Starving Listeria bug of oxygen makes it turn nasty

If the Listeria monocytogenes bacteria behind food poisoning are starved of oxygen, they are liable to turn really nasty according to research published in the online journal BMC Microbiology.

Limiting oxygen produces bacteria up to 100 times more invasive than similar bacteria grown with ample oxygen supplies.

Bjarke Christensen and Tine Licht together with colleagues from Denmark's National Food Institute set out to investigate whether the growth conditions of Listeria bacteria just prior to being eaten had an effect on their virulence once absorbed by the gut. Guinea pigs were fed food laced with L. monocytogenes, grown either in an oxygen-rich atmosphere, or starved of oxygen. The team used fluorescent labelling to tell the bacteria strains apart.

Bacterial oxygen restriction increased the number of animals carrying L. monocytogenes in their internal organs, although it did not affect the actual number of bacteria infecting each organ. It seems that oxygen restriction smoothes the bacteria's initial path from the gut into organs including the jejunum, liver and spleen, but does not help the bugs to multiply on arrival.

With better success surviving the gastric barrier, the oxygen-restricted bacteria have a greater chance of causing infection. The authors suggest that oxygen restriction may lead to increased levels of InteralinA (InlA) protein on the invading bacteria's cell walls, InlA being a key factor in L. monocytogenes virulence.

These findings are particularly relevant to help assess the risk of Listeria in food, especially the highly processed foods with long shelf lives that are popular breeding grounds for Listeria.