Iron is an essential nutrient for bacterial life but it is not highly bioavailable. To overcome this, bacteria secrete small, high-affinity iron chelating molecules called siderophores. Though siderophores are crucial for bacterial iron acquisition, they have the potential to cause adverse effects such as iron toxic overload. A tight regulatory control for siderophore production is therefore essential to maintain an appropriate amount of intracellular iron. Our current understanding of siderophore regulation is incomplete as it is mostly gathered from single gene studies in distantly-related bacteria. Recent innovations in high-throughput functional genomics technology provides a new tool for studying gene expression and regulation in bacteria.
In this study, we aimed to decipher the regulatory networks governing bacterial iron acquisition in hypervirulent Klebsiella pneumoniae, a critical public health threat in which siderophores are a major virulence factor. Hypervirulent K. pneumoniae produces four different siderophores – aerobactin, yersiniabactin, enterobactin and salmochelin – which make distinct contributions to infection. Transcriptomics showed numerous genes were differentially expressed on iron starvation, including all siderophore-encoding genes, other genes for anaerobic stress response, and acquisitions of other metals. Growing the strain under increasing levels of iron starvation showed sequential siderophore induction, with aerobactin produced first followed by enterobactin, yersiniabactin and, lastly, salmochelin. Expression of the plasmid siderophores salmochelin (iroB) and aerobactin (iucA) seemed plateau a lower level of expression than the chromosome-encoded ones (enterobactin/entC and yersiniabactin/irp2). To gain insights into the expression dynamics of siderophores during infection, we utilized a murine bacteremia model. The mice infected with the K. pneumoniae SGH10 strain rapidly developed illness, with bacteria disseminating to multiple sites within the host (in the blood, lung and liver 4Χ108 CFU/mL, 1.4Χ109 CFU/g, 1.8Χ109 CFU/g, respectively). The in vivo expression of the four siderophores was investigated and they were found to be differentially expressed in the different host niches (in the blood and lung, yersiniabactin, approx. 8 times and 12 times, respectively).
Overall, our findings support the hypothesis that K. pneumoniae uses distinct regulatory mechanisms to control each of its four siderophores. Further work will explore the mechanisms underpinning their differential expression in different environmental and niches.