Browsing by Subject "Soil adherence"

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Uptake of enterohemorrhagic Escherichia coli into the roots of lettuce plants
(2020) Eißenberger, Kristina; Schmidt, Herbert
Within the last 10 years, the annual numbers of human infections with enterohemorrhagic Escherichia coli (EHEC) in Germany increased by a factor of 2.4. The peak was reached during the large German outbreak in 2011. Intriguingly, the source of the outbreak was supposedly traced back to organic fenugreek sprouts. Moreover, the number of EHEC outbreaks traced back to plant-based foods, e.g. fresh produce, increased also in the United States. This trend poses a serious threat to public health as fresh produce is mostly consumed raw. Also, these observations gave rise to investigate the interactions of plants and human pathogens in more detail especially as fresh produce may be contaminated directly on the field. In the present thesis, the capability of different EHEC strains and an enteroaggregative/enterohemorrhagic E. coli (EAEC/EHEC) strain, to adhere to and to internalize into the roots of different lettuce plants was investigated. These studies conducted within the scope of this dissertation focused on different aspects of the mentioned processes, such as different bacterial strains, the bacterial genetic equipment, and different environmental conditions, such as plant variety, soil type used for plant growth, and the soil microbiota. To mimic the natural conditions as close as possible, plants were grown from unsterile plant seed in unsterile soil under greenhouse conditions. In the first publication, the overall ability of EHEC O157:H7 strain Sakai to adhere to and internalize into the roots of Valerianella locusta, also known as lamb’s lettuce, grown in diluvial sand soil was described. It was demonstrated that EHEC O157:H7 strain Sakai is indeed able to attach to and internalized into the lettuce roots under the conditions tested. Moreover, this paper shed light on potentially important intrinsic bacterial factors, i.e. genes/proteins, which are putatively involved in adherence and/or internalization. Therefore, deletion mutants lacking hcpA and/or iha, were also investigated regarding adherence to and internalization into the lamb’s lettuce roots. Both genes, coding for the major subunit of the hemorrhagic coli pilus HcpA and the adhesin Iha, respectively, are supposed to be associated with adherence and therefore called “adherence factors”. However, deletion mutants lacking one or both of these genes did not show significant differences in root attachment compared to the wild-type strain. Regarding internalization, deletion of either of these genes resulted in significantly lower numbers of internalized bacteria clearly indicating that both of these genes – or the proteins encoded by these genes – play an important role during invasion of EHEC O157:H7 strain Sakai into the roots of lamb’s lettuce. Interestingly, deletion of both genes did not result in further reduction of internalization compared to single deletion mutants. Hence, hcpA and iha encode rather internalization factors than adherence factors. Moreover, internalization does not solely depend on these two factors. The second paper focused on the influence of lettuce varieties and soil type on adherence and internalization of E. coli O104:H4 strain C227/11φcu. In this study, the lettuce varieties Valerianella locusta and Lactuca sativa, also known as lamb’s lettuce and lettuce, respectively, were both grown in two different soil types, diluvial sand (DS) and alluvial loam (AL), to address the impact of plant host and environment on bacterial attachment and invasion into lettuce roots. To approach the latter aspect in more detail, the composition of the soil microbial community was analyzed in parallel by partial 16S rRNA gene sequencing. Adherence to the roots was positively influenced by the soil type as the number of adherent E. coli O104:H4 strain C227/11φcu bacteria significantly rose by a factor of three to four when the plants were grown in DS compared to AL. However, when grown in the same type of soil, no statistically significant differences in attachment were detected between the distinct lettuce varieties. On the other hand, internalization significantly differed predominantly between the two types of lettuce. Internalization into the roots of L. sativa compared V. locusta was found to be increased by a factor of 12 upon growth in DS, and by a factor of 108 when the plants were grown in AL. Moreover, internalization into the roots of L. sativa was five-times higher in AL than in DS. Consequently, the lettuce variety significantly influences to ability of E. coli O104:H4 strain C227/11φcu to internalize into the lettuce roots, while the soil type affected bacterial invasion only at the roots of L. sativa under the conditions tested. Moreover, by microbiota analysis, the inoculated strain was found within the soil microbiota, and this analysis demonstrated that soil type, lettuce variety, and the combination of both result in large differences in the composition of the soil microbiota.