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  • 1.
    Eskils, Katarina
    Stockholms universitet, Naturvetenskapliga fakulteten, Mikrobiologiska institutionen.
    Microbial ecology and virulence gene studies of the insect pathogen Bacillus thuringiensis2001Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Bacillus thuringiensis (Bt) is a spore-forming bacterium used as a bio-pesticide due to its insect toxicity. Bt was used as a model organism in a risk evaluation project. Prevalence, survival and spreading, as well as genome stability, putative virulence genes, and gene transfer were examined. Field release of a marked Bt subsp. israelensis (Bti) strain resulted in a minor, transient increase of Bt-like bacteria.

    The bacterial population structure returned to as before release after seven weeks (paper I). Physical chromosome maps were established for Bt subsp. gelechiae, alesti and kurstaki using pulsed-field gel electrophoresis (PFGE). Co-expressed virulence genes are scattered on the chromosome and are not located in operons or pathogenicity islands (paper II). Bt subsp. alesti and kurstaki have identical chromosome maps although their serotypes differ. Sequences homologous to Bacillus cereus enterotoxin genes were found on both chromosomes and on a plasmid in Bt subsp. alesti/kurstaki (paper III). A 5.2 kb region from Bt subsp. alesti was cloned and sequenced, and found to contain three flagellin genes constituting a fla-operon, a putative flagellar motor switch protein gene fliM, and a transglycosylase-like protein gene tlp.

    Deletion mutants in the region are avirulent and do not express previously identified virulence factors (paper IV). Chromosomal genes were transferred by transduction to Bt soil isolate strains, using a bacteriophage originating from soil (paper V).

  • 2.
    Lundström, Annika
    Stockholms universitet, Naturvetenskapliga fakulteten, Mikrobiologiska institutionen.
    Identification and characterisation of five innate immune genes in the cabbage looper Trichoplusia ni2001Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    When insects are infected with bacteria, they respond by producing a range of different antibacterial peptides and by activating protease cascades leading to coagulation and melanisation. Cellular defence mechanisms are also involved in killing of microorganisms through phagocytosis and encapsulation.

    We have found several genes that are upregulated upon bacterial infection in the cabbage looper, Trichoplusia ni, by using the differential display method. Attacin and gloverin are antibacterial proteins previously known from other insect species. Both are glycine-rich proteins that act by disrupting the permeability barrier of the cell membrane.

    An azurocidin homologue without serine protease activity is expressed exclusively in the insect gut. It is induced by injection of bacteria and by feeding larvae with bacteria. Two amino acid substitutions, serine to glutamate and histidine to serine, in the catalytic triad explain the lack of protease activity.

    Another protein induced by a bacterial infection is a 3-dehydroecdysone 3b-reductase-like protein (DERH). This protein was detected in integument and hemolymph of T. ni larvae. In addition, it is developmentally regulated. A possible function for DERH is to convert 3-dehydroecdysone into ecdysone, regulating the titer of biologically active ecdysteroids in the larvae.

    Furthermore, a peptidoglycan recognition protein (PGRP) is upregulated in T. ni by bacterial infection. PGRP is a peptidoglycan- and bacteria binding protein, possibly involved in immune recognition. Homologues were also cloned from mouse and man. The mouse PGRP also binds peptidoglycan, suggesting a functional conservation of the protein from insects to humans.

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