Mixed-Species Biofilms and Disinfectant Tolerance

Communities, not single isolates

Mixed communities may have varied metabolic states and matrix composition. The surrounding water, deposits and surface condition can also shape their behaviour.

This complexity supports a multi-line approach: hygiene design, cleaning, monitoring and appropriate authorised treatment.

Why single-species tests can understate tolerance

A 2025 dairy study in npj Science of Food (Yuan et al., 10 November 2025; doi 10.1038/s41538-025-00581-x) grew Staphylococcus aureus and Pseudomonas fluorescens on stainless steel alone and together. The dual-species biofilm had significantly more biomass, cell activity and extracellular polymeric substance, a denser structure, and greater tolerance to both chlorine dioxide and quaternary ammonium compounds than either organism alone. P. fluorescens up-regulated the S. aureus ica genes that drive polysaccharide adhesin production. The lesson is general: a disinfectant test on one organism can overstate what the same exposure achieves against a community. The dairy-specific detail is in the mixed-species dairy biofilm guide.

Tolerance data from any study are specific to its organisms, surface, age and test method, and no study in this centre tested a ChloroKlean product.

Disinfectant stress and resistance gene transfer: a free-chlorine study

A 2026 study in Environmental Science & Technology (Li et al., 11 August 2026; doi 10.1021/acs.est.6c04433) examined how chlorination affects plasmid-mediated transfer of antibiotic resistance genes in single-species and multi-species biofilms of Escherichia coli, Pseudomonas putida and Pseudomonas aeruginosa. Free chlorine at an initial 5 mg Cl/L significantly enhanced conjugative transfer: partial disruption of the recipient biofilm let donor bacteria colonise deeper, and the hotspot for gene transfer moved from the biofilm surface (about 18 µm) to the inner layer (about 27 µm) in the multi-species model. A model of long-term dynamics predicted deeper donor colonisation and wider gene dissemination under that exposure.

This is free-chlorine research. It was not a comparison with chlorine dioxide and is not evidence that chlorine dioxide behaves differently or better; no equivalent chlorine dioxide experiment was run in that study. Its relevance here is the mechanism: a sub-eradicating oxidant exposure that opens biofilm structure without removing it can change the community in unwanted ways. That argues for treatments that are verified to reach an endpoint, for physical removal where feasible, and for monitoring after any partial-kill event, whatever the oxidant.

Species-specific resistance, EPS and quorum sensing: a free-chlorine pipeline study

A 2026 paper in Applied and Environmental Microbiology (Niu et al., 13 March 2026; doi 10.1128/aem.01531-25) isolated five bacteria from biofilm and scale inside municipal drinking-water pipes that had been in service for over a decade: Sphingomonas ursincola, Sphingobium amiense, Gordonia amicalis, Microbacterium saccharophilum and Hydrogenophaga laconesensis. The isolates differed sharply in how much biofilm they formed (S. ursincola the most, H. laconesensis the least), and the authors measured both quorum-sensing signal molecules (N-acyl homoserine lactones C6-HSL and 3-oxo-C14-HSL, roughly 0.22 to 0.25 µg/L) and extracellular polymeric substance (about 20 to 32 mg/L), reporting that the signalling molecules influenced EPS composition. In other words, the organisms were coordinating the matrix that later shields them.

Chlorine tolerance was species-specific and concentration-dependent. At free chlorine of 1.0 mg/L or below, resistance ranked M. saccharophilum > S. amiense > G. amicalis > S. ursincola > H. laconesensis; above 1.0 mg/L the two sphingomonads, S. ursincola and S. amiense, were the most robust. A low residual of 0.6 mg/L damaged only about 26 to 35 per cent of cells, which the authors judged insufficient to control biofilm formation by these isolates, while 1.0 to 1.5 mg/L controlled biofilm biomass under their test conditions. The point is not the specific numbers, which belong to five isolates in one laboratory, but the shape of the result: the ranking of 'most resistant' changed with concentration, and the strongest biofilm former was not the most chlorine-tolerant organism at low doses.

This is a free-chlorine study of drinking-water isolates. It did not test chlorine dioxide, and nothing in it transfers to a chlorine dioxide dose, a ChloroKlean product or a UK building system. Its value here is mechanistic: it shows that 'resistance' in a biofilm is a property of the community, its EPS and its signalling, not a single number for a species, and that a residual which is too low to reach an endpoint can leave the most matrix-rich organisms in place. The same logic underlies the HSE approach to building water systems, where a measured residual is treated as one control among several rather than as proof that attached growth is under control.

What tolerance does and does not mean

Tolerance in a study is not a universal resistance value, and it should not be used to predict an outcome without site evidence. Across the three studies above, tolerance depended on which organisms were present together, how much matrix they had built, whether signalling was active, and what concentration was applied. Any one of those can change between a laboratory model and a working system.

Evidence and uncertainty

Published biofilm studies are valuable for understanding mechanisms, but their conditions may not match a particular installation. Species, surfaces, deposits and operating conditions should be recorded when interpreting evidence.

For safety-critical systems, decisions should be documented through the relevant risk assessment and management plan.

Choosing the next question

A useful next step is to identify what is known, what is inferred and what needs verification. This avoids treating a general reference as a site diagnosis.

Where a product is considered, confirm the intended use, authorisation and label directions independently of this educational guide.

Use this sequence to frame investigation and control; it is not a dosing protocol.

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