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Salmonella and E. coli O157:H7 bacteria can survive on lettuce

Researcher Inge van der Linden confirmed in her promotion research that Salmonella and E. coli O157:H7 can survive on lettuce seeds, in the irrigation water of greenhouses and on head lettuce from the Belgian greenhouse cultivation. But Van der Linden proved through genetic research that the bacteria are mainly found in the inner crop part of the lettuce, as the conditions there are reasonably damp and constant. The biggest risk of a food infection is therefore present if the lettuce plants are contaminated just before the harvest. Van der Linden hopes to graduate from the University Gent on the 9th of October

An increase in the number of food infection after consuming fruit and vegetables has been noted over the last few years. In many cases this was due to Salmonella and E. coli O157:H7, two pathogenic bacteria that are mainly associated with animal products such as meat, eggs and dairy products. Although most reports are from the United States, Europe is not free of incident. The 'EHEC crisis' in France and Germany in 2011 was caused by sprouts contaminated by E. coli O104:H4, an until then lesser known pathogenic relation of E. coli O157:H7. In Belgium head lettuce is an economically important vegetables, but until now little was known about possible infection with Salmonella and E. coli O157:H7.

Pathogen can survive for a long time

Inge Van der Linden has mapped out which factors can aid contamination during the cultivation in the greenhouse as a first step. She did this by infecting seeds, irrigation water and growing lettuce plants artificially and in a controlled environment. The test results confirmed that both seeds and irrigation water can form a possible source of contamination. A more important finding was that the pathogens can survive for longer than two years on lettuce seeds and increase in number when they sprout. In contamination through irrigation water it was shown that the survival of the pathogens was strongly dependent on the differences in water quality between the various greenhouses.

Bacteria survives especially well in head

In the second part of the research the survival of the bacteria on the lettuce plant was studied. The bacteria were applied to the leaves of the lettuce plants, after which their survival was studied on various leaves of the head of lettuce and on both young and almost harvest ripe lettuce plants. The survival and growth of the bacteria on the lettuce leaves was very variable, and was dependent on the growing stages of the plant, and mainly on the environmental conditions, particularly the moisture. The bacteria appeared to survive well within the head, where it is relatively moist. Changes in conditions reduced the survival: in a laboratory test with a constant high air moisture content the bacteria survived on the lettuce, whereas this was much less the case in greenhouse conditions when there are more fluctuations.

Expression of pathogens reduces

In the third phase of the research Van der Linden studied the genetic mechanisms that the bacteria use to survive on the lettuce. Gene expression experiments brought to light that E. coli O157:H7 is in a stressed condition on the plant, and that there was a reduction in the expression of various genes that regulate the pathogenic capability, which includes the genes for the Shiga toxins. These are the substances that can cause symptoms such as kidney failure and bloody stool in an advanced stage of food infection.

Source: ILVO
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