Advisor at Japan Food Research Laboratories
Human beings are social creatures. At the moment, 7.2 billions of people live on this planet. Some places are densely-populated, some are not. Very few people, I believe, live off the land away from other people. Some microorganisms float along by themselves, but others live in groups. Especially in water-supply and sewerage systems or rivers where nutrients are limited, microorganisms get together and help each other in order not to be swept away. Not only congeneric microorganisms but different species of them live helping each other in some cases as if they form a multicellular organism. Shingen Takeda, a famous feudal warlord, is quoted as saying that “Men are a castle, Men are a stone wall.” He learned from microorganisms, didn’t he?
Recent researches have revealed that microorganisms live in groups in an environment with low water content, too. Figure1 shows microorganisms adhered to alfalfa sprouts. Such a group living of microorganisms is called “biofilm”. In the United States, mass food-poisoning incidents caused by Salmonella or O157 transmitted by alfalfa sprouts happened in a row, and the Western and Eastern Regional Research Centers of Agricultural Research Service (ARS) conducted thorough researches on the countermeasures. Along the way it became clear that biofilms were formed on alfalfa sprouts as shown in Figure1 and they showed resistance to washing or microbiocide.
As shown in Figure1, microorganisms gain a foothold, gather peers, and start to spread filmily. They take a steric constitution under some conditions. Aerobic bacteria are arranged on the surface (outside) of the constitution, and anaerobic ones at the bottom (inside)
Same phenomenon happens inside animate beings. It is known that biofilms called “plaque” are deeply committed to the development of dental decay. Some food-poisoning or putrefactive bacteria form biofilms. Their detached bacteria will form their own biofilms at their destination, and this fission-fusion process will be repeated. Hygiene control that prevents biofilm formation is needed at both food-handling facilities (food factories, etc.) and home kitchens.
Biofilms are gradually formed out of fungus bodies and sticky substances such as polysaccharide produced by microorganisms. Biofilms will increasingly be resistant to thermal sterilization, sanitizers and bactericides. Heat or chemical agents might kill bacteria at the surface of biofilms, but bacteria inside will remain alive.
Many of gram-negative bacteria such as Escherichia coli produce capsules or mucous materials outside their bodies and tend to be origins of biofilms. Some gram-positive bacteria form biofilms, too. Table1 shows microorganisms to watch out for at food-handling facilities. Food-poisoning bacteria in the photos are in a floating situation called “plankton”. If their floating ends up in a growth environment more suitable for bilfilms, they turn themselves into one.
Wet locations are prone to be breeding grounds for biofilms. Biofilms can develop on stainless steels and plastics, too. They can form themselves inside pipes, on wood, stone and concrete. Just as alfalfa sprouts in Figure1, food materials such as vegetables or animals can develop biofilms, which will contaminate food products
Equipment at food-handling facilities should be made of materials that are resistant to biofilms. Residues of attached organic substances can be a starting point of biofilm formation. Basic measures are washing and microbiocide. Substances first attach to the surface of newly-introduced equipments, such as pipes, are not microorganisms but a little amount of organic substances (Figure2). In washing equipments, it is necessary to choose proper methods and washing agents. Combination use with physical washing on the surface such as brushing or high-pressure washing will increase the effect. Surfaces with scratches or dents require careful washing. In order to make the washing and microbiocide easier to maintain clean working environment, equipments should be easily-assembled/disassembled ones.
Kokubo, Y. ed., Genba de Yakudatsu Shokuhin Biseibutsu Q&A (Q&A on food microorganism useful on the spot), 3rd ed., Chuohoki Publishing, p.220 (2011)
W.F.Fett: J.Food Protection, 63(8),625-632(2000)
Tshuchido, T., Journal of the Food Hygienic Society of Japan, 43, J-283 (2002)