18 The Family Phyllobacteriaceae

The family Phyllobacteriaceae belongs to the order Rhizobiales in the Alphaproteobacteria and currently comprises the 72 species in 13 genera: Ahrensia, Aliihoeflea, Aminobacter (including Chelatobacter), Aquamicrobium (including Defluvibacter), Chelativorans, Hoeflea, Lentilitoribacter, Mesorhizobium, Nitratireductor, Phyllobacterium, Pseudahrensia, Pseudaminobacter, and Thermovum. They form a single cluster within the 16S rRNA gene phylogeny. The family consists of environmental (soil, water) and plant-associated bacteria that have a heterotrophic respiratory metabolism with oxygen as terminal electron acceptor. One Aquamicrobium species can use nitrate as an alternative terminal electron acceptor. One Mesorhizobium species is facultatively chemolithotrophic using thiosulfate or elemental sulfur as sole energy source. Candidatus Liberibacter, a group of uncultivated phloem-inhabiting bacteria that are associated with various plant diseases in citrus and Solanaceae or are endophytic in pear plants, is also associated with the family. However, comprehensive phylogenetic analyses indicate the position of this group as a member of the Phyllobacteriaceae is uncertain.