The Invasin and Complement-Resistance Protein Rck of Salmonella is More Widely Distributed Than Previously Expected

The open reading frame (ORF) on the operon encodes an outer membrane protein responsible for invasion of nonphagocytic cell lines and resistance to complement-mediated killing. Until now, the ORF was only detected on the virulence plasmids of three serovars of Salmonella subsp. (i.e., Bovismorbificans, Enteritidis, and Typhimurium). The increasing number of Salmonella genome sequences allowed us to use a combination of reference sequences and whole-genome multilocus sequence typing (wgMLST) data analysis to probe the presence of the operon and of in a wide array of isolates belonging to all Salmonella species and subspecies. We established the presence of partial or complete operons in 61 subsp. serovars as well as in 4 other subspecies with various syntenies and frequencies. The ORF itself was retrieved in 36 subsp. serovars and in two subspecies with either chromosomal or plasmid-borne localization. It displays high conservation of its sequence within the genus, and we demonstrated that most of the allelic variations identified did not alter the virulence properties of the protein. However, we demonstrated the importance of the residue at position 38 (at the level of the first extracellular loop of the protein) in the invasin function of Rck. Altogether, our results highlight that is not restricted to the three formerly identified serovars and could therefore have a more important role in virulence than previously expected. Moreover, this work raises questions about the mechanisms involved in acquisition and about virulence plasmid distribution and evolution. The foodborne pathogen Salmonella is responsible for a wide variety of pathologies depending on the infected host, the infecting serovars, and its set of virulence factors. However, the implication of each of these virulence factors and their role in the specific host-pathogen interplay are not fully understood. The significance of our research is in determining the distribution of one of these factors, the virulence plasmid-encoded invasin and resistance to complement killing protein Rck. In addition to providing elements of reflection concerning the mechanisms of acquisition of specific virulence genes in certain serotypes, this work will help to understand the role of Rck in the pathogenesis of Salmonella.