Procaryotic species and subspecies delineation using average nucleotide identity and gene order conservation

Genome-based studies indicate that the Enterococcus faecium Clade B strains belong to Enterococcus lactis species and lack of the hospital infection associated markers

Enterococcus lactis and the heterotypic synonym Enterococcus xinjiangensis from dairy origin have recently been identified as a novel species based on 16S rRNA gene sequence analysis. Enterococcus faecium type strain NCTC 7171^T was used as the reference genome for determining E. lactis and E. faecium to be separate species. However, this taxonomic classification did not consider the diverse lineages of E. faecium, and the double nature of hospital-associated (clade A) and community-associated (clade B) isolates. Here, we investigated the taxonomic relationship among isolates of E. faecium of different origins and E. lactis, using a genome-based approach. Additional to 16S rRNA gene sequence analysis, we estimated the relatedness among strains and species using phylogenomics based on the core pangenome, multilocus sequence typing, the average nucleotide identity and digital DNA-DNA hybridization. Moreover, following the available safety assessment schemes, we evaluated the virulence profile and the ampicillin resistance of E. lactis and E. faecium clade B strains. Our results confirmed the genetic and evolutionary differences between clade A and the intertwined clade B and E. lactis group. We also confirmed the absence in these strains of virulence gene markers IS16, hylEfm and esp and the lack of the PBP5 allelic profile associated with ampicillin resistance. Taken together, our findings support the reassignment of the strains of E. faecium clade B as E. lactis.

Genomic differences within the phylum Marinimicrobia: From waters to sediments in the Mariana Trench

Marinimicrobia are widespread from the marine surface to the hadal zone. Major clades of Marinimicrobia have evolved to different ecotypes along with energy gradients, but their genomes in deeper waters and sediments have rarely been studied. Here we obtained 11 Marinimicrobia draft genomes from the water column in the full-ocean depth and the hadal sediments in the Mariana Trench. All the predicted genomic capabilities of the metagenome-assembled genomes (MAGs) are indicative of heterotrophic lifestyle. The MAGs from the hadal depths are distinct from those from the mesopelagic and bathypelagic depths by enrichment of the genes involved in amino acids metabolism and mismatch repair. Compared with the MAGs from waters, those from the sediments were dramatically expanded by acquiring the genes responsible for chemotaxis, mobility and the two-component systems. Marinimicrobia were apparently differentiated in the environments with different depths, organic matters and electronic acceptors. Our results also posit a potential evolutionary relationship between the species inhabiting the waters and sediments, indicating the occurrence of allopatric speciation in Marinimicrobia.