Glutamate in foods increases food poisoning by protecting bacteria from acid

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http://www.sciencedaily.com/releases/2010/09/100905231235.htm

Talented Bacteria Make Food Poisoning Unpredictable
ScienceDaily (Sep. 6, 2010) While we are often exposed to bacteria in
our food which could cause food poisoning, we don't always become ill --
why should this be so?
Professor Colin Hill, who is presenting his work at the Society for
General Microbiology's autumn meeting in Nottingham, describes how
bacteria use different tricks to aid their survival inside the body,
helping to explain why food poisoning can be so unpredictable.
One of the biggest challenges faced by food-borne bacteria is acid.
Acidic conditions, particularly in the stomach and in the gut will kill
most microbes found in contaminated food.
Professor Hill's group at University College Cork has revealed that
Listeria bacteria, which may be found in soft cheeses and chilled
ready-to-eat products, can overcome harsh acidic conditions by
exploiting key food ingredients. Listeria that survive are able to cause
serious and sometimes fatal infections, particularly in the elderly and
pregnant women.
Certain food constituents such as the amino acid, glutamate, can help
the bacteria neutralise acid, allowing the bacteria to pass through the
stomach unscathed. Professor Hill explains the significance of this.
"People who consume foods that are contaminated with Listeria and are
also high in glutamate, such as soft cheese or meat products, have a
higher chance of developing serious infection than someone eating the
same quantity of bacteria in a low-glutamate food," he said. "Of course
this is further complicated by the fact that a contaminated,
low-glutamate food could be eaten in combination with a high-glutamate
food such as tomato juice, which could also increase the risk of
infection."
Listeria can also take advantage of food processing and storage
conditions to help them survive. "Bacteria that are exposed to low pH
before entering the body may adapt to become more acid-tolerant and
therefore better equipped to deal with acidic conditions in the body.
For example, Listeria contaminating naturally acidic foods such as
cheese may be more likely to cause infection than Listeria carried at a
more neutral pH in water.
Professor Hill explains how his group's work could help reduce the
incidence of Listeria infections. "The number of cases of listeriosis
has nearly doubled in the last decade in Europe. This is because the
bacterium is so good at overcoming the challenges it faces in food and
in the body," he said. "Our studies show that consuming Listeria in one
food may be quite safe, while eating the same amount in another food
might be lethal. By understanding the role of the food matrix we may be
able to identify and eliminate high-risk foods from the diet of
susceptible people."