The Hydroxamate Siderophore Rhequichelin Is Required for Virulence of the Pathogenic Actinomycete Rhodococcus equi

Abstract

We previously showed that the facultative intracellular pathogen Rhodococcus equi produces a nondiffusible and catecholatecontaining siderophore (rhequibactin) involved in iron acquisition during saprophytic growth. Here, we provide evidence that the rhbABCDE cluster directs the biosynthesis of a hydroxamate siderophore, rhequichelin, that plays a key role in virulence. The rhbC gene encodes a nonribosomal peptide synthetase that is predicted to produce a tetrapeptide consisting of N 5 -formyl-N 5 -hydroxyornithine, serine, N 5 -hydroxyornithine, and N 5 -acyl-N 5 -hydroxyornithine. The other rhb genes encode putative tailoring enzymes mediating modification of ornithine residues incorporated into the hydroxamate product of RhbC. Transcription of rhbC was upregulated during growth in iron-depleted medium, suggesting that it plays a role in iron acquisition. This was confirmed by deletion of rhbCD, rendering the resulting strain R. equi SID2 unable to grow in the presence of the iron chelator 2,2-dipyridyl. Supernatant of the wild-type strain rescued the phenotype of R. equi SID2. The importance of rhequichelin in virulence was highlighted by the rapid increase in transcription levels of rhbC following infection and the inability of R. equi SID2 to grow within macrophages. Unlike the wild-type strain, R. equi SID2 was unable to replicate in vivo and was rapidly cleared from the lungs of infected mice. Rhequichelin is thus a key virulence-associated factor, although nonpathogenic Rhodococcus species also appear to produce rhequichelin or a structurally closely related compound. Rhequichelin biosynthesis may therefore be considered an example of cooption of a core actinobacterial trait in the evolution of R. equi virulence.

Effect of Macrolide and Rifampin Resistance on the Fitness of Rhodococcus equi

“…The defect in in vitro growth of dual-resistant clinical R. equi isolates was more severe in the absence of iron. R. equi is a facultative intracellular pathogen of host macrophages (20), an environment deprived of iron; to compensate, the bacterium utilizes several mechanisms for iron acquisition (21,22). Due to the low-iron environment in macrophages, it is possible that dual macrolide and rifampin resistance diminishes the ability of R. equi to survive in macrophages.…”

Repurposing of gallium‐based drugs for antibacterial therapy

“…Given the essential role of iron in bacterial metabolism, and considering the very low amounts of free iron in the mammalian host, it is not surprising that, in different bacterial pathogens, mutations in specific iron-uptake mechanisms have been found to dramatically reduce the ability to cause infection in a variety of animal models [65][66][67][68][69][70][71]. Consequently, inhibition of iron uptake or disruption of bacterial iron metabolism has been proposed as a promising target for the development of new antibacterial drugs [72][73][74][75].…”

Transcriptome Reprogramming by Plasmid-Encoded Transcriptional Regulators Is Required for Host Niche Adaption of a Macrophage Pathogen

“…In addition to the plasmid-encoded virulence factors, a number of virulence-associated factors, including cytoadhesive pili (24), the hydroxamate siderophore rhequichelin (25), and isocitrate lyase (26), are encoded by the chromosome. Comparative genome analysis demonstrated that many of these have orthologues in nonpathogenic actinobacteria.…”