Abstract

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All over the world, vegetables are routinely eaten raw or partially exposed to heat as part of food garnishing. Used in these ways, these vegetables can become sources of infection with various food borne pathogenic organisms. This study assessed the contamination of vegetables and related environmental materials with L. monocytogenes and the thermal adaptive responses, heat resistance (D-values) and antibiotic resistance profile of the isolates. The bacterium was present in 92% of 250 vegetable and 65.52% of 290 environmental samples. Highest incidence occurred in leafy vegetables (100%) which also bore the heaviest load (>5log cfu g1). Among the environmental samples, highest incidence was found in soil (100%) with contamination load greater than 6log cfu g-1. These indicate that raw vegetables are of public health interest for transmission of L. monocytogenes and genesis of listeriosis. The mean thermal death times (D-value) of representative isolates were 1.82 min, 1.27 min and 56.4 sec at 60o, 65o and 70oC respectively in fish soup. Heat shocking L. monocytogenes cells at 45oC / 30 min resulted in significant increase in D-value at 60oC but not 65o and 70oC. Antimicrobial susceptibility profile of the isolates showed resistance to two or more antibiotics suggesting prior exposure of the organisms to antimicrobial agents. The public health implications of the findings are discussed.

Keywords: Listeria monocytogenes, D-value, heat shock, soup model, vegetable, Antibiotic susceptibility