Interplay between siderophores and colibactin genotoxin in Escherichia coli.

Highly pathogenic Escherichia coli strains that belong to the phylogenetic group B2 have developed a greater ability to acquire iron (heme receptor and numerous siderophores), to produce the genotoxin colibactin and to synthesize antimicrobial siderophore-microcins. There is an increased prevalence of these E. coli strains over the last 30 years in the intestinal microbiota in industrialized countries. Integrating the regulation of fitness/virulence factors, such as siderophores, colibactin and siderophore-microcins into networks that respond to specific environmental signals, such as the local iron concentration, could result in an accurate production of specific fitness/virulence factors, so that the E. coli can adapt to the competitive environment that is the gut and/or the blood. Iron deficiency is common in infancy, even in industrialized countries. Usual strategies for anemia correction are iron supplementation and iron fortification of foods. The long-term consequences and risks associated with high iron supply in the light of this iron-dependent network described in this review could explain at least in part the increased prevalence of E. coli B2 in the gut of people in industrialized countries. © 2017 IUBMB Life, 69(6):435-441, 2017.

The Bacterial Stress-Responsive Hsp90 Chaperone (HtpG) Is Required for the Production of the Genotoxin Colibactin and the Siderophore Yersiniabactin in Escherichia coli.

The genotoxin colibactin, synthesized by Escherichia coli, is a secondary metabolite belonging to the chemical family of hybrid polyketide/nonribosomal peptide compounds. It is produced by a complex biosynthetic assembly line encoded by the pks pathogenicity island. The presence of this large cluster of genes in the E. coli genome is invariably associated with the high-pathogenicity island, encoding the siderophore yersiniabactin, which belongs to the same chemical family as colibactin. The E. coli heat shock protein HtpG (Hsp90Ec) is the bacterial homolog of the eukaryotic molecular chaperone Hsp90, which is involved in the protection of cellular proteins against a variety of environmental stresses. In contrast to eukaryotic Hsp90, the functions and client proteins of Hsp90Ec are poorly known. Here, we demonstrated that production of colibactin and yersiniabactin is abolished in the absence of Hsp90Ec We further characterized an interplay between the Hsp90Ec molecular chaperone and the ClpQ protease involved in colibactin and yersiniabactin synthesis. Finally, we demonstrated that Hsp90Ec is required for the full in vivo virulence of extraintestinal pathogenic E. coli This is the first report highlighting the role of heat shock protein Hps90Ec in the production of two secondary metabolites involved in E. coli virulence.

Iron Homeostasis Regulates the Genotoxicity of Escherichia coli That Produces Colibactin.

The genotoxin colibactin is a secondary metabolite produced by a variety of pathogenic enterobacteria. Its biosynthesis requires the enzymatic activity of the phosphopantetheinyl transferase (PPTase) ClbA. We previously showed that ClbA can also contribute to the production of siderophores. Because the biosynthesis of siderophores is regulated by iron availability, we hypothesized that iron could also modulate the production of colibactin through the transcriptional regulation of clbA This study revealed an increased transcription of clbA under iron-limiting conditions and a decrease of clbA expression in iron-rich media. We demonstrate that clbA transcription is regulated by both the ferric uptake regulator (Fur) and the small regulatory noncoding RNA RyhB. We evidenced that the regulation of the transcription of clbA by Fur and RyhB leads to the regulation of colibactin production. This work highlights the complex mechanism of regulation of an important virulence factor by the two major regulators of bacterial iron homeostasis, making iron a key environmental factor contributing to bacterial virulence and carcinogenesis.

Interplay between siderophores and colibactin genotoxin biosynthetic pathways in Escherichia coli.

In Escherichia coli, the biosynthetic pathways of several small iron-scavenging molecules known as siderophores (enterobactin, salmochelins and yersiniabactin) and of a genotoxin (colibactin) are known to require a 4'-phosphopantetheinyl transferase (PPTase). Only two PPTases have been clearly identified: EntD and ClbA. The gene coding for EntD is part of the core genome of E. coli, whereas ClbA is encoded on the pks pathogenicity island which codes for colibactin. Interestingly, the pks island is physically associated with the high pathogenicity island (HPI) in a subset of highly virulent E. coli strains. The HPI carries the gene cluster required for yersiniabactin synthesis except for a gene coding its cognate PPTase. Here we investigated a potential interplay between the synthesis pathways leading to the production of siderophores and colibactin, through a functional interchangeability between EntD and ClbA. We demonstrated that ClbA could contribute to siderophores synthesis. Inactivation of both entD and clbA abolished the virulence of extra-intestinal pathogenic E. coli (ExPEC) in a mouse sepsis model, and the presence of either functional EntD or ClbA was required for the survival of ExPEC in vivo. This is the first report demonstrating a connection between multiple phosphopantetheinyl-requiring pathways leading to the biosynthesis of functionally distinct secondary metabolites in a given microorganism. Therefore, we hypothesize that the strict association of the pks island with HPI has been selected in highly virulent E. coli because ClbA is a promiscuous PPTase that can contribute to the synthesis of both the genotoxin and siderophores. The data highlight the complex regulatory interaction of various virulence features with different functions. The identification of key points of these networks is not only essential to the understanding of ExPEC virulence but also an attractive and promising target for the development of anti-virulence therapy strategies.

The Polyamine Spermidine Modulates the Production of the Bacterial Genotoxin Colibactin.

Colibactin is a polyketide/nonribosomal peptide produced by Escherichia coli strains that harbor the pks island. This toxin induces DNA double-strand breaks and DNA interstrand cross-links in infected eukaryotic cells. Colibactin-producing strains are found associated with colorectal cancer biopsy specimens and promote intestinal tumor progression in various murine models. Polyamines are small polycationic molecules produced by both microorganisms and eukaryotic cells. Their levels are increased in malignancies, where they contribute to disease progression and metastasis. In this study, we demonstrated that the endogenous spermidine synthase SpeE is required for full genotoxic activity of colibactin-producing E. coli Supplying spermidine in a ΔspeE pks+E. coli strain restored genotoxic activity. Spermidine is involved in the autotoxicity linked to colibactin and is required for direct damaging activity on DNA. The production of the colibactin prodrug motif is impaired in ΔspeE mutants. Therefore, we demonstrated that spermidine has a direct impact on colibactin synthesis.IMPORTANCE Colibactin-producing Escherichia coli strains are associated with cancerous and precancerous colorectal tissues and are suspected of promoting colorectal carcinogenesis. In this study, we describe a new interplay between the synthesis of the genotoxin colibactin and the polyamine spermidine. Polyamines are highly abundant in cancer tissue and are associated with cell proliferation. The need for spermidine in genotoxic activity provides a new perspective on the role of these metabolites in the pathogenicity of colibactin-producing E. coli strains in colorectal cancer.

High iron supply inhibits the synthesis of the genotoxin colibactin by pathogenic Escherichia coli through a non-canonical Fur/RyhB-mediated pathway.

The genotoxin colibactin is a secondary metabolite produced by a variety of pathogenic Enterobacteria, and is associated with colon cancer development and acute systemic infections. The colibactin biosynthesis requires the enzymatic activity of the phosphopantetheinyl transferase ClbA. We recently evidenced that two master regulators of bacterial iron homeostasis, i.e. the ferric uptake regulator (Fur) and the small regulatory non-coding RNA RyhB, were involved in the regulation of the clbA transcription and of the colibactin production. In this study, we investigated the impact of high iron supply on clbA transcription and colibactin production in wild type, ΔryhB, Δfur and ΔryhB Δfur strains. This revealed that high iron resulted in decreased synthesis of the genotoxin colibactin through both pathways dependent and independent of Fur/RyhB. This work highlights the complex regulatory mechanism that controls an important bacterial virulence and carcinogenesis factor by regulators of bacterial iron homeostasis.