Attachment of Listeria monocytogenes to corn salad

Listeria monocytogenes is an important foodborne pathogen because of its high hospitalization and mortality rate. The Commission Regulation (EU) 2024/2895 tightens the detection limit for L. monocytogenes at the end of food's shelf life to not detectable in 25 g food. In this context, food hygiene measures and storage conditions need to be reconsidered. Little is known about how L. monocytogenes attaches to vegetables such as corn salad. Therefore, the aim of this study was to investigate the attachment and colonization of L. monocytogenes on corn salad and the ability of L. monocytogenes to grow and form biofilm. Sterile corn salad was cultivated and colonization of L. monocytogenes was imaged by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Two vegetable isolates and one of mammalian origin were tested for their potential to form biofilm in BHI and LB broth as well as in corn salad medium. For qualitative investigations, biofilm formation of the green fluorescent protein expressing strain NCTC 10887 was imaged via 3D-CLSM, and quantitative image analysis was performed with the “BiofilmQ” software. The biofilm formed in corn salad medium showed different architectural features than in BHI and LB broth. L. monocyotgenes colonized the leaf surface in a homogenous, undirected manner. The stomata of corn salad were densely colonized and the cells formed string-like structures. Furthermore, CLSM showed that layer formation of cells occurred which gives rise to the hypothesis that a biofilm was formed.