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Bloom of Filamentous Bacteria in a Mesotrophic Lake: Identity and Potential Controlling Mechanism. Jakob Pernthaler, 2004.Ephemeral blooms of filamentous bacteria are a common phenomenon in the water column of oligo- to mesotrophic lakes . It is assumed that the appearance of such morphotypes is favored by selective predation of bacterivorous protists and that filter-feeding zooplankton plays a major role in suppressing these bacteria . The phylogenetic affiliation of the important bloom-forming filamentous bacteria in freshwaters is presently unknown . Here we report the identification of dominant members of a filamentous bacterial assemblage during a bloom of such morphotypes in a mesotrophic lake . By molecular cloning and fluorescence in situ hybridization with specific oligonucleotide probes, up to 98% of filamentous cells in lake water could be assigned to a clade of almost identical (99% similarity) 16S rRNA gene sequence types, the cosmopolitan freshwater LD2 cluster . For a period of less than 1 week, members of the LD2 clade constituted >40% of the total bacterial biomass, potentially favored by high grazing of planktivorous protists . This is probably the most pronounced case of dominance by a single bacterioplankton species ever observed in natural freshwaters . In enclosures artificially stocked with the metazoan filter feeder Daphnia, bacteria related to the LD2 clade formed a significantly larger fraction of filaments than in enclosures where Daphnia had been removed . However, in the presence of higher numbers of Daphnia individuals, the LD2 bacteria, like other filaments, were eventually eliminated both in enclosures and in the lake . This points at the potential importance of filter-feeding zooplankton in controlling the occurrence and species composition of filamentous bacterial morphotypes in freshwater plankton . Development of an In Vitro Integration Assay for the Bacteroides Conjugative Transposon CTnDOT. Qi Cheng, 2002.Integrated self-transmissible elements called conjugative transposons (CTns) are responsible for the transfer of antibiotic resistance genes in many different species of bacteria . One of the best characterized of these newly recognized elements is the Bacteroides CTn, CTnDOT . CTnDOT is thought to have a circular transfer intermediate that transfers to and integrates into the genome of the recipient cell . Previous investigations of the mechanism of CTnDOT integration have been hindered by the lack of an in vitro system for checking this model of integration and determining whether the CTnDOT integrase alone was sufficient to catalyze the integration reaction or whether host factors might be involved . We report here the development of an in vitro system in which a plasmid containing the joined ends of CTnDOT integrates into a plasmid carrying a CTnDOT target site . To develop this in vitro system, a His-tagged version of the integrase gene of CTnDOT was cloned and shown to be active in vivo . The protein produced by this construct was partially purified and then added to a reaction mixture that contained the joined ends of the circular form of CTnDOT and a plasmid carrying one of the CTnDOT target sites . Integration was demonstrated by using a fairly simple mixture of components, but integration was stimulated by a Bacteroides extract or by purified Escherichia coli integration host factor . The results of this study demonstrate both that the circular form of CTnDOT is the form that integrates into the target site and that host factors are involved in the integration process . UV Resistance of Bacillus anthracis Spores Revisited: Validation of Bacillus subtilis Spores as UV Surrogates for Spores of B . anthracis Sterne. Wayne L. Nicholson, 2003.Recent bioterrorism concerns have prompted renewed efforts towards understanding the biology of bacterial spore resistance to radiation with a special emphasis on the spores of Bacillus anthracis. A review of the literature revealed that B . anthracis Sterne spores may be three to four times more resistant to 254-nm-wavelength UV than are spores of commonly used indicator strains of Bacillus subtilis . To test this notion, B . anthracis Sterne spores were purified and their UV inactivation kinetics were determined in parallel with those of the spores of two indicator strains of B . subtilis, strains WN624 and ATCC 6633 . When prepared and assayed under identical conditions, the spores of all three strains exhibited essentially identical UV inactivation kinetics . The data indicate that standard UV treatments that are effective against B . subtilis spores are likely also sufficient to inactivate B . anthracis spores and that the spores of standard B . subtilis strains could reliably be used as a biodosimetry model for the UV inactivation of B . anthracis spores .
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