Role of post-harvest abiotic factors on interactions of Salmonella typhimurium with lettuce leaves ( Lactuca sativa L. var crispa)

Foodborne gastroenteritis outbreaks of Salmonella Typhimurium infections are often associated with leafy vegetables. In addition to biotic factors, attachment and adhesion to the vegetable leaves and subsequent survival and internalization in the tissues in the post-harvest stages are mediated by abiotic factors, mainly light, temperature and time. This study combined quantitative cultural methods and Scanning Electron Microscopy (SEM) to investigate the influence of light (dark = 0 mu E m-2s-1 and light = 100 mu E m-2s-1), temperature (4 degrees C, 12 degrees C and 25 degrees C) and storage time (0 h, 24 h, 48 h and 72 h) on the interactions of Salmonella Typhimurium UVBioTagTM with experimentally contaminated lettuce leaves (Lactuca sativa L. var crispa), preconditioned for 20 min in the tested conditions before contamination. After 2h, an intense adhesion of S. Typhimurium to the leaves was observed, increasing during storage for 72 h (estimated shelf life), regardless the temperature and light conditions. SEM microphotographs indicated that the adhesion to the leaf surfaces was followed by internalization into stomata and formation of biofilm-like bacterial aggregates, in all tested conditions. The study highlighted that Salmonella was capable to colonize the leaf surfaces even under the absence of light. Understanding how abiotic factors influence these processes is essential to develop more effective strategies to minimize the risks associated with consumption of raw leafy vegetables.