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Sodalis glossinidius is a facultative intracellular bacterium that is a secondary symbiont of the tsetse fly (Diptera: Glossinidae). Since studies with other facultative intracellular bacteria have shown that high-affinity iron acquisition genes are upregulated in vivo, we investigated the regulation of several Sodalis genes that encode putative iron acquisition systems. These genes, SG1538 (hemT) and SG1516 (sitA), are homologous to genes encoding periplasmic heme and iron/manganese transporters, respectively, hemT promoter- and sitA promoter-gfp fusions were constructed, and in both Escherichia coli and Sodalis backgrounds, expression levels of these fusions were higher when the bacteria were grown in iron-limiting media than when the bacteria were grown in iron-replete media. The Sodalis promoters were tested for iron regulation in an E. coli strain that lacks the fur gene, which encodes the iron-responsive transcriptional repressor Fur. Expression of the promoter-gfp fusions in the E. coli fur mutant was constitutively high in both iron-replete and iron-deplete media, and addition of either Shigella flexneri fur or Sodalis fur to a plasmid restored normal regulation. A Sodalis fur mutant was constructed by intron mutagenesis, and semiquantitative reverse transcription-PCR (RT-PCR) showed that iron repression of sitA expression was also abolished in this strain. In vivo expression analysis showed that hemT and sitA are expressed when Sodalis is within tsetse fly hosts, suggesting a biological role for these genes when Sodalis is within the tsetse fly. doi: 10.1128/JB.00161-10

The catalase gene katA of the medicinal leech symblont Aeromonas veronii bv. sobria was cloned, sequenced, and functionally characterized. Southern hybridization, using an A. veronii katA-specific hybridization probe, suggested the presence of a single gene copy in many Aeromonas species. A. veronfi katA consisted of 1446 nt encoding a protein with a high degree of similarity to the small-subunit group III bacterial catalases. A catalase-null mutant (JG186) was constructed through gene-replacement mutagenesis. In the parent strain (HM21R), catalase activity was only detected in extracts of cells grown to early exponential phase following [H.sub.2][O.sub.2] induction, in which the ability to induce activity was inversely related to optical density. In contrast, induced JG186 cells were very sensitive to oxidative stress, with survival being affected even at low [H.sub.2][O.sub.2] concentrations. In contrast to the findings of previous reports of other symbiotic systems, the catalase mutant was not defective in its ability to competitively colonize or persist within its host, in both co-inoculation and sole-colonization assays. This body of evidence suggests either that oxidative stress, in the form of [H.sub.2][O.sub.2] exposure, is not encountered by the microbial partner under the examined symbiotic conditions or that compensatory mechanisms exist. The data suggest that although many colonization factors reoccur, each symbiotic system has also evolved specific mechanisms that affect symbiont-host dynamics.

Research describes the Sodalis glossinidius genomic size, its overall coding capacity, and the plasmid it harbors. Inflence of symbiotic association with the tsetse fly on the genomic size and content of the bacterium is also discussed.

A combination of diagnostic and quantitative PCR techniques is employed to explain the history and growth dynamics observed in the symbionts during the embryonic development of Hirudo verbana, the European medicinal leech. Leech development is shown to lead to diversity in the temporal establishment of the symbiont species.

A study determined that Sodalis isolates from several tsetse fly species are virtually identical based on a phylogenetic analysis of their ftsZ gene sequences. Quantitative PCR analysis showed that transinfected flies maintained their Sodalis populations at densities comparable to those in flies harboring native symbionts.

A study is conducted to gain insight into forces driving the evolutionary transition to symbiosis and their consequences on the genomic composition of microbial participants. The symbiotic genomes were comparatively analyzed by a heterologous array hybridization approach using Escherichia coli macroarrays to understand how host ecology might influence selection for adaptations in microbial genomes and how initial genome reductions might harness organisms into symbiotic lifestyles.