Systematic analyses of factors required for adhesion of Salmonella enterica serovar Typhimurium to corn salad (Valerianella locusta).

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 non-animal 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 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. Here, we systematically analyzed the contribution of the complete adhesiome, of LPS, and of flagella-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 flagella-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 contribution 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 the experimentally controlled surface expression of individual adhesive structures, and the analyses of the contribution to binding to leave surfaces of corn salat under laboratory conditions. The model systems allowed the determination of the relative contribution of fimbrial and non-fimbrial adhesins, the type III 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.