Environmental Microbiology (2006) 8(12), 2179–2188 doi:10.1111/j.1462-2920.2006.01101.x © 2006 The Authors Journal compilation © 2006 Society for Applied Microbiology and Blackwell Publishing Ltd Blackwell Publishing LtdOxford, UKEMIEnvironmental Microbiology1462-2912© 2006 The Authors; Journal compilation © 2005 Society for Applied Microbiology and Blackwell Publishing Ltd ? 200681221792188Original ArticlePred- atory bacteria and the origin of mitochondriaY. Davidov, D. Huchon, S. F. Koval and E. Jurkevitch Received 17 February, 2006; accepted 16 June, 2006. *For corre- spondence. E-mail jurkevi@agri.huji.ac.il; Tel. (+972) 8 9489167; Fax (+972) 8 94666794. A new a-proteobacterial clade of Bdellovibrio-like predators: implications for the mitochondrial endosymbiotic theory Yaacov Davidov, 1 Dorothee Huchon, 3 Susan F. Koval 4 and Edouard Jurkevitch 1,2 * 1 Department of Plant Pathology and Microbiology and 2 The Otto Warburg Center for Biotechnology in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot, Israel. 3 Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel. 4 Department of Microbiology and Immunology, University of Western Ontario, London, Ontario, Canada. Summary Bdellovibrio -and-like organisms (BALOs) are peculiar, ubiquitous, small-sized, highly motile Gram-negative bacteria that are obligatory predators of other bacte- ria. Typically, these predators invade the periplasm of their prey where they grow and replicate. To date, BALOs constitute two highly diverse families affili- ated with the d - proteobacteria class. In this study, Micavibrio spp., a BALO lineage of epibiotic preda- tors, were isolated from soil. These bacteria attach to digest and grow at the expense of other prokaryotes, much like other BALOs. Multiple phylogenetic analy- ses based on six genes revealed that they formed a deep branch within the a - proteobacteria , not affiliated with any of the a -proteobacterial orders. The presence of BALOs deep among the a - proteobacteria suggests that their peculiar mode of parasitism maybe an ancestral character in this proteobacterial class. The origin of the mitochondrion from an a - proteobacte- rium endosymbiont is strongly supported by molecu- lar phylogenies. Accumulating data suggest that the endosymbiont’s host was also a prokaryote. As prokaryotes are unable to phagocytose, the means by which the endosymbiont gained access into its host remains mysterious. We here propose a scenario based on the BALO feeding-mode to hypothesize a mechanism at play at the origin of the mitochondrial endosymbiosis. Introduction Predation among prokaryotes has not been extensively explored and the only well-known obligate predatory- bacteria group is the Bdellovibrio -and-like organisms (BALOs). Bdellovibrio -and-like organisms are small, highly motile Gram-negative bacteria that obligatorily prey on other Gram-negative bacteria. In their typical life cycle free-swimming cells invade the periplasm of the prey, grow, replicate and then differentiate to progeny cells that lyse the host to start a new cycle (Jurkevitch, 2000). Bdellovibrio -and-like organisms are commonly found in diverse habitats including soil, fresh water, seawater, sew- age and animal feces. They form the two distant and internally diverse families, Bacteriovoracaceae and Bdell- ovibrionaceae, classified as the order Bdellovibrionales, and cluster within the δ - proteobacteria class (Davidov and Jurkevitch, 2004). Their high diversity, ubiquity and pre- sumed long evolutionary history suggest a possible eco- logical and evolutionary impact on microbial communities. Given that phagocytosis is unknown in prokaryotes, the association of BALOs with their hosts was proposed to be analogous to early events of the endosymbiotic acquisi- tion of eukaryotic cell organelles. If prey bacteria could have avoided digestion by the predators, stable symbio- ses between the two types of organisms might have devel- oped (Guerrero et al ., 1986; Margulis, 1993). Molecular data support an origin of the mitochondrion in an α - pro- teobacterium (Gray et al ., 1999). This, and the discovery of amitochondriate phagocytotic eukaryotes (Cavalier- Smith, 1987) caused the abandon of the ‘predatory endo- symbiosis hypothesis’. However, recent and accumulating data suggest that all characterized amitochondrial eukary- otes are derived from mitochondriate ancestors (van der Giezen et al ., 2005), implying that the proteomitochon- drion was acquired by a prokaryotic host (Embley and Martin, 2006; Martin and Koonin 2006). In this article, we report on Bdellovibrio- like predatory bacteria that form a deep branch within the α - proteobac- teria . As it is widely accepted that mitochondria evolved from an α - proteobacterium , a re-examination of a possible