2.1 The plant environment as a habitat for human pathogens

Agricultural plants have become a source of human pathogens, especially emerging ones that belong to the group of Shiga toxin-producing Escherichia coli (STEC) strains. The threat of human pathogens in freshly consumableproducts of plant origin became apparent during the outbreak caused by E.coli O104:H4 in Germany and France in 2011, where almost 4000 persons became infected leading to 54 casualties and over 900 incidences of hemolytic uremic syndrome. The most likely transmission route of the pathogen to consumable products was remarkable as the source was Fenugreek seeds that were transported from Egypt to Rotterdam harbor, the Netherlands, approximately 17 months before appearance of the first disease incidences in Hamburg and the surrounding area. Although the pathogen was neither found in Fenugreek sprouts, nor in the seeds themselves, epidemiological facts revealed that the pathogen must have been associated with seeds over a relatively long period of time.

This raised the questions on how a human pathogen can persist as a viable entity in a hostile environmentfor such a long period and why similar observations had not been made before. Must this be considered as the first incidence of a disease outbreak caused by a human pathogenic bacterium that was adapted to the plant environment?To address this question, it must be referred to the nature of the causative agent that is atypical of pathogenic E. coli strains commonly occurring in Europe and the USA. TheE. coliO104: H4outbreak strain was an enteroaggregative strain that, unlike other typical E. coli O type strains, does not have animals as a majorreservoir, but only humans instead. Outbreaks caused by this type of pathogen are rare in Western societies, whereas those caused byE. coli O157:H7 and S. enterica are more common.This indicates that particular features in these human pathogens already exist, extending their life-time in the plant environment. The question is whether these features were intrinsic to particular subsets of human pathogens or were recently gained, e.g., via horizontal gene transfer. The strain that caused the outbreak in the Hamburg area must be considered as a highly virulent pathogen and it must have acquired its virulence and antibioticresistance traits via horizontal gene transfer events like transduction (phage infection) and conjugation (plasmid transfer). Acquisition of new virulence traits is one aspect in theevolution of a new pathogen, but selection pressure is another, and the outbreak strain must have evolved by increasing its virulence in humans side-by-side with improvement of its ecological competence in plants. The threat of this development is the emergence of new types of highly virulent human bacterial pathogens that are fully adapted to life near, or maybe inside agricultural plants.