Phages offer extra security for processors
A natural approach to controlling Listeria may be an appealing option for processors.
“We need to reduce costs,” echoes like a daily mantra in processing plants around the world. With budgets being squeezed to suffocation, could food safety fall victim to the global credit crunch in the hunt for cheaper pathogen control?
The risks are amplified when cost efficiencies result in reformulation, as well as potentially cheaper sources of raw material, both of which will demand new efficacy tests for Listeria-fighting products, such as becteriocins and essential oils. For a pathogen such as Listeria monocytogenes, these circumstances present an ideal opportunity to multiply.
Not to be ignored
According to the US Department of Health and Human Services, an estimated 2,500 people become seriously ill with listeriosis each year in the USA, of which 500 die as a result.
The statistics do not show Europe any more favourably. The European Food Safety Authority (EFSA) and the European Centre for Disease Prevention and Control (ECDC) highlight that the number of human listeriosis cases rose by 8.6% to reach 1,583 in 2006, with the number of cases per 100,000 having risen by 59% over the preceding five years. Ready to eat foodstuffs, such as cheeses, fish and meat/poultry products were the oprigins of most human infections.
For any company already under financial stress, a voluntary recall could be grave, and an outbreak of listeriosis traced to the processing plant could be a disaster. In the current climate, control methods that can be fully relied on, come reformulation or reduced staff, could be judged a priority.
Listeria contamination of cooked, fresh and ready to (h)eat products can take place any time between slaughter, quartering, processing and packaging. De-boning machines, bowl choppers, vacuum packers, slicers, grinders, meat saws, ice machines, smokehouses, filleting machines, skinning machines, and many other types of food processing machinery can all harbour Listeria. Even an insignificant (and legal) level of bacteria on a finished chicken product can bloom in a consumer’s refrigerator or survive in a microwave oven.
Whilst organic acids, nisin and similar compounds inhibit the outgrowth of Listeria and even kill the bacteria, as ingredients they can affect the organoleptic qualities of the end-product and may not work as well in some products as in others. For example, when reformulating a ready-to-eat chicken slice or turkey burger, taste issues may need to be addressed to mask the bacteriocin – all of which equals time, effort and money and there is, therefore, a trade-off between quality and food safety.
Bacteriophages (phages) may be an alternative approach. These micro-organisms attack only bacteria, and do not affect mammalian or plant cells.
Phages are estimated to be the most widely distributed entities in the biosphere, and humans are routinely exposed to them at high levels through food and water without adverse effect. They are, in fact, already common residents of the intestines.
On many food products, more than 100 million viable phages per gram can often be identified, and high numbers are routinely consumed through food without any impact on taste or product quality.
Much smaller than the bacteria they destroy, phages enter a host cell by attaching to receptors on the bacteria cell surface. As each phage recognizes a particular receptor, it is specific only to a certain type of bacteria.
Once attached to its particular host bacterium, the phage punctures the cell wall and releases its genetic material into the bacterium. The host’s normal synthesis of proteins and nucleic acids is disrupted and the phage is replicated within the bacterial cell, eventually leading to cell lysis and destruction of the bacterium. The replicated phages are released and can go on to seek more of the same bacterial cells. If a phage is unable to connect to a host cell, it will break down into common biological particles that are naturally absorbed back into the environment.
EBI Food Safety of Wageningen, The Netherlands, has taken advantage of this highly specific solution. Its concentrated anti-Listeria phages-containing Listex has FDA and USDA GRAS status for use on all food products susceptible to Listeria. The product can be sprayed or added as a dip at the critical point of contamination, whereby the phages, depending on the level of contamination, will kill most, if not all, of the Listeria bacteria.
EBI points to experiments conducted by the Ghent University Laboratory of Food Microbiology and Preservation. One of the experiments tested the lytic activity of Listex against L monocytogenes on vacuum-packaged cooked chicken fillet stored at 7C – the equivalent of a fridge in the home that is not working well and where warmer conditions can cause outgrowth at low levels of bacteria that were only inhibited in the processing plant rather than killed.
The industrially prepared and sliced cooked chicken fillet was treated in three ways: non-inoculated product (control); product inoculated with a cocktail of three L monocytogenes strains at 10 cfu/g; and, product inoculated with the same three strain cocktail of L monocytogenes at 10 cfu/g and subsequently treated with Listex at 1x107 pfu/cm2.
Inocluted products were vacuum packaged and stored at 7±1C. At regular intervals during storage, samples were analyzed to determine the number of lactic acid bacteria, the presence of L monocytogenes (in25g) and/or the number of L monocytogenes.
The results showed that addition of Listex at 1x107 pfu/cm2 resulted in a significant reduction of the L monocytogenes count with 3.32 log10 (cfu/g) compared to the untreated control after 21 days of storage.