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Applied and Environmental Microbiology, December 2002, p. 6043-6050, Vol. 68, No. 12
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.12.6043-6050.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

The Genus Caedibacter Comprises Endosymbionts of Paramecium spp. Related to the Rickettsiales (Alphaproteobacteria) and to Francisella tularensis (Gammaproteobacteria)

Cora L. Beier,1 Matthias Horn,1* Rolf Michel,2 Michael Schweikert,3 Hans-Dieter Görtz,3 and Michael Wagner1

Lehrstuhl für Mikrobiologie, Technische Universität München, D-85350 Freising,1 Central Institute of the Federal Armed Forces Medical Service, D-56065 Koblenz,2 Biologisches Institut, Universität Stuttgart, D-70569 Stuttgart, Germany3

Received 23 May 2002/ Accepted 2 September 2002

Obligate bacterial endosymbionts of paramecia able to form refractile inclusion bodies (R bodies), thereby conferring a killer trait upon their ciliate hosts, have traditionally been grouped into the genus Caedibacter. Of the six species described to date, only the Paramecium caudatum symbiont Caedibacter caryophilus has been phylogenetically characterized by its 16S rRNA gene sequence, and it was found to be a member of the Alphaproteobacteria related to the Rickettsiales. In this study, the Caedibacter taeniospiralis type strain, an R-body-producing cytoplasmatic symbiont of Paramecium tetraurelia strain 51k, was investigated by comparative 16S rRNA sequence analysis and fluorescence in situ hybridization with specific oligonucleotide probes. C. taeniospiralis is not closely related to C. caryophilus (80% 16S rRNA sequence similarity) but forms a novel evolutionary lineage within the Gammaproteobacteria with the family Francisellaceae as a sister group (87% 16S rRNA sequence similarity). These findings demonstrate that the genus Caedibacter is polyphyletic and comprises at least two phylogenetically different bacterial species belonging to two different classes of the Proteobacteria. Comparative phylogenetic analysis of C. caryophilus, five closely related Acanthamoeba endosymbionts (including one previously uncharacterized amoebal symbiont identified in this study), and their hosts suggests that the progenitor of the alphaproteobacterial C. caryophilus lived within acanthamoebae prior to the infection of paramecia.

* Corresponding author. Mailing address: Lehrstuhl für Mikrobiologie, Technische Universität München, Am Hochanger 4, D-85350 Freising, Germany. Phone: 49 8161 715457. Fax: 49 8161 715475. E-mail: horn{at}microbial-ecology.de.

Applied and Environmental Microbiology, December 2002, p. 6043-6050, Vol. 68, No. 12
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.12.6043-6050.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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