Transposon Mutagenisis of Serratia marcescens strain SCBI, a Nematode Symbiont that Enables Insect Killing
Entomopathogenic nematodes form symbiotic relationships with bacteria such as Photorhabdus, Xenorhabdus, and Serratia. The nematodes and bacteria associate to become a better parasitoid to insect larva. The nematodes provide protection for the bacteria to gain entry to the insect, and the bacteria kill the insect and act as the food source for the nematodes. Since entomopathogenic nematodes and their bacterial counterparts are very efficient insect killers this relationship can potentially very useful as a biological control of insect pests. Serratia marcescens strain South African Caenorhabditis briggsae (SCBI), a recently discovered entomopathogenic bacterium, is unique in that it can either form an entomopathogenic relationship with the nematode C. elegans or a pathogenic one. Strain SCBI is the first bacteria known to either form a pathogenic or entomopathogenic relationship, it is the perfect model to better understand the evolution of entomopathogenic relationships. Entomopathogenic relationships are thought to be closely linked to swarming, the movement of bacteria on a solid surface from a single colony, which is driven by quorum sensing. Quorum sensing is chemical communication between cells, and since swarming involves the differentiation between cells, the mode of communication is key. This is thought to be central to entomopathogenic relationships because of the stages involved; the bacteria go from a low population in the intestine of a nematode to a high density traveling through the insect. This is most likely done by quorum sensing. To study this entomopathogenic relationship, a library of transposon mutants of strain SCBI was developed using the pMiniHimar RBI plasmid, a plasmid, which uses a transposase gene to create mutants. These mutants were then tested for their swarming capabilities, a function that is closely linked to quorum sensing, because of the essential differentiation of the cells in a colony. The mutation of the swarming function may test the ability of bacteria to quorum sense, which should affect the bacteria's ability to form relationship with nematodes. From 800 mutants created and collected in this project, 5 were found to have different swarming patters from wild type Strain SCBI. These will then be introduced to nematodes to test if whether changes in swarming affect the relationship and how. This intercellular communication may be a driving factor in entomopathogenic relationships, and more generally in many bacterial-host relationships.
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