Journal of Bacteriology

ABSTRACTTo survive in host cells, intracellular pathogens or symbiotic bacteria require protective mechanisms to overcome the oxidative stress generated by phagocytic activities of the host. By genomic library tagging, we cloned adps(stands forDNA-bindingprotein fromstarved cells) gene of the symbiotic “CandidatusLegionella jeonii” organism (called the X bacterium) (dpsX) that grows inAmoeba proteus. The gene encodes a 17-kDa protein (pI 5.19) with 91% homology to Dps and DNA-binding ferritin-like proteins of other organisms. The cloned gene complemented thedpsmutant ofEscherichia coliand conferred resistance to hydrogen peroxide. DpsXproteins purified fromE. colitransformed with thedpsXgene were in oligomeric form, formed a complex with pBlueskript SKII DNA, and protected the DNA from DNase I digestion and H2O2-mediated damage. The expression of thedpsXgene in “CandidatusLegionella jeonii” was enhanced when the host amoeba was treated with 2 mM H2O2and by phagocytic activities of the host cell. These results suggested that the Dps protein has a function protective of the bacterial DNA and that its gene expression responds to oxidative stress generated by phagocytic activities of the host cell. With regard to the fact that invasion ofLegionellasp. into respiratory phagocytic cells causes pneumonia in mammals, further characterization ofdpsXexpression in theLegionellasp. that multiplies in a protozoan host in the natural environment may provide valuable information toward understanding the protective mechanisms of intracellular pathogens.

ABSTRACT “ Candidatus Glomeribacter gigasporarum” is an endocellular β-proteobacterium present in the arbuscular mycorrhizal (AM) fungus Gigaspora margarita . We established a protocol to isolate “ Ca . Glomeribacter gigasporarum” from its host which allowed us to carry out morphological, physiological, and genomic investigations on purified bacteria. They are rod shaped, with a cell wall typical of gram-negative bacteria and a cytoplasm rich in ribosomes, and they present no flagella or pili. Isolated bacteria could not be grown in any of the 19 culture media tested, but they could be kept alive for up to 4 weeks. PCR-based investigations of purified DNA from isolated bacteria did not confirm the presence of all genes previously assigned to “ Ca . Glomeribacter gigasporarum.” In particular, the presence of nif genes could not be detected. Pulsed-field gel electrophoresis analyses allowed us to estimate the genome size of “ Ca . Glomeribacter gigasporarum” to approximately 1.4 Mb with a ca. 750-kb chromosome and a 600- to 650-kb plasmid. This is the smallest genome known for a β-proteobacterium. Such small genome sizes are typically found in endocellular bacteria living permanently in their host. Altogether, our data suggest that “ Ca . Glomeribacter gigasporarum” is an ancient obligate endocellular bacterium of the AM fungus G. margarita .