The role of L-arabinose metabolism for Escherichia coli O157:H7 in edible plants

Arabinose is a major plant aldopentose in the form of arabinans complexed in cell wall polysaccharides or glycoproteins(AGP), but comparatively rare as a monosaccharide. l-arabinose is an important bacterial metabolite, accessed by pectolyticmicro-organisms such as Pectobacterium atrosepticum via pectin and hemicellulose degrading enzymes. However, not all plantassociated microbes encode cell-wall-degrading enzymes, yet can metabolize l-arabinose, raising questions about their useof and access to the glycan in plants. Therefore, we examined l-arabinose metabolism in the food-borne pathogen Escherichiacoli O157:H7 (isolate Sakai) during its colonization of plants. l-arabinose metabolism (araBA) and transport (araF) genes wereactivated at 18°C in vitro by l-arabinose and expressed over prolonged periods in planta. Although deletion of araBAD did notimpact the colonization ability of E. coli O157:H7 (Sakai) on spinach and lettuce plants (both associated with STEC outbreaks),araA was induced on exposure to spinach cell-wall polysaccharides. Furthermore, debranched and arabinan oligosaccharidesinduced ara metabolism gene expression in vitro, and stimulated modest proliferation, while immobilized pectin did not. Thus,E. coli O157:H7 (Sakai) can utilize pectin/AGP-derived l-arabinose as a metabolite. Furthermore, it differs fundamentally in aragene organization, transport and regulation from the related pectinolytic species P. atrosepticum, reflective of distinct plantassociated lifestyles.