Iron, copper, zinc, and manganese transport and regulation in pathogenic Enterobacteria: correlations between strains, site of infection and the relative importance of the different metal transport systems for virulence

Abstract

For all microorganisms, acquisition of metal ions is essential for survival in the environment or in their infected host. Metal ions are required in many biological processes as components of metalloproteins and serve as cofactors or structural elements for enzymes. However, it is critical for bacteria to ensure that metal uptake and availability is in accordance with physiological needs, as an imbalance in bacterial metal homeostasis is deleterious. Indeed, host defense strategies against infection either consist of metal starvation by sequestration or toxicity by the highly concentrated release of metals. To overcome these host strategies, bacteria employ a variety of metal uptake and export systems and finely regulate metal homeostasis by numerous transcriptional regulators, allowing them to adapt to changing environmental conditions. As a consequence, iron, zinc, manganese, and copper uptake systems significantly contribute to the virulence of many pathogenic bacteria. However, during the course of our experiments on the role of iron and manganese transporters in extraintestinal Escherichia coli (ExPEC) virulence, we observed that depending on the strain tested, the importance of tested systems in virulence may be different. This could be due to the different set of systems present in these strains, but literature also suggests that as each pathogen must adapt to the particular microenvironment of its site of infection, the role of each acquisition system in virulence can differ from a particular strain to another. In this review, we present the systems involved in metal transport by Enterobacteria and the main regulators responsible for their controlled expression. We also discuss the relative role of these systems depending on the pathogen and the tissues they infect.

Keywords: Enterobacteria; copper; iron; manganese; metal transporters; regulation; virulence; zinc.

Figure 1

Iron transporters in Enterobacteria and metal availability in the host during infection. In a healthy individual, Fe³⁺ is stored intracellularly in complex with ferritin (Fn), bound by serum transferrin (TF) or bound by lactoferrin (LTF) at mucosal surfaces. In the blood, Fe²⁺ is complexed with haem, which is bound by haemoglobin (Hb) within red blood cells. During infection, haemolytic bacterial cytotoxins damage host cells, leading to the release of ferritin, while hemolytic toxins lyse erythrocytes, liberating Hb, thus, bound by haptoglobin (HP). Free haem is scavenged by haemopexin (HPX). Secreted bacterial siderophores can remove iron from transferrin, lactoferrin and ferritin, and siderophore-iron complexes are then recognized by cognate receptors at the bacterial surface. Similarly, secreted haemophores can remove haem from haemoglobin or haemopexin. Enterobacteria also possess receptors for free haem. Outer membrane receptors for haem can also transport haem from haemoglobin, and HemR/HmuR can transport haem from haemopexin and haptoglobin. Enterobactin-mediated iron acquisition can be inhibited by the innate immune protein lipocalin-2 (NGAL, Neutrophil Gelatinase-Associated Lipocalin), which binds and sequesters siderophores (Skaar, ; Hood and Skaar, 2012). Transporter families are indicated below transporters. One representative outer membrane receptor that transports the ligand to the periplasm represents different siderophore transporters, the identity of the outer membrane receptor being shown at the bottom under the inner membrane transporter in brackets. ABC, ATP-binding cassette; ZIP, Zrt/Irt-like protein; NRAMP, natural resistance associated with macrophage protein; OFeT, oxidase-dependent iron transporter; ILT, iron/lead transporter superfamily; FeoB, ferrous iron uptake family; OM, outer membrane; PP, periplasm; IM, inner membrane; CP, cytoplasm.

Figure 2

Manganese, zinc, and copper transporters in Enterobacteria and metal availability in the host during infection. Each microorganism possesses a different complement of transporters to ensure a good balance between import and export of metals. In deep tissues, infection leads to the recruitment of neutrophils, which deliver calprotectin and S100A12 to the infected site. To compete with host-mediated zinc and manganese sequestration, bacteria express high affinity metal transporters. It is yet unclear if uptake of Mn²⁺ and Zn²⁺ is mediated by an outer membrane receptor (OMR) coupled to the TonB/ExbB/ExbD system or by diffusion through porins. Ceruloplasmin is a multi-copper oxidase produced in the liver of healthy individuals where it binds Cu²⁺. The complex is then released in blood to bring Cu²⁺ to tissues. Following phagocytosis of bacteria in macrophages, interferon-γ induces the import of Cu⁺ inside the phagolysosome to induce bacterial killing. However, pathogenic Enterobacteria have several systems to detoxify their periplasm or cytoplasm (Hood and Skaar, 2012). Transporter families are indicated in brackets. NRAMP, natural resistance associated with macrophage protein; ABC, ATP-binding cassette; MntP, manganese transporter efflux pump family; ZIP, Zrt/Irt-like protein; MFS, Major Facilitator Superfamily; RND, resistance and nodulation; CDF, cation diffusion facilitator; OM, outer membrane; PP, periplasm; IM, inner membrane; CP, cytoplasm.