Abstract
Enterobacteriaceae bacteria are prone to opportunistic pathogenic behavior during gut dysbiosis, potentially influenced by distinct environmental cues within the compromised gut. Chemosensing systems play a central role in allowing bacteria to perceive and react to physicochemical signals within the gastrointestinal (GI) milieu, influencing their motility, population organization, and colonization site preferences. We investigated the chemosensory behavior of Enterobacteriaceae, a bacterial family associated with gastrointestinal (GI) bleeding and sepsis, to learn whether these bacteria respond to human serum, a chemical stimulus they encounter during GI bleeding. We discover that clinical isolates of diverse non-typhoidal Salmonella enterica serovars, Escherichia coli , and Citrobacter koseri all sense human serum as a potent attractant. Using a custom injection-based microfluidics device we simulated GI bleeding and found that femtoliter volumes of human serum are sufficient to stimulate motile cells to rapidly swim toward, and congregate at, the serum source. With exceptional temporal resolution, we determined that this response is orchestrated through chemotaxis, with a major chemical cue driving chemoattraction being L-serine, an amino acid abundantly present in serum, recognized by the chemoreceptor Tsr. To gain deeper insight into the structural basis of this phenomenon, we successfully elucidated the crystal structure of Salmonella Typhimurium Tsr in complex with L-serine. This structure allowed for a precise characterization of the ligand-binding site, thereby correcting prior misconceptions regarding L-serine positioning and accurately defining the chemoreceptor’s ligand-binding interactions. Building upon these structural insights, we identified a conserved amino acid recognition motif for L-serine shared among Tsr orthologues and mapped the phylogenetic distribution of this chemoreceptor. We found that Tsr, which mediates serum chemoattraction, is widely conserved among Enterobacteriaceae and many World Health Organization priority pathogens associated with bloodstream infections. Given that Enterobacteriaceae are a primary cause of bacteremia and sepsis, particularly as a complication of inflammatory bowel diseases, their remarkable sensitivity to serum as a chemoattractant could relate to their proclivity for bloodstream entry.
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