The pathogenic actinobacterium Rhodococcus equi: what's in a name?

Abstract

Summary Rhodococcus equi is the only recognized animal pathogenic species within an extended genus of metabolically versatile Actinobacteria of considerable biotechnological interest. Best known as a horse pathogen, R. equi is commonly isolated from other animal species, particularly pigs and ruminants, and causes severe opportunistic infections in people. As typical in the rhodococci, R. equi niche specialization is extrachromosomally determined, via a conjugative virulence plasmid that promotes intramacrophage survival. Progress in the molecular understanding of R. equi and its recent rise as a novel paradigm of multihost adaptation has been accompanied by an unusual nomenclatural instability, with a confusing succession of names: "Prescottia equi", "Prescotella equi", Corynebacterium hoagii and Rhodococcus hoagii. This article reviews current advances in the genomics, biology and virulence of this pathogenic actinobacterium with a unique mechanism of plasmid‐transferable animal host tropism. It also discusses the taxonomic and nomenclatural issues around R. equi in the light of recent phylogenomic evidence that confirms its membership as a bona fide Rhodococcus.

Production of Lipopeptide Biosurfactant by a Hydrocarbon-Degrading Antarctic Rhodococcus

“…The genomic data reveals a typical Rhodococci feature with an approximate size of 8,440,759 bp, a G + C content of 63.28%, 8462 protein-coding sequences (CDs), 6 rRNA and 82 tRNAs genes. The large size and high CDs of the draft genome are believed to be associated with the extensive actinobacterial chromosome linearisation and dependent to the host itself, which are isolated from an extreme niche (Antarctic soil) [ 46 ]. The annotated data also shows a high similarity with two other known genomes of R. erythropolis (PR4 and CCM2925) ( Table 1 ).…”

Conservation of Rhodococcus equi (Magnusson 1923) Goodfellow and Alderson 1977 and rejection of Rhodococcus hoagii (Morse 1912) Kämpfer et al. 2014

“…nov. by Kämpfer et al [12], which involved polyphasic taxonomic analyses that included the type strains C. hoagii DSM 20295 T (=ATCC 7005 T ) and R. equi DSM 20307 T (=ATCC 6939 T ). These studies (i) confirmed the identity of R. equi DSM 20307 T and C. hoagii DSM 20295 T as the same species while (ii) they did not find distinct chemotaxonomic differences or 16S rDNA-based phylogenetic separation from other rhodococci to justify the reclassification of R. equi into a new genus 'Prescotella' [12] (consistent with compelling phylogenomic evidence [13,14]). In application of the principle of priority of the International Code of Nomenclature of Prokaryotes, aka the 'Prokaryotic code' [15], Kämpfer et al proposed to reclassify Corynebacterium hoagii and Rhodococcus equi as Rhodococcus hoagii comb.…”

“…In contrast, the equi epithet has been in widespread use and constantly associated with its cognate bacterial species since its discovery in 1923 by H. Magnusson as the causative agent of a severe infectious disease of foals [26,27]. While R. equi can also cause opportunistic human infections and colonize other animal species [14,17,18,[28][29][30], it remains best known as a major horse pathogen and thus the epithet is aptly descriptive of the species [31,32]. Indeed, the name R. equi has a solid standing in veterinary and human medicine, animal science and the equine industry, and the change to R. hoagii is disconcerting and likely to hamper the traceability and interpretation of the medical, scientific and technical literature regarding this pathogen.…”

“…Such mistakes are not uncommon, as illustrated e.g. by the whole genome sequence of the type strain of Rhodococcus rhodnii NRRL B-16535 T (GenBank accession number GCA_000720375.1), which phylogenetically does not belong to the rhodococci but to an unknown, distant actinobacterium [13,14].…”

“…While validly published, the use of the new name Rhodococcus hoagii is problematic for a number of reasons. We discussed these in a recent review on R. equi [ 14 ] and below we summarise the main points and expand on some additional key arguments, for consideration by the Judicial Commission of the International Committee on Systematics of Prokaryotes (ICSP) for rejection of the name R. hoagii according to Rule 56a of the Prokaryotic Code [ 15 ]. It is beyond this Request for an Opinion to address the issue of the potential illegitimacy of Rhodococcus Zopf 1981 (bacterial genus) because a later homonym of the algal genus Rhodococcus Hansgirg 1884; for more information about this question we refer to a note by Tindall [ 16 ].…”