Oxygen restriction feeds Listeria bacteria

By Charlotte Eyre

- Last updated on GMT

When Listeria pathogen are starved of oxygen,
they become 100 times more powerful, according to
a scientific study.

The study will give food manufacturers cause for concern, as safety measures used to prevent contaminated food from causing deadly diseases in the food chain may not be as reliable as previously thought. It may also give processors using modified atmosphere packaging (MAP) pause for thought, as the method reduces oxygen levels. The study was published in the current issue of BMC Microbiology journal. Scientists Bjarke Christensen and Tine Licht, together with colleagues from Denmark's National Food Institute, investigated whether the growth conditions of Listeria had an effect its virulence in the gut. Guinea pigs were fed food infected by Listeria monocytogenes, grown either in an oxygen-rich or oxygen free environment. Christenses and Licht then measured the presence of the bacteria in the intestine, liver and spleen after periods of four and seven days. Although oxygen-restriction does not help the bacteria to multiply in the gut, it speeds up its path into organs such as the liver and spleen, they said. Listeria grown under normal conditions was detected in the livers and intestines of two animals, and in the spleens of four animals, while Listeria grown under oxygen-restricted conditions prior to dosage was recovered from livers, spleens and intestines of 18, 12, and 14 animals respectively. Christenses and Licht concluded that cultures of L. monocytogenes under oxygen-restricted conditions were approximately 100 fold more invasive, and so more likely to cause infection. Fears over food-borne illnesses have increased over recently years, as studies have shown that conventional commercial washing and sanitising methods to remove microbial contaminants from produce surfaces are only marginally effective. In 2005, scientists from the US Agricultural Research Service (ARS) and the University of Navarra conducted experiments to understand how these microorganisms survive and grow on produce surfaces even after exposure to wash solutions like chlorine. They warned that bacteria often cling to food surfaces and form a "biofilm" - a hard to remove mass of microbes attached to a surface and to each other by complex sugars known as bacterial polymers. Systems thought by the industry to prevent food contamination include Leak-Master Mapmax micro-leak detection systems for rigid and flexible modified atmosphere packaging (MAP), which removes air from the space above food and flushes in inert gases such as carbon dioxide and nitrogen. However the new study suggests that processors may have to revise the way they use MAP. There are many opportunities for Listeria contamination during the food production process because the bacteria can occur in poultry, meat, dairy products, fruit and vegetables. It is particularly common in highly processed foods with long shelf-lives. Listeria are the bacteria responsible for listeriosis, a rare but lethal food-borne infection that causes septicemia, meningitis, stillbirth along with nausea, vomiting, and diarrhea. Listeriosis has a mortality rate of 25 per cent, compared to salmonella's mortality rate of one per cent. The bacteria are incredibly hardy and able to grow in temperatures ranging from 4°C, the temperature of a refrigerator, to 37°C, the body's internal temperature. Source: BMC Microbiology "Oxygen restriction increases the infective potential of Listeria monocytogenes in vitro in Caco-2 cells and in vivo in guinea pigs" ​Authors: J Bo Andersen, B B Roldgaard, B Bak Christensen, T R Licht

Related topics: Regulation & Safety

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