Members of the Cytophaga–Flavobacterium–Bacteroides phylum as intracellular bacteria of acanthamoebae: proposal of ‘Candidatus Amoebophilus asiaticus’

Abstract

Three Gram-negative, rod-shaped bacteria that were found intracellularly in two environmental and one clinical Acanthamoeba sp. isolates were analysed. Two endocytobionts showing a parasitic behaviour were propagated successfully outside their amoebal host cells and were identified subsequently by comparative 16S rRNA sequence analysis as being most closely affiliated with Flavobacterium succinicans (99% 16S rRNA sequence similarity) or Flavobacterium johnsoniae (98% 16S rRNA sequence similarity). One endocytobiont could neither be cultivated outside its original Acanthamoeba host (Acanthamoeba sp. TUMSJ-321) nor transferred into other amoebae. Electron microscopy revealed that the amoebal trophozoites and cysts were almost completely filled with cells of this endosymbiont which are surrounded by a host-derived membrane. According to 16S rRNA sequence analysis, this endosymbiont could also be assigned to the Cytophaga-Flavobacterium-Bacteroides (CFB) phylum, but was not closely affiliated to any recognized species within this phylogenetic group (less than 82% 16S rRNA sequence similarity). Identity and intracellular localization of this endosymbiont were confirmed by application of a specific fluorescently labelled 16S rRNA-targeted probe. Based on these findings, we propose classification of this obligate Acanthamoeba endosymbiont as 'Candidatus Amoebophilus asiaticus'. Comparative 18S rRNA sequence analysis of the host of 'Candidatus Amoebophilus asiaticus' revealed its membership with Acanthamoeba 18S rDNA sequence type T4 that comprises the majority of all Acanthamoeba isolates.

Hybridisation of picoeukaryotes by eubacterial probes is widespread in the marine environment

“…This could indicate that at least 44% of the picoeukaryotes hosted intracellular bacterial symbionts in the studied environment. Previous investigations showed that both picoeukaryotes and CFB were able to establish symbiotic relationships with either prokaryotes or protozoans (Guillou et al 1999, Horn et al 2001. The presence of intracellular bacteria could also be due to antagonist relationships, such as the predation of picoeukaryotes on bacteria, or the attack of bacteria on small algae (Cole 1982, Guillou et al 2001.…”

A mitochondrial-like aconitase in the bacterium Bacteroides fragilis : Implications for the evolution of the mitochondrial Krebs cycle

“…In support of the idea that a CFB group bacterium may have been a participant in such a relationship, present-day endosymbionts of some Acanthamoebae spp. can be unambiguously assigned to the CFB group of eubacteria (41).…”

Obligate bacterial endosymbionts of Acanthamoeba spp. related to the beta-Proteobacteria: proposal of 'Candidatus Procabacter acanthamoebae' gen. nov., sp. nov.

“…On: Fri, 11 May 2018 00:35:01 M. Horn and others 2000) or to the phylum Cytophaga-FlavobacteriumBacteroides (Horn et al, 2001).…”