Abstract
In a recent publication in PLOS ONE, Gabriele Margos and colleagues have questioned the division of the genus Borrelia into two genera on the basis that the differences in percentage of conserved proteins (POCP) between these two groups is >50%, which an earlier study has suggested as the threshold for differentiating prokaryotic genera. However, the POCP threshold is a poorly characterized and rarely used criterion for establishing distinction among prokaryotic genera. Detailed evaluation of the intergeneric POCP values for 37 genera from 3 different families (viz. Enterobacteriaceae- 24 genera, Morganellaceae-8 genera and Cystobacteraceae-5 genera) presented here shows that the POCP values for all genera within each of these families exceeded >58%. Thus, the suggested POCP threshold is not a useful criterion for delimitation of genus boundary and the objection by Margos et al. on this ground is invalid. Additionally, Margos et al. have questioned the specificities of ~15–20% of the conserved signature indels (CSIs) described in our work. However, as shown here, this concern is due to misunderstanding of the results and the CSIs in question are still highly-specific characteristics of the members of these genera and they provide important information regarding the evolutionary relationships of two new reptiles-echidna-related species viz. Borrelia turcica and Candidatus Borrelia tachyglossi to other Borrelia species. Results presented here show that both these species are deeper-branching members of the genus Borrelia and their placement within this genus is strongly supported by phylogenetic analyses and multiple uniquely shared CSIs with the other Borrelia species. Based on the large body of evidence derived from phylogenetic, genomic, molecular, phenotypic and clinical features, it is contended that the characteristics clearly distinguishing the Borrelia and Borreliella genera are far more numerous and of different kinds than those discerning most (all) other neighbouring genera of prokaryotes. Thus, the placement of these two groups of microorganisms into distinct genera, Borrelia and Borreliella, which clearly recognizes the differences among them, is highly appropriate and it should lead to a better understanding of the clinical, molecular and biological differences between these two important groups of microbes.
Highlights
The family Borreliaceae includes species that are causative agents of Lyme disease (LD) and others that are causative agents of tick- and louse-borne relapsing fever (RF) [1,2,3,4,5]
All of the species that are part of the RF group were retained within the genus Borrelia, whereas all species related to the LD group were placed into a new genus called Borreliella [6]
They state that the proposal by Adeolu and Gupta [6] to divide the genus Borrelia into two genera “was largely based on the identification of conserved signature insertions/deletions (CSIs) and conserved signature proteins (CSPs) that are differentially present in the LD or RF Borrelia genogroup, as well as average nucleotide identity (ANI) values calculated between whole genomes of 18 Borrelia species including eight LD species and ten RF species . . . it is uncontested that these differences exist between LD and RF Borrelia”
Summary
The family Borreliaceae includes species that are causative agents of Lyme disease (LD) and others that are causative agents of tick- and louse-borne relapsing fever (RF) [1,2,3,4,5]. Margos et al [10] acknowledge that the LD and RF groups of species “have different clinical, biological, and epidemiological characteristics, and phylogenetic data is concordant with this, demonstrating that these two groups are genetically similar yet distinct and form independent monophyletic sister clades that once shared a common ancestor” They state that the proposal by Adeolu and Gupta [6] to divide the genus Borrelia into two genera “was largely based on the identification of conserved signature insertions/deletions (indels) (CSIs) and conserved signature proteins (CSPs) that are differentially present in the LD or RF Borrelia genogroup, as well as average nucleotide identity (ANI) values calculated between whole genomes of 18 Borrelia species including eight LD species and ten RF species . I discuss below our responses to all of these criticisms and the problem of using or relying on the suggested POCP threshold as a criterion for the delimitation of prokaryotic genera, which is the main basis of Margos et al.’s [10] resistance to our division of the genus Borrelia into two genera
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