Publications

Iron Acquisition and Siderophore Release by Carbapenem-Resistant Sequence Type 258

Holden VI, Wright MS, Houle S, Collingwood A, Dozois CM, Adams MD, Bachman MA

PMID: 29669884

Abstract

is rapidly acquiring resistance to all known antibiotics, including carbapenems. Multilocus sequence type ST258 (sequence type 258), carrying a gene encoding the carbapenemase () on a transmissible plasmid, is the most prevalent carbapenem-resistant (CRE) in the United States and has disseminated worldwide. Previously, whole-genome sequencing identified core genome single nucleotide variants that divide ST258 into two distinct clades, ST258a and ST258b. Furthermore, a subset of ST258b strains have a 347-base deletion within the enterobactin (Ent) exporter gene Despite the predicted inability of these strains to secrete the siderophore Ent, this clade is prevalent among clinical isolates, indicating that a full-length gene is not necessary for infection. To compare the transcriptional responses of ST258 subtypes to iron limitation, we performed transcriptome sequencing (RNA-Seq) in minimal medium alone or supplemented with iron or human serum and measured gene expression patterns. Iron limitation induced differential expression of distinct iron acquisition pathways when comparing ST258a and ST258b strains, including the upregulation of the hemin transport operon in partial deletion isolates. To measure how strains vary in iron chelation and siderophore production, we performed chrome azurol S (CAS) and Arnow assays as well as mass spectrometry. We determined that both ST258a and ST258b strains grow under iron-depleted conditions, can utilize hemin for growth, and secrete Ent, despite the partial deletion in a subset of ST258b strains. All carbapenem-resistant (CR) strains tested were susceptible to growth inhibition by the Ent-sequestering innate immune protein lipocalin 2. Carbapenem-resistant , including , are a major health care concern worldwide because they cause a wide range of infection and are resistant to all or nearly all antibiotics. To cause infection, these bacteria must acquire iron, and a major mechanism of acquiring iron is by secreting a molecule called enterobactin that strips iron from host proteins. However, a subset of carbapenem-resistant strains that lack a portion of the gene that is required for enterobactin secretion was recently discovered. To understand how these mutant strains obtain iron, we studied their transcriptional responses, bacterial growth, and enterobactin secretion under iron-limited conditions. We found that strains both with mutated and intact genes grow under iron-limiting conditions, secrete enterobactin, and utilize an alternate iron source, hemin, for growth. Our data indicate that carbapenem-resistant can use varied methods for iron uptake during infection.