lethal biocide concentrations – conditions known to foster tolerance or decreased susceptibility.

This issue is particularly pronounced when looking at biofilms. These dense, surface-attached microbial communities are encased in a protective matrix and are naturally more resistant to both antibiotics and biocides, especially once they mature. This makes them harder to eradicate from surfaces, pipes, or medical devices, and increases the likelihood that survivor populations will persist and adapt.

Worryingly, recent evidence indicates that bacterial populations are indeed becoming less susceptible to commonly used biocides. Some studies show rising minimum inhibitory concentrations (MICs), and mobile genetic elements have been identified that carry genes linked to biocide resistance. These genetic traits can be transferred horizontally between bacteria and are increasingly found in species that also carry antibiotic resistance genes – including WHO-prioritised AMR pathogens and members of the ESKAPE group.

Although most bacteria still cannot survive the high concentrations used in well-formulated disinfectants and antiseptics, the overall trend of decreasing susceptibility threatens our long-term ability to rely on these products. Understanding the selective pressures and mechanisms at play is critical to preserving their efficacy.

Why Biofilms Matter

Biofilms are not just hard-to-kill structures - they are hotspots for adaptive stress responses. Deep within the extracellular matrix, cells experience protection from direct biocide contact and sustained sub-lethal exposure. This promotes horizontal gene transfer and long-term survival strategies, including expression of resistance phenotypes.

Given that bacteria in biofilms can be up to 1,000-fold more tolerant than planktonic cells, failure to assess formulations against biofilm-associated survival risks overlooking both tolerance and resistance dynamics.

Why Scientific Testing Matters

Biocides remain among our most effective tools for hygiene and infection control, but misuse or sub-lethal application can subtly erode their power. As highlighted by researchers in the Nature article, declining susceptibility – though often modest in magnitude – may signal adaptation pathways that, over time, could undermine current disinfection practices. The increasing overlap between biocide tolerance mechanisms and antibiotic resistance genes further amplifies this concern.

By testing real-world scenarios such as biofilm-associated survival, adaptation cycles, cross-resistance outcomes, labs like BluTest support evidence-based use of biocides that preserves efficacy and safety.

Understanding decreases in biocide susceptibility and how this may intersect with antibiotic resistance represents a critical frontier in understanding responsible antimicrobial use. The Nature paper emphasises the importance of rigorous testing frameworks such as existing biocide efficacy standards in compliance with BPR, biofilm research, and awareness of genetic mechanisms that may drive resistance.

BluTest Laboratories, as a scientific CRO specialising in antimicrobial efficacy testing, is proud to support this expanding field through methodologically sound biofilm studies, adaptation modelling, and cross-resistance profiling. These data empower antimicrobial developers, regulators, and users to deploy biocides responsibly - and maintain their effectiveness for the long term.

At BluTest Laboratories – a UKAS-accredited microbiology CRO (Lab No. 4597) - we bridge the gap between emerging science and real-world testing. Key capabilities include:

• Biofilm efficacy testing, from simple to complex interkingdom models using sessile MIC assays and standardised assays such as the CDC Reactor ASTM E2871.
• Standard biocide efficacy tests using methods such as EN 1276, EN 13697 and EN 14476 with high-relevance pathogens, ensuring compatibility with existing regulatory frameworks.
• Adaptation modelling studies, where organisms are repeatedly exposed to singular or formulated biocides to assess shifts in susceptibility.
• Cross-resistance profiling, evaluating whether reduced biocide susceptibility coincides with elevated antibiotic MICs in relevant species.

Together, this approach allows developers to identify potential efficacy drift early, adjust formulations, and guide recommended application protocols, while providing regulators and users with scientifically grounded risk assessments.

About BluTest Laboratories

BluTest Laboratories is a UKAS-accredited microbiology CRO specialising in testing disinfectants, biofilm models, antimicrobial efficacy, virology, and product development services. Learn more at www.blutest.com.

The article is written by Daniel Yaxley, General Manager at BluTest Laboratories

References

[1] O’Reilly P, Loiselle G, Darragh R, Slipski C, Bay DC. Reviewing the complexities of bacterial biocide susceptibility and in vitro biocide adaptation methodologies. npj Antimicrobials and Resistance. 2025;3:39. https://doi.org/10.1038/s44259-025-00108-0