Mixed-Species Dairy Biofilms: Why Single-Species Tests Can Understate Resistance
An evidence-led guide for understanding biofilm in managed water and hygiene systems.
Dairy plant biofilms are rarely one organism. A 2025 npj Science of Food study found that Staphylococcus aureus and Pseudomonas fluorescens grown together on stainless steel built a denser, more tolerant biofilm than either alone, against both chlorine dioxide and quaternary ammonium compounds. This guide explains what that means for PT4 food-area hygiene programmes.
Why dairy surfaces grow communities
Milk residues supply protein, fat and lactose; stainless steel welds, gaskets, valve seats and dead legs supply shelter; and cyclical warm-cool operation supplies time. Psychrotrophic pseudomonads such as Pseudomonas fluorescens grow at refrigeration temperatures and produce heat-stable enzymes that spoil product, while Staphylococcus aureus is a persistent contaminant that can produce toxins. In practice these and other organisms colonise the same surfaces, and the community that results is what a cleaning-in-place or open-plant disinfection step actually meets.
Most disinfectant efficacy testing, including the standardised suspension and surface tests used to support product claims, is run on single reference strains. That is appropriate for a defined regulatory purpose, but it is not a prediction of performance against a mature, mixed community on plant.
The 2025 npj Science of Food study
Yuan and colleagues at a dairy research institute in Shanghai (npj Science of Food, 10 November 2025; doi 10.1038/s41538-025-00581-x) grew S. aureus and P. fluorescens on stainless steel coupons as single cultures and as a co-culture, then compared the biofilms' structure and their tolerance of chlorine dioxide and quaternary ammonium compounds. The dual-species biofilm had significantly higher biomass, cell activity and extracellular polymeric substance production, a denser structure under microscopy, and increased tolerance of both disinfectants relative to the single-species biofilms.
The study also identified a mechanism. Reverse-transcription qPCR showed that the presence of P. fluorescens markedly up-regulated the icaA and icaD genes in S. aureus. Those genes drive production of polysaccharide intercellular adhesin, a key matrix component, and the increase correlated with greater EPS and a more robust biofilm. In other words one organism changed the other's behaviour in a way that protected both.
What this means for single-species efficacy data
A log-reduction achieved against a single S. aureus or P. fluorescens biofilm in a laboratory test is a measurement of that test, not a ceiling or a floor for plant performance. The study shows that co-culture can raise tolerance to two chemically unrelated disinfectants at once, which points to the matrix and structure, rather than any specific chemistry, as the protective factor. Programmes that rely on a single-species claim to set a contact time or concentration on plant therefore risk understating what mature mixed biofilm requires.
The practical response is not a higher dose by default. It is to combine effective cleaning that removes soil and matrix before disinfection, hygienic design that limits harbourage, an authorised disinfectant applied per its label, and verification that includes surface sampling from known harbourage points rather than only rinse-water results.
What the study does and does not establish
It establishes that, on stainless steel under the study's conditions, a two-species dairy biofilm was denser and more tolerant of chlorine dioxide and quaternary ammonium compounds than the corresponding single-species biofilms, and it identifies a gene-regulation mechanism behind the difference.
It does not rank chlorine dioxide against quaternary ammonium compounds; both were less effective against the co-culture, and the paper is about the biofilm rather than the disinfectants. It does not report a chlorine dioxide dose or contact time that can be transferred to plant. It tested two organisms; real dairy communities contain many more. And no ChloroKlean product was tested, so the results neither support nor undermine any product-specific claim.
Where PT4 fits
Disinfection of food and feed area surfaces and equipment, including CIP circuits, falls under product-type 4 of the GB Biocidal Products Regulation. Any product used must be compliant for that use under GB BPR, whether fully authorised or lawfully supplied under transitional arrangements while the active substance is under review, and applied according to its label, including concentration, contact time, temperature, and any required rinse. Disinfection of drinking water for people or animals is PT5; other water uses are classified by their intended purpose. This guide is educational and sets no doses.
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.
A proportionate biofilm-management approach
Use this sequence to frame investigation and control; it is not a dosing protocol.
Define the system and risk
Map wetted surfaces, operating conditions, users and relevant legal or sector guidance.
Gather evidence
Review inspection, operational, residual and microbiological records rather than relying on one indicator.
Address contributing conditions
Consider cleaning, hydraulics, nutrients, stagnation and equipment condition alongside any authorised biocide programme.
Verify and review
Document the intervention and review results through the site’s written scheme or hygiene plan.
Expert Insights
"Biofilm control is a system-management question: chemistry, surfaces, flow, cleaning and verification all matter."
ChloroKlean Technical Team
Technical review team
About the Reviewer
Gavin Owen
Managing Director, ChloroKlean
Gavin Owen leads ChloroKlean's technical and commercial operations, bringing over 20 years of experience in industrial chemical distribution and water treatment. He oversees product development, regulatory compliance strategy, and the company's BPR compliance programme across PT2, PT4, PT5, and PT11 product types. Gavin works directly with water treatment professionals, facilities managers, and public health engineers across healthcare, leisure, food processing, and industrial sectors.
Frequently Asked Questions
Common questions about this topic, answered by our technical team.
Scope and safe-use note
- This is general educational information, not a dosing instruction or a product label.
- Use only a biocidal product authorised for its intended product type and follow its label, Safety Data Sheet and site risk assessment.
- Investigate system design, cleaning, monitoring and microbiological findings with a competent person where there is a health risk.
Published evidence about a disinfectant or another product does not establish efficacy, authorisation or an appropriate use pattern for any ChloroKlean product.
Related Resources
Continue exploring our knowledge base and product information.
Biofilm learning hub
Browse the connected biofilm guides.
Mixed-species biofilms and disinfectant tolerance
The general principle across water and hygiene systems.
What is EPS in biofilm?
The matrix that protects mixed communities.
Why chemical disinfection alone may not remove biofilm
Kill, reduction and removal are different endpoints.
Food safe disinfectant guide
PT4 food-area disinfection in the UK framework.
PT4 food and feed area hygiene
Product-type information for food-area disinfection.
Sources & References
This article references guidance from the following authoritative sources:
- Biofilms: survival mechanisms of clinically relevant microorganisms
Industry Standard - Frontiers in Microbiology (PMC)
- Legionella and the prevention of legionellosis
WHO - World Health Organization
- ACOP L8: Legionnaires' disease
HSE - Health and Safety Executive
- Biocidal Products Regulation
ECHA - European Chemicals Agency
- The dual-species biofilm formed by Staphylococcus aureus and Pseudomonas fluorescens exhibited enhanced resistance to disinfectants (10 November 2025)
Industry Standard - npj Science of Food
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