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

Salmonella enterica is a foodborne pathogen leading to gastroenteritis and is commonly acquired by consumption of contaminated food of animal origin. However, numbers of outbreaks linked to the consumption of fresh or minimally processed food of non-animal origin are 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 consumed unprocessed. 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 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 Human gastrointestinal pathogens are often transmitted by animal products, but recent outbreaks show increasing importance of vegetables as source of infection by pathogenic E. coli or Salmonella enterica. The mechanisms of binding of S. enterica to vegetables such as salad are only poorly understood. We established an experimental model system to systematically investigate the role of adhesive structures of S. enterica serovar Typhimurium in binding to corn salad leaves. The contributions of all members of the complex adhesiome, flagella, and O-antigen were evaluated. We identified that Saf fimbriae, type 1 secretion system-secreted BapA, an intact LPS, and flagella-mediated motility contribute to adhesion of Salmonella to corn salad leaves. These results will enable future investigations on factors contributing to contamination of vegetables under agricultural conditions.