Protistology 5 (4) 303–312 (2008)
First evidence of bacterial endocytobionts in the lobose testate amoeba Arcella (Amoebozoa, Arcellinida)*
Júlia Katalin Török,1 Beatrix Pollák,2 Zsófia Heéger,2
György Csikós 3 and Károly Márialigeti 2
1 Department of Systematic Zoology and Ecology,
Summary
While prokaryotic symbionts in ciliates are extensively searched, free-living amoeba species of no importance to human health have been widely neglected in
symbiont studies. Te present paper gives the first evidence of bacterial endocytobionts in Arcella, a lobose testate amoeba. Clonal cultures of different
Arcella species were investigated for the presence of endocytobionts using 16S rRNA gene sequencing, fluorescent in situ hybridization (FISH) and
transmission electron microscopy (TEM). A rich diversity of eubacterial sequences have been identified by amplification of partial 16S rRNA gene sequences either
by direct isolation of DNA from the testacean cell or following cultivation of bacteria from individual Arcella cells on agar plates. FISH with different probes
against various eubacterial targets demonstrated the presence of single bacterial cells scattered in the cytoplasm that were clearly different from those aggregated
within the food vacuoles. α-Proteobacteria and Gram-positive bacteria were visualized in different host species kept in clonal cultures. Transmission electron
microscopical surveys revealed single rod-shaped bacteria located in different parts of the cytoplasm. Bacteria from the same host clone appear to cover a wide
phylogenetic range. Although some belong to the same taxa as symbiotic bacteria of other eukaryotic organisms, others are close relatives of human pathogens,
suggesting the potential role of Arcella species as reservoires. Tis idea was supported by the presence of Legionella sp. in 3 different Arcella samples from
natural environment. In symbiont-bearing Arcella clones lysis of host cell has not been detected, but cells in older cultures occasionally started cyst formation.
This phenomenon might indicate that the endocytobionts detected so far are not harmful to the host cells, though their possible beneficial role is still to be proved.
Key words: Bacteria, symbiosis, endocytobionts, testate amoebae, Arcella
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