Inter- and Intra-kingdom Signaling in Bacterial Chemotaxis, Biofilm Formation, and Virulence

Cell-cell communication between bacteria, belonging to the same species or to different species (Intra-kingdom signaling), or communication between bacteria and their animal host (Inter-kingdom signaling) is mediated through different chemical signals that are synthesized and secreted by bacteria or the host and is crucial for the survival of bacteria inside their host. The overall goal of this work was to understand the role of inter- and intra-kingdom signaling in phenotypes such as chemotaxis, colonization and biofilm formation, and virulence that are associated with infections caused by the human gastrointestinal (GI) tract pathogens. A part of our work also aimed at developing microfluidics-based models to study inter- and intra-kingdom signaling in biofilm formation, inhibition, and dispersal.

We showed that norepinephrine (NE), an important host signal produced during stress, increases human opportunistic pathogen Pseudomonas aeruginosa growth, motility, attachment, and virulence, and also showed that the actions of NE are mediated primarily through the LasR, and not the RhlR QS system. We investigated the molecular mechanism underlying the chemo-sensing of the intra-kingdom signal autoinducer-2 (AI-2) by pathogens Escherichia coli and Salmonella typhimurium by performing different chemotaxis assays (capillary, microPlug and microFlow assays), and discovered that AI-2 is a potent attractant for E. coli and S. typhimurium, and that the Tsr chemoreceptor and periplasmic AI-2 binding protein LsrB are necessary for sensing AI-2, although uptake of AI-2 into the cytoplasm is not required. We concluded that LsrB, when bound to AI-2, interacts directly with the periplasmic domain of Tsr primarily at the Thr-61 and Asp-63 residues of LsrB, making LsrB the first known periplasmic-protein partner for Tsr.

We fabricated a simple user-friendly microfluidic flow cell (microBF) device that can precisely measure the effect of a wide range of concentrations of single or combinations of two or more soluble signals on bacterial biofilm formation and development. We also constructed a synthetic biofilm circuit that utilizes the Hha and BdcA dispersal proteins of E. coli along with a quorum sensing (QS) switch that works based on the accumulation of the signal N-(3-oxo-dodecanoyl)-L-homoserine lactone (3-o-C12HSL) and implemented it in an upgraded ?BF device. We showed that a QS system may be utilized with biofilm dispersal proteins to control consortial biofilm formation by removing an existing biofilm and then removing the biofilm that displaced the first one. These types of synthetic QS circuits may be used to pattern biofilms by facilitating the re-use of platforms and to create sophisticated reactor systems that will be used to form bio-refineries.