Evidence of Transmission of Listeria monocytogenes in Seeds and Placentome of Red Capsicum annuum

Introduction: The saprophytic lifestyle of Listeria monocytogenes in growing plants has remained a relatively, unexplored area. Salad vegetables are a recognised vehicle for transmission of L. monocytogenes and the route of contamination is often assumed to be via surface water. An investigation of the source of contamination of chopped red bell peppers (Capsicum annum) within a local food processing factory found that the most likely site associated with L. monocytogenes contamination was the waste bins containing the seeds and placentome.

Purpose: The goals of this study were to survey commercial, fresh peppers in order to provide more evidence that the primary site of contamination of Capsicum annum was the seeds/placentome and to understand the mechanism of entry of the bacteria.

Methods:  Listeria monocytogenes strains were grown at 22^oC in TSB broth. Chemotaxic responses were investigated using agar plugs supplement with 20 mM of plant sugars. Cells were mixed with an equal amount of soft agar (0.6 %, [w/v]) to a final cell density of ~10⁵ cfu/ml. Chemo-attraction was detected by movement of the bacteria towards the sugar source. Extracts of Capsicum flesh were also tested. Listeria monocytogenes detection was carried out using the ISO 11290-1-1996 method. Flesh and seeds/placentome were tested, separately.

Results: A survey of bell peppers purchased from local shops (n =80) found that 5 peppers imported from The Netherland contained L. monocytogenes (5/40); but it was not detected in samples imported from Spain or Morocco (n=20 of each). The positive samples came from seed/placentome samples. The chemotaxis experiments suggests that the bacteria are repelled by the flesh of the peppers, possibly due the presence of capsaicinoids, but are attracted to the sugars contained within the placentome of the peppers.

Significance:   Isolating L. monocytogenes from the seeds of the bell peppers and not from the flesh suggest that the bacteria can penetrate the fruit.