When neighbours matter: reviewing Pseudomonadaceae and Listeria sanitizer tolerance

Food-processing surfaces are ecological rather than sterile by default. Bacteria arriving from water, produce, equipment and the wider environment can attach, grow and form communities. That matters for Listeria monocytogenes, a foodborne pathogen whose persistence is often considered in relation to biofilms. A 2025 Food Microbiology paper asks a focused and useful question: does the company L. monocytogenes keeps in a multi-species biofilm alter its response to sanitizers?

The short answer from this laboratory model is yes, in particular where Pseudomonadaceae were present. That is an important observation, but it needs to be read at the scale at which it was made. It does not mean every Pseudomonadaceae-containing surface will protect Listeria, nor does it establish how a particular hygiene programme will perform in a working factory. This review sets the paper alongside a relevant dual-species study and separates the evidence from the inferences it cannot support.

For a primer on how surface-associated communities differ from free cells, see biofilm. The practical issue is not simply whether Listeria is detected alongside another organism; it is whether the community, surface, soil and cleaning conditions together affect removal or survival.

The 2025 study at a glance

Voloshchuk and colleagues examined biofilms containing L. monocytogenes and bacterial families previously reported to co-occur with it in tree-fruit packing facilities: Pseudomonadaceae, Xanthomonadaceae, Flavobacteriaceae and Microbacteriaceae. The researchers constructed single-family and multi-family assemblages with Listeria, rather than relying on an undefined environmental biofilm. This is a strength for comparison: the presence or absence of particular family-level partners can be assessed in a controlled system.

The paper exposed multi-family biofilms to benzalkonium chloride (BAC) at 200 ppm, and peroxyacetic acid (PAA) at 250 or 500 ppm, then measured die-off kinetics. Total bacteria were assessed by aerobic plate count and Listeria by a most probable number method. The authors also assessed a commercial biofilm remover across their assemblages, but the central sanitizer comparison is BAC and PAA.

Terminology deserves care here. These experiments used food-industry sanitizers including BAC and PAA. They did not test chlorine dioxide and did not test ChloroKlean. Results about one active chemistry, formulation and experimental set-up should not be transferred automatically to another.

What the research shows

Within the tested model, assemblages that included Pseudomonadaceae were associated with greater tolerance of L. monocytogenes to both BAC and PAA than assemblages without Pseudomonadaceae. The conclusion rests on the authors’ measured Listeria die-off over sanitizer exposure, not merely on a snapshot count after treatment. It therefore supports a specific, evidence-led proposition: community composition can be relevant when interpreting sanitizer response in a mixed biofilm.

The finding is especially useful because it does not frame biofilm tolerance as a fixed property of a single Listeria isolate. A companion population may change spatial arrangement, matrix characteristics, access of a chemical to cells, physiological state, or several of these factors at once. The 2025 paper demonstrates the outcome in its model; it does not identify a single causal mechanism by which Pseudomonadaceae produced it.

Supporting dual-species work by Rodríguez-López and colleagues provides a more architectural perspective. Their study grew L. monocytogenes with Pseudomonas aeruginosa, Pseudomonas fluorescens or Pseudomonas putida on stainless steel. Epifluorescence microscopy found different surface distributions depending on the Pseudomonas species. Confocal microscopy reported similar maximum thicknesses across the biofilms, while colocalisation analyses indicated a tendency for Listeria to share locations with the Pseudomonas strains tested.

That dual-species study also modelled dose-response data for BAC and neutral electrolysed water. Listeria was less susceptible to BAC when co-cultured with P. aeruginosa or P. fluorescens; its susceptibility to neutral electrolysed water was reduced in all three dual-species biofilms. Neutral electrolysed water is a chlorine-based sanitizer system, not chlorine dioxide. Taken together, the two papers show that the pattern is not confined to a discussion of one sanitizer class: tested Pseudomonas partners were associated with altered Listeria response under the particular BAC, PAA and chlorine-based neutral-electrolysed-water conditions investigated.

Neither paper turns this into a universal hierarchy of organisms or chemistries. The species-specific BAC pattern in the dual-species work is itself a useful caution. Different Pseudomonas partners did not produce the same result in every comparison. The 2025 work similarly points to Pseudomonadaceae within defined assemblages, not to every environmental organism that might be labelled Pseudomonas.

Why study design matters

Both studies are controlled biofilm experiments, which makes them valuable for isolating variables but limits direct extrapolation. The 2025 assemblages were selected from families associated with tree-fruit packing environments. A dairy line, a ready-to-eat facility or a drain biofilm may have a different succession of organisms, nutrient supply, temperature history, surface damage and residue load. A family-level assemblage is not the same thing as the full diversity of a facility microbiome.

The dual-species work has a different deliberate simplification: one Listeria strain was paired with one of three named Pseudomonas species on stainless steel. It adds microscopy and spatial information, but a two-species stainless-steel model cannot reproduce every mixed community or every material in use. Similar maximum thickness also should not be treated as evidence that the biofilms were functionally identical; thickness is only one physical descriptor.

Sanitizer experiments are likewise conditional. Concentration, contact time, temperature, application method, water chemistry, organic matter, pre-cleaning, surface condition and recovery method can affect an observed response. Culture-based enumeration quantifies recoverable organisms under the stated method; it does not reveal all viable states, matrix chemistry or the complete mechanism of tolerance. “Tolerance” in these papers is therefore an outcome under specified exposure conditions. It should not be casually relabelled as genetic resistance, permanent adaptation or failure of an entire sanitation programme.

What it does not prove

• It does not prove that Pseudomonadaceae always increase Listeria survival in every facility, on every surface or in every multi-species biofilm.
• It does not prove that a detected Pseudomonas population caused a Listeria persistence event in a real plant. Association in a constructed biofilm is not a field-causation study.
• It does not prove that BAC, PAA, neutral electrolysed water, chlorine dioxide or any other sanitizer is ineffective. The papers tested defined conditions and report comparative responses within them.
• It does not provide evidence about chlorine dioxide or ChloroKlean performance, compatibility, authorisation, dose, contact time or superiority. No such product claim follows from these studies.
• It does not establish one mechanism—such as matrix shielding or co-localisation—as the explanation for the 2025 observations.

Implications for evidence-led food hygiene

The operational lesson is to investigate the system, rather than to make a chemistry-only assumption. Environmental monitoring, hygienic design, physical cleaning, residue control and verification are all relevant when a surface repeatedly harbours biofilm-associated organisms. If Listeria or indicator organisms recur, species composition may be a worthwhile line of enquiry alongside review of equipment niches and cleaning execution.

This is also why claims need disciplined boundaries. A study using BAC, PAA and chlorine-based neutral electrolysed water can inform questions about mixed-species biofilm behaviour. It cannot validate an untested chlorine-dioxide approach. For background on the regulatory and application context that must be considered separately, see chlorine dioxide for PT4 food hygiene. For a wider discussion of mixed-community behaviour, see mixed-species biofilms and disinfectant tolerance.

The evidence is most useful when it prompts proportionate questions: which organisms are recurring, under what production conditions, on which surfaces, and after which cleaning steps? Answering those with site-specific investigation is more defensible than treating a laboratory association as a universal prediction.

References

1. Voloshchuk O, Rolon ML, Bartlett KV, Mendez Acevedo R and colleagues. Pseudomonadaceae increased the tolerance of Listeria monocytogenes to sanitizers in multi-species biofilms. Food Microbiology 128 (2025), 104687. PubMed record and abstract; publisher DOI record.
2. Rodríguez-López P, Rodríguez-Herrera JJ and López Cabo M. Architectural Features and Resistance to Food-Grade Disinfectants in Listeria monocytogenes-Pseudomonas spp. Dual-Species Biofilms. Frontiers in Microbiology 13 (2022), 917964. Full primary paper in PubMed Central; publisher DOI record.