Biofilm is described as a are complex microbial ecosystems formed from one or several microbes bound in a complex extracellular matrix of their own secretions and formations. In earlier discussions in this biofilm series, we have talked about general formation of the complex ecosystems, the detection, classical and modern control methods of biofilms in these ecosystems. In this part, some key microbes identified as clinically significant in food borne pathogens will be summarised.
Infection by the microorganisms or intoxication by the toxins they secret are two main routes of food borne diseases linked to biofilms. Bacteria and fungi secret toxins in food matrices or in human or animal body. Some of the toxins can be fatal if not identified early and treated appropriately. Disease outbreaks and food poisoning are not uncommon. The level of risk and attendant healthcare challenge will depend on the type of microorganisms in the biofilm ecosystem and the toxins they secret.
What are the key microbes identified in food, processing and serving environment and their impact?
Bacillus cereus is an anaerobic or facultative anaerobic gram positive and spore-forming bacterium able to grow in diﬀerent environments over a wide temperatures range of about 4◦C to 50◦C. It is known to be resistant to heat, chemical treatments and radiation with persistent vegetative forms in food processing surfaces. B. cereus can to survive industrial pasteurisation processes due the endospore, which is capable of a later growth after a non-sporicidal treatment.
The persistence of the spores complicates eradication of the bioﬁlm with cleaning procedures. This is important in dairy factories where it can also reduce pasteurised milk and cream shelf-life leading to outbreaks of infections. Some strains produce toxins that cause diarrhoea and abdominal pain.
Enterohemorrhagic Escherichia coli
Human intestinal microbiota contains majority of E. coli strains without presenting any health problem. However, other noxious foodborne strains found in drinking water, fruits and vegetables including tomatoes, melons, parsley, cilantro, lettuce, spinach, etc., raw milk or fresh meat constitute adverse health conditions.
The carrier products could be contaminated from point of origin as part of the food manufacturing process. In the food industry, this contamination may take place during the pre-harvest period, due to the use of a contaminated water supply when cultivating the vegetables. Contamination may also occur in post-harvest environments, following washing and processing of the raw material such as carcasses, vegetables, etc. The storage temperatures encourage fast growth of certain pathogenic strains of E. coli in contaminated food and food materials.
Though not resistant to pasteurisation treatment, L. monocytogenes, is a gram-positive, ubiquitous and dangerous foodborne pathogen. Examples of food products known to transmit this pathogen include seafood, dairy products, meat, ready-to-eat products, fruits, soft cheeses ice-cream, unpasteurised milk, can died apples, frozen vegetables, and poultry. Common contaminated foods are smoked ﬁsh, cold cuts and fresh cheese.
L. monocytogenes bioﬁlms are mainly composed of teichoic acids able to grow on polypropylene, steel, rubber or glass surfaces throughout the industry and therefrom spreads to food batches, where it can replicate at refrigeration temperatures. Its ability to replicate at low temperatures enhances its hydrophilicity and induces bioﬁlm status as a response to cold temperatures, increasing its attachment to surfaces and its resistance to cleaning procedures in many food factories.
Resistance to treatments at up to 60◦C further complicates eradication strategies in food industries.L. monocytogenes may gastroenteritis in healthy individuals. Pregnant women, infants, the elderly and immunocompromised individuals, are known to be particularly susceptible to listeriosis, a critical disease which also involves septicaemia and meningitis. In pregnant women, listeriosis can lead to spontaneous abortion or damage to the foetus. Common contaminated foods are smoked ﬁsh, cold cuts and fresh cheese.
Some strains of S. enterica cause gastroenteritis or septicaemia when contaminated food is consumed. S. enterica serovar Enteritidis is recorded as the most frequent strain causing nausea, vomiting, fever, diarrhoea and abdominal pain as the main symptoms. Poultry meat is an identified common reservoir for these bacteria in processed food.
The biofilms can form on multiple types of surfaces including cooked, pre-cooked food, raw food and cross contamination is possible between manufacturing plants and supermarkets as with other gram-positives. Cross country outbreaks occur due to the international nature of some of the products particularly poultry and meat. Inter-country control measures may be necessary to ensure eradication.
Staphylococcus aureus is a Gram-positive, non-spore forming, non-motile, facultative anaerobic bacterium. It is a human opportunistic pathogen, largely due to its characteristic production of enterotoxins at temperatures between 10◦C and 46◦C. Strains of S. aureus are able to multiply on the mucous membranes and skin of food handlers, a major issue for food factories because staphylococcal enterotoxins are heat-stable and are secreted during growth of this bacterium in a food matrix, eventually contaminated by the food handler or an animal. The bacteria grow well in food with a low water activity, such as those with high sugar or salt content.
Methicillin-resistant S. aureus (MRSA) has been noticed in farm animals and in farm animal-derived foods, which are a primary contamination sources for this resistant pathogen. S. aureus is able to form bioﬁlms on many diﬀerent kinds of animal surfaces, whether biotic or abiotic surfaces along the food production chain. This factor is of considerable economic importance since the removal measure is diﬀerent and dependent on the inherent composition of the surface.
Potential microbial biofilm sources
Improperly preserved processed, cooked, pre-cooked and raw meat and dairy products can be a breeding ground for biofilms and the origin of contamination and potential poisoning from secreted toxins.
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