Root mediated uptake of Salmonella is different from phyto-pathogen and associated with the colonization of edible organs

Abstract

Background Pre-harvest contamination of fruits and vegetables by Salmonella in fields is one of the causes of food-borne outbreaks. Natural openings like stomata, hydathodes and fruit cracks are known to serve as entry points. While there are reports indicating that Salmonella colonize and enter root through lateral root emerging area, further investigations regarding how the accessibility of Salmonella to lateral root is different from phyto-pathogenic bacteria, the efficacy of lateral root to facilitate entry have remained unexplored. In this study we attempted to investigate the lateral root mediated entry of Salmonella, and to bridge this gap in knowledge. Results Unlike phytopathogens, Salmonella cannot utilize cellulose as the sole carbon source. This negates the fact of active entry by degrading plant cellulose and pectin. Endophytic Salmonella colonization showed a high correlation with number of lateral roots. When given equal opportunity to colonize the plants with high or low lateral roots, Salmonella internalization was found higher in the plants with more lateral roots. However, the epiphytic colonization in both these plants remained unaltered. To understand the ecological significance, we induced lateral root production by increasing soil salinity which made the plants susceptible to Salmonella invasion and the plants showed higher Salmonella burden in the aerial organs. Conclusion Salmonella, being unable to degrade plant cell wall material relies heavily on natural openings. Therefore, its invasion is highly dependent on the number of lateral roots which provides an entry point because of the epidermis remodeling. Thus, when number of lateral root was enhanced by increasing the soil salinity, plants became susceptible to Salmonella invasion in roots and its transmission to aerial organs. Background Salmonella serovars are recognized as important food-borne pathogens associated with poultry [1, 2] and raw plant products [3, 4]. There has been an increase in human infections with various serovars linked to raw produce [5]. The raw plant products get contaminated during shipping and processing [6]. However, in the last few decades, reports have shown the evidence of pre-harvest contamination of salad vegetables [7, 8]. Irrespective of the species, plants grown in Salmonella-contaminated soil became colonized with the organism [9, 10]. Various biotic and abiotic components play an important role in successful colonization of Salmonella in roots. De-novo factors like flagella [11], fimbriae [12], and exopolysaccharides [13] are known to enable pathogen to colonize the host. Presence of a phytopathogen like Xanthomonas [14] in vicinity can lead to higher colonization of Salmonella. But, beneficial organisms like Sinorhizobium meliloti is known to reduce the burden of Salmonella in plants [15]. Gu et al reported the presence of Salmonella in the vasculature of the leaves and the fruits without causing any visible symptoms [16].