According to the World Health Organization, up to 30% of people in industrialized countries suffer from foodborne illness every year.1 A significant source of infection is bacterial contamination of food contact surfaces in food processing settings. Since many pathogens possess the ability to form biofilms — matrices of extracellular polymeric substance that promote bacterial adhesion and protect pathogens from environmental stressors — demonstrate little efficacy against the biofilm architecture. Recently, Bridier et al. composed a review explicating primary mechanisms of biofilm formation and survival as well as novel control strategies.2
The heterogenous nature of the biofilm is key to its success. Interior microenvironments — which exist as three-dimensional layers of resources (oxygen, nutrients, metabolites, etc) — lend useful traits like biocide resistance or mechanical integrity to the community. This means that nutrient or oxygen depletion in one part of the biofilm can trigger gene-regulated expression of protective factors that increase resistance for the biofilm as a whole. Strident conditions like local oxidative stress can also boost genetic mutations in favor of variants with beneficial characteristics.
Because food processing environments house a broad range of bacteria, surface-associated communities often contain multiple species whose interactions shape the biofilm and lend it specific functions. In most cases, multispecies biofilms are more resistant to antimicrobials than monospecies biofilms since species with lesser resistance benefit from spatial protection — i.e. living in close proximity to species with enhanced resistance, and/or sharing matrix components.
Some species may even actively work together. Multi-species interactions can elicit increased production of extra-polymeric substances and/or physicochemical changes that enhance community resistance. Some distant bacterial relatives can “talk” directly and exchange materials (DNA, proteins, etc) through nanotubes. Researchers believe that this type of cooperation safeguards against adverse conditions. Understanding the mechanisms that underlie these interactions could assist researchers in targeting these functions for biofilm control.
Bridier et al. state that, while industry professionals may rely on regular disinfection techniques to control contamination of equipment and food products, this strategy is not generally effective against biofilms and can even lead to selection of resistant phenotypes. Some emerging options for targeting biofilms include eco-friendly biocides generated on-site (like ozone or acidic electrolysed water), natural plant extracts (such as essential oils), and enzyme-based detergents that disrupt the biofilm and release individual cells.
Biological control strategies include the application of natural or engineered bacteriophages, which diffuse through the biofilm and eliminate pathogens. One natural phage (LISTEX P100) has already received recognition by the United States Department of Agriculture for use as a processing aid against foodborne pathogens. Researchers can also harness hyper-swimming flagellated bacteria to tunnel through a biofilm, creating pores that enhance the efficacy of biocidal disinfectants.
Another approach aims at preventing biofilm adhesion and formation by modifying industrial surfaces. Examples include enhancing the hydrophobicity of polystyrene to elicit antibacterial properties, coating materials with lectin inhibitors or biosurfactants with anti-adhesive characteristics, and applying nano-materials (silver, cobalt, iron mixed oxides) to surfaces.
Finally, molecules antagonistic to the quorum-sensing system can be employed to disrupt bacterial communication and prevent biofilm formation. One potential target for this strategy is the cyclic-di-GMP pathway, which regulates cellular processes necessary for biofilm architecture. Thus far, brominated furanones have been specifically successful in antagonizing this system.
According to the authors, biofilm resistance to antimicrobials — including standard disinfectants — and potential for cross-contamination renders a thorough understanding of how biofilms function imperative for the optimization of disinfectant protocols for food industry applications. Standardized diagnostic guidelines for the detection of foodborne biofilms could assist this goal. In addition to culturing tools, some bacteria may require immunological or molecular techniques for detection. On the processing line, ex-situ coupons can enable the characterization of industrial biofilms via fluorescence in situ hybridization analysis. Molecular typing can also identify the origin of contamination and distinguish between persistent and non-persistent strains. This can be done using PCR-RFLP (restriction fragment length polymorphism), RAPD (random amplified polymorphism DNA) or PFGE (pulsed field gel electrophoresis).
Read the recent Culture article ‘Strategies to combat microbial hazards in fresh produce‘ for further discussion.
1 World Health Organization (2002) ‘Fact Sheet N237: Food Safety and Foodborne Illness.’
2 Bridier, A. et al. (2015) ‘Biofilm-associated persistence of food-borne pathogens.’ Food Microbiology 45: 167-178.