Wageningen, The Netherlands
June 18, 2007
Can lettuce grown on soil infected
by Salmonella bacteria itself be infected? Michel Klerks,
scientist at Plant
Research International, part of Wageningen UR, discovered
that Salmonella bacteria spread on the plant as well as within
the plant. Internal reproduction and spreading increase the risk
of food poisoning through lettuce consumption. Prevention of
Salmonella infection in lettuce plants and the application of
molecular detection methods during routine screening for
pathogens in the food production chain can reduce the risk of
food poisoning by eating contaminated fresh leafy vegetables.
This is the subject on which Michel Klerks will on 20 June take
his doctoral degree at Wageningen University.
Klerks investigated the physiological and molecular interactions
between Salmonella bacteria and lettuce varieties. He discovered
that Salmonella can actively move to the roots of the lettuce
plant. The bacterium then reproduces and spreads itself on the
plant. Spreading of the bacterium does not lead to visible
differences between healthy and infected soil-grown plants. The
natural defence mechanism of lettuce, however, is activated
during this process. Interesting fact is that Salmonella was not
only found on the plant but also within the plant itself.
In the field lettuce can be infected by using manure
contaminated by Salmonella- and E. coli bacteria. Earlier
research has shown that the risk of Salmonella infection can be
reduced by decreasing the amount of pathogens in dairy cattle
manure. This can be achieved by feeding cattle more hay or
straw. Breeding new lettuce varieties that are resistant against
the pathogens may further reduce the risk of infection. Hygienic
measures after harvest are of little use because washing does
not remove the bacteria from internally infected vegetables.
In his research Klerks also focussed on the development of more
sensitive and more accurate molecular detection methods to
determine small numbers of Salmonella bacteria and pathogenic E.
coli bacteria. Testing manure or infected soil for the extent of
Salmonella and/or E. coli infestation before seedlings are
planted out enables a good estimate of the risk of crop
contamination. The methods can also be applied after harvest in
the routine diagnosis of these pathogens in the food production
chain. The analytical time required for testing food products by
means of molecular methods is reduced to two days whereas
current standard procedures require five days.
The work of Klerks is part of the Wageningen UR research
programme headed by Ariena van Bruggen, professor Biological
Farming Systems of Wageningen University. The research is
financed by the Netherlands STW foundation for technology and
the Product Board for Horticulture and is carried out in
cooperation between the Biological Farming Systems Group and the
Louis Bolk Institute. Prof. Van Bruggen is also the thesis
supervisor of Michel Klerks.
On 20 June Michel Klerks will take his doctoral degree at
Wageningen University by defending his thesis Quantitive
detection of Salmonella enterica and the specific interaction
with Lactuca sativa. |
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