Part of the food microbiology pillar of the TAAG Learning Center. It connects with environmental monitoring in plants and with Listeria monocytogenes, the pathogen most associated with persistence in plants.
Table of contents
Bacteria do not live free-floating
In the laboratory we tend to picture bacteria as cells floating in a broth. In a food plant the most common situation is different: cells attached to a surface, grouped together and wrapped in a matrix that they produce themselves. That is a biofilm. A review published in Biofouling defines it as a community of microorganisms attached to a surface and surrounded by a complex matrix of extracellular polymeric substances.
The difference is not just one of form. A bacterium inside a biofilm tolerates desiccation, the flow of a rinse and disinfectants better than the same bacterium in suspension. That is why a line can look clean, pass a visual inspection and still keep releasing microorganisms onto the product.
What a biofilm is made of
Flemming and Wingender described in Nature Reviews Microbiology the biofilm matrix as a hydrated network of extracellular polymeric substances (EPS), made up mostly of polysaccharides, proteins, nucleic acids and lipids. That matrix serves several functions at once:
- It provides mechanical stability and anchors the cells to the surface.
- It forms a three-dimensional network that holds the cells together.
- It acts as an external digestive system: it retains enzymes close to the cells to take advantage of organic residues.
In a plant, that organic matter is abundant: remnants of milk, meat, juice or dough that remain in gaskets, valves and scratched surfaces after production.
How it forms, step by step
The review by Galié and colleagues in Frontiers in Microbiology summarizes the sequence in the food industry:
- Surface conditioning: proteins, fats and other food residues coat the material and change its properties.
- Reversible adhesion: the first cells approach and attach weakly; they can still be detached with a rinse.
- Irreversible adhesion and microcolonies: the cells produce matrix and begin to multiply in place.
- Maturation: a three-dimensional structure forms, with nutrient and oxygen gradients.
- Dispersal: some of the cells are released and colonize other surfaces or pass into the product.
The practical conclusion is that the time to act is early. Cleaning that arrives while the cells are still in the reversible phase is far more successful than cleaning that faces a mature biofilm. That is why cleaning frequency and the time a piece of equipment stays dirty between the end of production and the start of heavy sanitation weigh as much as the chemical chosen.
The dispersal stage also explains a pattern that many quality teams recognize: product contamination that appears intermittently, with negative results for weeks and an isolated positive with no apparent cause. An established biofilm releases cells irregularly, and that irregularity means sporadic sampling can miss it.
Where they appear in a plant
The same review mentions, in the dairy industry, pipes, butter centrifuges, cheese vats, pasteurizers and packaging tools, with species as different as psychrotrophic Pseudomonas and the thermophile Geobacillus stearothermophilus. In fresh fish it cites biofilms with Aeromonas hydrophila, L. monocytogenes, Salmonella enterica or Vibrio spp. The risks specific to each sector are developed in the article on microbiology in the dairy industry.
The material and its finish matter
Biofilms form on stainless steel, polyethylene, polypropylene, rubber, PVC, glass and wood. The surface finish changes the outcome: in the studies reviewed, untreated or sanded steel was the easiest for Salmonella to colonize, while electropolished steel was colonized very little. A scratched surface, a cracked gasket or a poorly finished weld offer refuge that no chemical product fully reaches.
Biofilm, persistence and Listeria
Listeria monocytogenes is the most studied example of persistence in plants: the same strain appears for months or years on the same equipment. Carpentier and Cerf, in a review in the International Journal of Food Microbiology, analyzed why this happens and reached a conclusion that changes the approach. The factors associated with the presence of Listeria in plants are those that favor its growth, and those conditions occur in so-called harborage sites: places protected by unhygienic design of equipment and facilities, or by damaged materials, that are difficult to eliminate.
The authors proposed that there are no L. monocytogenes strains with unique properties that make them persistent, but rather harborage sites where any strain can persist. They also argued that, when the initial load is low, cleaning and disinfecting early is the only way to prevent it from becoming established. For the quality team, the message is that persistence is fought mainly with hygienic design and maintenance, not just with a more powerful disinfectant.
Why it tolerates disinfectants better
The Biofouling review describes the resistance of biofilms to disinfection as a multifactorial process, linked to the spatial organization of the community. Among the most cited mechanisms are:
- The matrix as a barrier: the extracellular polymers hinder the diffusion of the biocide and can react with it before it reaches the cells.
- Physiological heterogeneity: oxygen and nutrient gradients create zones with slow-growing cells and persister cells, less sensitive to many agents.
- Mixed communities: Sánchez-Vizuete and colleagues reviewed in Frontiers in Microbiology how non-pathogenic bacteria can protect pathogens. One example: a Bacillus subtilis isolated from equipment protected Staphylococcus aureus against peracetic acid.
- Genetic adaptation: some L. monocytogenes strains isolated from plants carry genes such as qacH or bcrABC, pumps that expel quaternary ammonium compounds.
Mixed communities are the norm in plants, not the exception. Galié and colleagues cite a study in which the survival of E. coli O157:H7 on industrial surfaces increased up to sixfold in the presence of other species.
How it is controlled
Prevent before eliminating
Control starts with design: smooth, crack-free surfaces, gaskets in good condition, demountable equipment and no dead zones where product accumulates. Good Manufacturing Practices and preventive maintenance are, in practice, the first tool against biofilms.
Clean first, disinfect afterwards
A disinfectant applied over soil or over intact matrix loses a good part of its effect. That is why cleaning, with a suitable detergent and mechanical action or turbulence in closed circuits, has to remove the organic matter and break up the matrix before disinfection. The Galié review mentions among the disinfectants used in industry sodium hypochlorite, peracetic acid, chlorine dioxide, hydrogen peroxide, ozone and quaternary ammonium compounds.
Complementary tools
For difficult biofilms, enzymes that degrade the matrix (proteases, cellulases, glycosidases, DNases), bacteriophages such as P100 against Listeria, bacteriocins such as nisin and essential oils are studied and used. They are complements: none replaces a well-designed cleaning program.
How to know whether control is working
A biofilm is not visible to the naked eye until it is very large, so verification depends on sampling. Some guidelines:
- Sample after cleaning and disinfection, and also before production starts, as described in the article on post-sanitation surfaces.
- Include points that can be dismantled: gaskets, valve interiors, rollers, scrapers and drains, with the techniques of surface sampling.
- Treat repeated positives at the same point as a sign of a harborage site and not as an isolated finding, within the environmental monitoring program.
- When an isolate reappears, compare it with previous ones; genomic sequencing makes it possible to tell whether it is the same resident strain.
Biofilms do not only affect food safety. They also explain many recurring spoilage problems, such as those caused by the resistant yeasts described in the article on Zygosaccharomyces.
Conclusion
A biofilm turns a one-off contamination into a recurring problem: cells attached and protected by their matrix survive cleanings that would eliminate the same bacteria in suspension. The evidence points to persistence depending more on harborage sites than on special strains, so control combines hygienic design, maintenance, cleaning before disinfecting and sampling that looks where the biofilm really hides.
About TAAG
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Frequently asked questions
What is a biofilm in the food industry?
It is a community of microorganisms attached to a plant surface, such as stainless steel, rubber or plastic, and wrapped in a matrix of polysaccharides, proteins, nucleic acids and lipids that the cells themselves produce.
Why does a biofilm resist disinfectants more?
Because the matrix hinders the disinfectant from reaching the cells, inside the biofilm there are slow-growing cells that are less sensitive, companion species can protect pathogens and some strains carry tolerance genes for certain biocides.
Does Listeria persist in a plant because it has special strains?
The review by Carpentier and Cerf proposes that it does not: persistence is explained by harborage sites in equipment and facilities with unhygienic design or damaged materials, where any strain can grow and survive cleaning.
Is it enough to increase the disinfectant concentration?
No. Without prior cleaning that removes the organic matter and breaks up the matrix, the disinfectant loses effect. Control relies on hygienic design, maintenance, cleaning, disinfection and verification by sampling.