Salmonella enterica is a foodborne pathogen often leading to gastroenteritis and is commonly acquired by consumption of contaminated food of animal origin. However, frequency of outbreaks linked to the consumption of fresh or minimally processed food of nonanimal origin is increasing. New infection routes of S. enterica by vegetables, fruits, nuts, and herbs have to be considered. This leads to special interest in S. enterica interactions with leafy products, e.g., salads, that are mainly consumed in a minimally processed form. The attachment of S. enterica to salad is a crucial step in contamination, but little is known about the bacterial factors required and mechanisms of adhesion. S. enterica possesses a complex set of adhesive structures whose functions are only partly understood. Potentially, S. enterica may deploy multiple adhesive strategies for adhering to various salad species and other vegetables. In this study, we systematically analyzed the contributions of the complete adhesiome, of lipopolysaccharide (LPS), and of flagellum-mediated motility of S. enterica serovar Typhimurium (STM) in adhesion to Valerianella locusta (corn salad). We deployed a reductionist, synthetic approach to identify factors involved in the surface binding of STM to leaves of corn salad, with particular regard to the expression of all known adhesive structures, using the Tet-on system. This work reveals the contribution of Saf fimbriae, type 1 secretion system-secreted BapA, an intact LPS, and flagellum-mediated motility of STM in adhesion to corn salad leaves. IMPORTANCE Transmission of gastrointestinal pathogens by contaminated fresh produce is of increasing relevance to human health. However, the mechanisms of contamination of, persistence on, and transmission by fresh produce are poorly understood. We investigated the contributions of the various adhesive structures of STM to the initial event in transmission, i.e., binding to the plant surface. A reductionist system was used that allowed experimentally controlled surface expression of individual adhesive structures and analyses of the contribution to binding to leave surfaces of corn salad under laboratory conditions. The model system allowed the determination of the relative contributions of fimbrial and nonfimbrial adhesins, the type 3 secretion systems, the O antigen of lipopolysaccharide, the flagella, and chemotaxis of STM to binding to corn salad leaves. Based on these data, future work could reveal the mechanism of binding and the relevance of interaction under agricultural conditions.

27Salmonella enterica is a foodborne pathogen leading to gastroenteritis and is commonly 28 acquired by consumption of contaminated food of animal origin. However, numbers of 29 outbreaks linked to the consumption of fresh or minimally processed food of non-animal origin 30 are increasing. New infection routes of S. enterica by vegetables, fruits, nuts and herbs have to 31 be considered. This leads to special interest in S. enterica interactions with leafy products, e.g. 32 salads, that are consumed unprocessed. The attachment of S. enterica to salad is a crucial step 33 in contamination, but little is known about the bacterial factors required and mechanisms of 34 adhesion. S. enterica possesses a complex set of adhesive structures whose functions are only 35 partly understood. Potentially, S. enterica may deploy multiple adhesive strategies for adhering 36 to various salad species, and other vegetables. Here, we systematically analyzed the 37 contribution of the complete adhesiome, of LPS, and of flagella-mediated motility of S. enterica 38 serovar Typhimurium (STM) in adhesion to corn salad. We deployed a reductionist, synthetic 39 approach to identify factors involved in the surface binding of STM to leaves of corn salad with 40 particular regard to the expression of all known adhesive structures using the Tet-on system. 41 This work reveals the contribution of Saf fimbriae, type 1 secretion system-secreted BapA, an 42 intact LPS, and flagella-mediated motility of STM in adhesion to corn salad leaves. 43 44 Importance 45 Human gastrointestinal pathogens are often transmitted by animal products, but recent 46 outbreaks show increasing importance of vegetables as source of infection by pathogenic E. 47 coli or Salmonella enterica. The mechanisms of binding of S. enterica to vegetables such as 48 salad are only poorly understood. We established an experimental model system to 49 systematically investigate the role of adhesive structures of S. enterica serovar Typhimurium 50 in binding to corn salad leaves. The contributions of all members of the complex adhesiome, 51 24.11.2019S. Typhimurium adhesion to corn salad 3 flagella, and O-antigen were evaluated. We identified that Saf fimbriae, type 1 secretion system-52 secreted BapA, an intact LPS, and flagella-mediated motility contribute to adhesion of 53 Salmonella to corn salad leaves. These results will enable future investigations on factors 54 contributing to contamination of vegetables under agricultural conditions. 55 56 24.11.2019 S. Typhimurium adhesion to corn salad Furthermore, SR11 ∆12 strains with additional deletions of single adhesive structures showed 131 no altered levels of adhesion compared to SR11 ∆12, except for deletion of Salmonella 132 157