The availability of knowledge of the route of infection and critical plant and microbe factors influencing the colonization efficiency of plants by human pathogenic bacteria is essential for the design of preventive strategies to maintain safe food. This research describes the differential interaction of human pathogenic Salmonella enterica with commercially available lettuce cultivars. The prevalence and degree of endophytic colonization of axenically grown lettuce by the S. enterica serovars revealed a significant serovar-cultivar interaction for the degree of colonization (S. enterica CFUs per g leaf), but not for the prevalence. The evaluated S. enterica serovars were each able to colonize soil-grown lettuce epiphytically, but only S. enterica serovar Dublin was able to colonize the plants also endophytically. The number of S. enterica CFU per g of lettuce was negatively correlated to the species richness of the surface sterilized lettuce cultivars. A negative trend was observed for cultivars Cancan and Nelly, but not for cultivar Tamburo. Chemotaxis experiments revealed that S. enterica serovars actively move toward root exudates of lettuce cultivar Tamburo. Subsequent micro-array analysis identified genes of S. enterica serovar Typhimurium that were activated by the root exudates of cultivar Tamburo. A sugar-like carbon source was correlated with chemotaxis, while also pathogenicity-related genes were induced in presence of the root exudates. The latter revealed that S. enterica is conditioned for host cell attachment during chemotaxis by these root exudates. Finally, a tentative route of infection is described that includes plant-microbe factors, herewith enabling further design of preventive strategies.

This paper describes the physiological and molecular interactions between the human-pathogenic organism Salmonella enterica serovar Dublin and the commercially available mini Roman lettuce cv. Tamburo. The association of S. enterica serovar Dublin with lettuce plants was first determined, which indicated the presence of significant populations outside and inside the plants. The latter was evidenced from significant residual concentrations after highly efficient surface disinfection (99.81%) and fluorescence microscopy of S. enterica serovar Dublin in cross sections of lettuce at the root-shoot transition region. The plant biomass was reduced significantly compared to that of noncolonized plants upon colonization with S. enterica serovar Dublin. In addition to the physiological response, transcriptome analysis by cDNA amplified fragment length polymorphism analysis also provided clear differential gene expression profiles between noncolonized and colonized lettuce plants. From these, generally and differentially expressed genes were selected and identified by sequence analysis, followed by reverse transcription-PCR displaying the specific gene expression profiles in time. Functional grouping of the expressed genes indicated a correlation between colonization of the plants and an increase in expressed pathogenicity-related genes. This study indicates that lettuce plants respond to the presence of S. enterica serovar Dublin at physiological and molecular levels, as shown by the reduction in growth and the concurrent expression of pathogenicity-related genes. In addition, it was confirmed that Salmonella spp. can colonize the interior of lettuce plants, thus potentially imposing a human health risk when processed and consumed.

Three randomly amplified polymorphic DNA (RAPD) markers, OPA-12420, OPB-061200 and OPA-01700, species specific to the root-knot nematode species Meloidogyne arenaria, M. incognita and M. javanica respectively, were identified. After sequencing these RAPD-PCR products, longer primers of 18 to 23 nucleotides were designed to complement the terminal DNA sequences of the DNA fragments. This resulted in three pairs of species specific primers that were used to amplify the sequence characterised amplified regions (SCARs). The developed sets of SCAR primers were successfully used in straightforward, fast and reliable PCR assays to identify M. incognita, M. javanica and M. arenaria. The length variant SCAR markers can be amplified from DNA from egg masses, second stage juveniles and females. This species identification technique is therefore independent of the nematode's life cycle stage. Moreover the SCAR-PCR assay was successfully applied using DNA extracts from infested plant material. The method has potential to be optimised for routine practical diagnostic tests facilitating the control of these economically important pest organisms. Identification de Meloigyne incognita, M. javanica et M. arenaria au moyen de l'amplification de régions de séquences caractéristiques (SCAR) par une technique PCR - Trois marqueurs d'ADN polymorphique amplifiée au hasard (RAPD) OPA-12420, OPB-O61200 et OPA-OI700, respectivement spécifiques des espèces de nématodes Meloidogyne arenaria, M. incognita et M. javanica, ont été identifiés. Après le séquençage de ces produits RAPD-PCR, les amorces les plus longues de 18 à 23 nucléotides ont été choisies pour compléter les séquences terminales d'ADN des fragments d'ADN. Cela a conduit à trois paires d'amorces spécifiques de l'espèce, utilisées pour amplifier les régions des séquences caractéristiques (SCAR). Les lots d'amorces SCAR mis au point ont été utilisés avec succès lors d'essais directs, rapides et surs pour identifier M. incognita, M. javanica et M. arenia. Les marqueurs peuvent être amplifiés à partir de l'ADN des masses d'oeufs, des juvéniles de deuxième stade ou des femelles. Cette technique d'identification spécifique est donc indépendante des différents états de développement du nématode. De plus la technique SCAR-PCR a été appliquée avec succès à l'ADN extrait du matériel végétal infesté. Cette méthode présente des potentialités d'amélioration permettant d'envisager des tests pratiques d'identification de routine, facilitant ainsi le contrôle de ces parasites économiquement importants.

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