Analysis of quorum sensing regulatory systems in the human isolate Lactobacillus plantarum

Bacteria can interact with each other via specific cell-to-cell communication systems that are used to regulate the expression of genes involved in diverse functions, such as virulence, genetic competence, or production of antimicrobial compounds. Modulation of the genes involved is done in a co-ordinatedand cell density-dependent way and has been termed quorum sensing (QS). Regulation of QS systems is mediated via specificextracellularsignaling molecules, which in Gram-positive bacteria generally areautoinducingpeptides (AIPs). QSsystemsare expected to evolve in high cell-density ecosystems with many microbial interactions, where they would enable bacterial populations to co-ordinate responses that improve their competitiveness, adaptation to changing environmental conditions, or interactions between bacteria and theirabioticor biotic environment. Lactobacillusplantarum is a lactic acid bacterium (LAB) that is encountered in ecological niches like fermented food products or plant material, but it is also a natural inhabitant of the human gastrointestinal tract (GI-tract). Intestinal LAB face large changes in environmental conditions during GI-tract passage and inhabit densely populated GI-tract compartments, while in the nutrient-rich industrial and food environments LAB can grow to high cell densities.The characteristics of these natural environments are therefore expected to be favorable for the development and presence of QS systems in LAB. This thesis describes the functional analysis of genes that were annotated as QS systems on the completed genome sequence of L.plantarum . A combination of functional genomics and bioinformatics approaches was used to predict and confirm their putative QS function. Usinga comparative genomics approach 5 two-component regulatory systems(TCS), consisting of ahistidineproteinkinase(HPK) and response regulator (RR), which could be involved in peptide-based QS were predicted from the genome sequence. Functional analysis of one of these TCS, designated L actobacillusa gr-like module ( lam ), indicated its putative regulatory function in adherence and identified anAgrD-like AIP encoded by it. Mutational analysis of a gene coding for a transcriptional regulator that contained anAraC-type DNA-binding domain and was flanked by a HPK-encoding gene, showed its regulatory role in adherence and cell-aggregation. The results demonstrate the potential of genomics and molecular tools for elucidating the functions of uncharacterized QS genes and identifying newAIPsin the L.plantarum genome, as well as their implications on future QS research in lactobacilli.