There is considerable interest in the formation of biofilms on equipment in the food industry, because of their role in microbial contamination of processed foods, including dairy products.
What is a biofilm?
Biofilms are three dimensional aggregations of microorganisms attached to surfaces. Bacteria in the biofilm join together and form a protective matrix around each other. It is estimated that up to 90% of microbial populations exist as biofilms, rather than as discrete organisms (planktonic cells) floating around in the environment.
We experience biofilms every day. The plaque on teeth is a form of biofilm, as is the slime in a water bottle, or in
the shower or your sink drainpipe. Biofilms have a big impact on human health, as biofilms can also colonise artificial heart valves, catheters, and hip-replacements.
Bacterial cells may detach and be dispersed from an established biofilm, and may rapidly multiply and contaminate adjacent surfaces and foods.*
What organisms are involved in biofilms?
A range of microorganisms can form biofilms – including both pathogenic and spoilage microorganisms. In the dairy industry we are particularly concerned about biofilms that contain organisms such as Salmonella spp., Staphylococcus aureus, Bacillus cereus, Escherichia coli, and Listeria monocytogenes. For example, Salmonella readily forms a biofilm on food contact surfaces and this contributes to the long-term persistence of this organism in food processing environments. Such a film then acts as a reservoir for recurrent Salmonella contamination of food products.
Bacteria in biofilms do not generally exist in pure culture, instead there is an amazingly diverse range of microorganisms in biofilms and they sometimes establish symbiotic relationships. The organisms communicate by means of a process called quorum sensing, where electronic signals between cells facilitate the production of the extracellular polymeric substances (EPS) that form a sticky gel holding the biofilm together.
Where do biofilms form?
In the food industry, biofilms can form on any surface (stainless steel, wood, plastic, epoxy resins, etc). The biofilm protects its resident microorganisms from the effects of sanitisers, and other antimicrobial substances. Depending on the microorganisms present, biofilms may increase the risk of a food becoming contaminated by pathogens or reduce product shelf-life due to the activity of spoilage organisms. Biofilms may also lead to corrosion on metal surfaces, blockages of pipes, and reduced heat transfer.
In dairy processing plants, biofilms may occur on any wet surface, including in storage tanks, on conveyor belts, in packaging equipment, in drains, and on trolley wheels. The challenge to control these films is to hunt down all possible environments and niches.
How can I prevent the formation of biofilms?
The control of biofilms starts with an effective cleaning procedure. Cleaning must break up or dissolve the EPS matrix associated with the biofilm, to physically remove the film, and to enable sanitising agents to gain access to any remaining viable cells. Some of the methods used to control biofilm formation include mechanical and manual cleaning, chemical cleaning, and the application of hot water. The effectiveness of chemical sanitisers can be limited by the presence of soil, water hardness, temperature of application, and the ability to physically make contact with the surviving microorganisms.
Are all sanitisers effective against biofilms?
Unfortunately the scientific literature shows that sanitiser activity against biofilms can be inconsistent. Reports vary on the bacteriocidal efficacy of acidic peroxygen-based disinfectants, hypochlorite, and benzalkonium chloride products. These agents have different mechanisms of action and different pH, and this contributes to variations in action and lethality.
What is the best strategy for managing biofilms?
The important issues for the dairy industry are to recognize the importance of biofilms; to know your plant well enough to know where films may be forming; and to put in place cleaning and sanitation regimes which limit the possibility of biofilms forming, persisting, and contaminating products. Environmental monitoring for specific pathogens, such as Listeria spp. will enable the processor to determine how well they are doing.
When looking for sites where biofilms may have formed, it is important to get down and under equipment and actively look for visible evidence of films. It may be necessary to dismantle complex equipment in order to locate hard to find niches where films may form. Swab those difficult to reach and clean areas, and check that that physical and chemical cleaning of equipment is sufficiently effective to remove biofilms.
Marchand, S et al., (2012). Biofilm Formation in Milk Production and Processing Environments; Influence on Milk Quality and Safety. Comprehensive Reviews in Food Science and Food Safety, Vol. 11, Issue 2, pp 133–147.
Chmielewski, R.A.N. and Frank, J.F. (2003). Biofilm Formation and Control in Food Processing Facilities. Comprehensive Reviews in Food Science and Food Safety, Vol. 2, Issue 1, pp 22–32.
Proal A, (2008). Understanding Biofilms. Bacteriality− Exploring Chronic Disease.
*Image: Montana State University, Center for Biofilm Engineering
Schematic diagram showing the life cycle of a biofilm (including detachment and dispersal)