ORIGINAL ABSTRACT

Two novel bacterial strains isolated from root nodules of white sweet clover (Melilotus albus) plants grown at a Canadian site were previously characterized and placed in the genus Phyllobacterium. Here, we present phylogenomic and phenotypic data to support the description of strain T1293T as representative of a novel species and present the first complete closed genome sequence of a bacterial strain (T1018) representing the species 'Phyllobacterium pellucidum'. Phylogenetic analysis of genome sequences, as well as analysis of 53 core genes, placed novel strain T1293T in a highly supported cluster of strains distinct from named Phyllobacterium species with Phyllobacterium myrsinacearum and Phyllobacterium calauticae as closest relatives. The highest average nucleotide identity and digital DNA-DNA hybridization values of genome sequences of T1293T compared to closest species type strains (84.1 and 26.5%, respectively) are well below the threshold values for bacterial species circumscription. The genome of strain T1293T has a size of 5,074,034 bp with a DNA G+C content of 55.1 mol% and possesses three plasmids with sizes of 397,619 bp, 476,847 bp and 519,835 bp. Detected in the genome were type III and type VI secretion system genes, implicated in plant-microbe and microbe-microbe interactions, but key nodulation, nitrogen-fixation and photosystem genes were not detected. A novel prophage (size ~41.5 kb) was also detected in the genome of T1293T. Tests using defined culture media revealed that novel strains T1293T and T1018 were highly resistant to, and able to metabolize glyphosate, a widely used herbicide that has negative consequences for the environment and human health. Data for multiple morphological, physiological, biochemical and plant tests complemented the sequence-based data. The data presented support the description of a new species, and the name Phyllobacterium meliloti sp. nov. is proposed with T1293T=LMG 32641T=HAMBI 3765T as the species type strain.