Possibilities for Contamination of Tomato Fruit by <i>Listeria monocytogenes</i> during Cultivation

Abstract

Outbreaks of food-borne illness caused by Listeria monocytogenes in or on fresh produce are sometimes reported. Tomatoes have been considered as one of the most implicated vehicles for produce-associated outbreaks. In the present paper, using tomato plants and three isolates of L. monocytogenes showing different serotypes (1/2a, 1/2b, 4b), viability and injury of L. monocytogenes in soil and in or on tomato plants during cultivation was investigated. Soil was artificially contaminated with L. monocytogenes at levels of 2, 4, 6 or 8 log CFU/g, followed by cultivation of tomato plants in the contaminated soils. The population of L. monocytogenes in the soil decreased to less than the detection limit (< 2 log CFU/g) 6 – 8 weeks after the start of cultivation. L. monocytogenes was not detected in any harvested fruit after a 16-week cultivation, although it was detected qualitatively from soil samples. Artificial injury of the root did not induce contamination of tomato fruit by L. monocytogenes via vessels. Thus, the possibility of internalization or contamination of tomato fruit by L. monocytogenes from contaminated soil is considered quite low during cultivation. However, L. monocytogenes on the fruit surface survived up to 2 – 3 weeks. Regardless of contamination level, hygienically inappropriate handling by workers might lead to contamination of tomato fruit by bacteria such as L. monocytogenes and thus to food poisoning.