Shigella flexneri adherence factor expression in in vivo-like conditions

The Shigella species are Gram-negative, facultative intracellular pathogens that invade the colonic epithelium and cause significant diarrheal disease. Despite extensive research on the pathogen, comprehensive understanding of how Shigella initiates contact with epithelial cells remains unknown. Shigella maintains many of the same Escherichia coli adherence gene operons; however, at least one critical gene component in each operon is currently annotated as a pseudogene in reference genomes. These annotations, coupled with a lack of structures upon microscopic analysis following growth in laboratory media, have led the field to hypothesize that Shigella is unable to produce fimbriae or other “traditional” adherence factors. Nevertheless, our previous analyses have demonstrated that a combination of bile salts and glucose induce both biofilm formation and adherence to colonic epithelial cells. Through a two-part investigation, we first utilized various transcriptomic analyses to demonstrate that S. flexneri strain 2457T adherence gene operons are transcribed. Subsequently, we performed mutation, electron microscopy, biofilm, infection, and proteomic analyses to characterize three of the structural genes. In combination, these studies demonstrate that despite the gene annotations, S. flexneri 2457T uses adherence factors to initiate biofilm formation as well as epithelial cell contact. Furthermore, host factors, namely glucose and bile salts in the small intestine, offer key environmental stimuli required for proper adherence factor expression in S. flexneri . This research may have a significant impact on vaccine development for Shigella and further highlights the importance of utilizing in vivo -like conditions to study bacterial pathogenesis.