Cadmium removal from contaminated soil by thermally responsive elastin (ELPEC20) biopolymers

Cadmium contamination of soil is a major concern in the biosphere. Beyond the suite of available physico-chemical treatment methods, green and more efficient technologies are desired to reduce cadmium and other heavy metal contaminants to acceptable levels. Elastin-like polypeptides (ELP) composed of a polyhistidine domain (ELPH12) can be used as an environmentally benign chelating agent for ex situ soil washing. However, ELPH12 is relatively non-selective. A biopolymer with metal-binding domains that have stronger affinity, capacity, and selectivity would have distinct advantages. The aim of this work is to investigate the use of a new generation of ELP biopolymer, ELPEC20, containing synthetic phytochelatin (EC) as the metal-binding domain for ex situ soil washing. ELPEC20 was shown to bind cadmium more effectively and selectively than ELPH12. The binding constant of ELPEC20 is an order of magnitude higher and the binding capacity is fivefold higher than ELPH12. In contrast to ELPH12, no decrease in cadmium binding was observed in the presence of other competing metal ions. The improved selectivity and binding capacity provided by ELPEC20 were directly reflected in the enhanced cadmium extraction efficiency from contaminated soil. In batch washing studies up to 62% of the bound cadmium was removed by ELPEC20 while less than 12% was removed by ELPH12. Cadmium was removed not only from the exchangeable fraction but also the oxidizable fraction. The high-affinity binding sites of ELPEC20 also results in very rapid extraction with complete removal achieved within 1 h, suggesting that ELPEC20 could be used as part of a rapid (short retention time) technology with minimum possibility for the biodegradation of biopolymers.     

Bioremediation of polyaromatic hydrocarbon contaminated soils by native microflora and bioaugmentation with Sphingobium chlorophenolicum strain C3R: A feasibility study in solid- and slurry-phase microcosms

The aim of the research was to verify if a Sphingobium chlorophenolicum strain C3R was effective in the degradation of phenanthrene (Ph) in agricultural soil co-contaminated by metals and mixtures of PAHs. The presence of PAHs in mixtures produced interactive effects which could either increase or decrease the utilization rate of Ph by C3R and by the native bacterial microflora. Bioaugmentation significantly improved the biodegradation rate of Ph in the presence of both cadmium and arsenic and PAH mixtures. The augmented C3R strain persisted in inoculated microcosms as monitored by the DGGE analysis and outcompeted some indigenous bacteria. The potential role of the soil bacteria in PAH degradation could be envisaged. The results indicate the applicability of S. chlorophenolicum C3R toward in situ bioremediation of sites contaminated with phenanthrene alone or co-contaminated with low molecular weight PAHs and with cadmium and arsenate.     

两株多环芳烃降解菌协同对菲-镉污染的去除特性北大核心

【目的】从污染土壤中分离筛选一株多环芳烃降解菌,并探究其与Pseudomonas aeruginosa B6-2构建的混菌体系对菲-镉复合污染的修复效能,以及微生物代谢特性对不同镉浓度赋存的响应特性,以期为复合污染的生物修复提供优良菌株资源及应用技术参考。【方法】采用富集驯化、筛选纯化方法得到一株多环芳烃降解菌,通过生理生化特征和16S rRNA基因序列分析进行鉴定。利用高效液相色谱法和电感耦合等离子体质谱法评估不同镉浓度赋存下各反应体系对菲和镉的去除效能;通过菌体细胞形态的扫描电镜观测及菌株代谢活性检测,探讨镉胁迫对菲生物降解过程的影响机制。【结果】筛选得到一株具有重金属耐受性和多环芳烃高效降解菌SZ-3,经鉴定为节杆菌属;降解菌协同体系(M)具有良好的菲降解效能和抗镉胁迫优势。镉胁迫浓度为0.5、10 mg/L时,M对菲和镉的去除率分别高于85%、80%;镉胁迫浓度为25、50 mg/L时,2种污染物的去除率均大于65%。扫描电镜分析表明,镉胁迫导致菌体表面粗糙且出现不同程度变形,菌体间黏附性和聚集性提高。反应周期内,邻苯二酚1,2-双加氧酶活性与电子传递体系活性随镉浓度增加而降低,两者变化与菲降解速率变化一致。【结论】Arthrobacter sp.SZ-3是一株PAHs高效降解菌,能与Pseudomonas aeruginosa B6-2协同高效修复菲-镉复合污染,随着初始镉胁迫浓度增加,混菌协同对目标污染物去除的优势显著。     

Mechanisms of cadmium induced genomic instability

Cadmium is an ubiquitous environmental contaminant that represents hazard to humans and wildlife. It is found in the air, soil and water and, due to its extremely long half-life, accumulates in plants and animals. The main source of cadmium exposure for non-smoking human population is food. Cadmium is primarily toxic to the kidney, but has been also classified as carcinogenic to humans by several regulatory agencies. Current evidence suggests that exposure to cadmium induces genomic instability through complex and multifactorial mechanisms. Cadmium dose not induce direct DNA damage, however it induces increase in reactive oxygen species (ROS) formation, which in turn induce DNA damage and can also interfere with cell signalling. More important seems to be cadmium interaction with DNA repair mechanisms, cell cycle checkpoints and apoptosis as well as with epigenetic mechanisms of gene expression control. Cadmium mediated inhibition of DNA repair mechanisms and apoptosis leads to accumulation of cells with unrepaired DNA damage, which in turn increases the mutation rate and thus genomic instability. This increases the probability of developing not only cancer but also other diseases associated with genomic instability. In the in vitro experiments cadmium induced effects leading to genomic instability have been observed at low concentrations that were comparable to those observed in target organs and tissues of humans that were non-occupationally exposed to cadmium. Therefore, further studies aiming to clarify the relevance of these observations for human health risks due to cadmium exposure are needed.     

Assessing Genotypic Diversity and Symbiotic Efficiency of Five Rhizobial Legume Interactions Under Cadium Stress for Soil Phytoremediation

In the framework of soil phytoremediation using local legume plants coupled with their native root-nodulating bacteria to increase forage yields and preserve contaminated soils in arid regions of Tunisia, we investigated the diversity of bacteria from root nodules of Lathyrus sativus, Lens culinaris, Medicago marina, M. truncatula, and M. minima and the symbiotic efficiency of these five legume symbiosis under Cadmium stress. Fifty bacterial strains were characterized using physiological and biochemical features such heavy metals resistant, and PCR-RFLP of 16S rDNA. Taxonomically, the isolates nodulating L. sativus, and L. culinaris are species within the genera Rhizobium and the ones associated to Medicago sp, within the genera Sinorhizobium. The results revealed also that the cadmium tolerance of the different legumes-rhizobia interaction was as follows: M. minima<M. truncatula<M. marina<L. sativus<L. culinaris indicating that the effect of Cadmium on root nodulation and biomass production is more deleterious on M. minima-S. meliloti and M. truncatula-S. meliloti than in other symbiosis. Knowledge on genetic and functional diversity of M. marina, L. sativus and L. culinaris microsymbiotes is very useful for inoculant strain selection and can be selected to develop inoculants for soil phytoremediation.     

工业污染土壤中镉的化学形态及植物修复研究

对工业污染地中重金属镉的分布及存在的化学形态进行了系统研究.结果表明,土壤中吸附的镉可被水所溶出,pH值越低（酸度越强）,镉的溶出率越大,移动性越强,越容易被作物所吸收.中性条件下,此类污染土壤可溶态（水溶态、交换态和络合态）镉含量较低,分别为0.68%、12.70%和12.40%;不可溶态镉（颗粒态）约为74.40%.箩卜的茎部为此类镉污染地中镉的超富集植物器官.     

Short-Rotation Coppice of Willow for Phytoremediation of a Metal-Contaminated Agricultural Area: A Sustainability Assessment

Large areas of land contaminated with cadmium (Cd), lead (Pb), and zinc (Zn) are currently in agricultural production in the Campine region in Belgium. Cadmium contents in food and fodder crops frequently exceed legal threshold values, resulting in crop confiscation. This imposes a burden on agriculture and regional policy and, therefore, encourages proper soil management. One way to increase agricultural income and improve soil quality is by growing alternative nonfood crops such as willows in short-rotation coppice (SRC) systems that remediate the soil. This paper compares SRC of willow with rapeseed and energy maize regarding four attributes: metal accumulation capacity, gross agricultural income per hectare, CO₂ emission avoidance potential, and agricultural acceptance. Based on multicriteria decision analysis, we conclude that, although SRC of willow has a high potential as an energy and remediating crop, it is unlikely to be implemented on the short term in Flanders unless the economic incentives for the farmers are improved.     

Effects of lead and cadmium on oxygen consumption and life expectancy of the pupal parasitoid, Pimpla turionellae

The parasitoid Pimpla turionellae L. (Hym., Ichneumonidae) was fed either with Cd, Pb, and Cd+Pb —contaminated food or water. Body concentrations of heavy metal and effects on lifetime and respiration were studied. Cadmium at a concentration of 33 g per gram food or per ml water reduced life expectancy; whereas lead did such at 82 g per ml water only. Differences were found between sexes and modes of contamination concerning body concentrations of Cd and Pb. Lead affected respiration only via water contamination; whereas cadmium did such also via food contamination. Ecological consequences of the observations are discussed.

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Zusammenfassung Pimpla turionellae L. (Hym., Ichneumonidae) wurde mit Blei (82 g/g) und Cadmium (33 g/g) bzw. deren Kombination über Futter oder Wasser belastet. Schwermetallgehalte sowie Auswirkungen auf Lebensdauer und Sauerstoffverbrauch wurden untersucht. Der Einfluß der Schwermetalle war nach Geschlecht bzw. nach Art der Kontamination verschieden. Cadmium wirkt schon bei geringen Konzentrationen verkürzend auf die Lebensdauer, Blei erst bei relativ hohen Konzentrationen. Bei Blei ergab sich eine Abnahme des Sauerstoffverbrauches nur bei Belastung über das Wasser, bei Cadmium hingegen bei beiden Kontaminationsarten. Überlegungen ökologischer Konsequenzen werden angestellt.

     

Effect of cadmium on enzymatic digestion and sugar transport in the small intestine of rabbit

Cadmium compounds are found widely in our environment: for example, in food, water, soil, and ambient air. The most important exposure route of animals to cadmium in the general environment is via oral exposure. In oral cadmium intoxication, the immediate target organ is the gastrointestinal tract. The aim of the present work was to determine how cadmium acts on the intestinal absorption of sugars and on the sucrase activity through rabbit jejunum, after in vitro administration and/or oral administration of CdCl₂ in drinking water. Results obtained show that cadmium decreasesD-galactose accumulation in the jejunum tissue. This effect seems to be the result of an action mainly located on Na⁺-dependent sugar transport of the mucosal border of the intestinal epithelium, because cadmium seemnnot to modify the sugar diffusion across the intestinal epithelium. Cadmium has also been shown to inhibit the (Na⁺-K⁺)-ATPase activity of the enterocyte, which might explain the inhibition of theD-galactose Na⁺-dependent transport. Nevertheless, a direct action of the cadmium molecule on the Na⁺-dependent carrier cannot be discarded. Cadmium altered the sucrose activity when it was administered in the drinking water for 4 d.     

Culturable Heavy Metal-Resistant and Plant Growth Promoting Bacteria in V-Ti Magnetite Mine Tailing Soil from Panzhihua,China

To provide a basis for using indigenous bacteria for bioremediation of heavy metal contaminated soil, the heavy metal resistance and plant growth-promoting activity of 136 isolates from V-Ti magnetite mine tailing soil were systematically analyzed. Among the 13 identified bacterial genera, the most abundant genus was Bacillus (79 isolates) out of which 32 represented B. subtilis and 14 B. pumilus, followed by Rhizobium sp. (29 isolates) and Ochrobactrum intermedium (13 isolates). Altogether 93 isolates tolerated the highest concentration (1000 mg kg⁻¹) of at least one of the six tested heavy metals. Five strains were tolerant against all the tested heavy metals, 71 strains tolerated 1,000 mg kg⁻¹ cadmium whereas only one strain tolerated 1,000 mg kg⁻¹ cobalt. Altogether 67% of the bacteria produced indoleacetic acid (IAA), a plant growth-promoting phytohormone. The concentration of IAA produced by 53 isolates was higher than 20 µg ml⁻¹. In total 21% of the bacteria produced siderophore (5.50–167.67 µg ml⁻¹) with two Bacillus sp. producing more than 100 µg ml⁻¹. Eighteen isolates produced both IAA and siderophore. The results suggested that the indigenous bacteria in the soil have beneficial characteristics for remediating the contaminated mine tailing soil.     

A New Klebsiella planticola Strain (Cd-1) Grows Anaerobically at High Cadmium Concentrations and Precipitates Cadmium Sulfide

Heavy metal resistance by bacteria is a topic of much importance to the bioremediation of contaminated soils and sediments. We report here the isolation of a highly cadmium-resistant Klebsiella planticola strain, Cd-1, from reducing salt marsh sediments. The strain grows in up to 15 mM CdCl₂ under a wide range of NaCl concentrations and at acidic or neutral pH. In growth medium amended with thiosulfate, it precipitated significant amounts of cadmium sulfide (CdS), as confirmed by x-absorption spectroscopy. In comparison with various other strains tested, Cd-1 is superior for precipitating CdS in cultures containing thiosulfate. Thus, our results suggest that Cd-1 is a good candidate for the accelerated bioremediation of systems contaminated by high levels of cadmium.     

A possible mechanism for cadmium-induced hypertension in rats

The mechanism of cadmium-induced hypertension was explored by measuring noradrenaline metabolism. Cadmium $\text{in vitro}$ was shown to inhibit both monoamine oxidase and catechol-O-methyltransferase, the two enzymes which inactivate the neurotransmitters noradrenaline and adrenaline. However, rats which were injected or fed (via the drinking water) with cadmium showed that, among the tissues surveyed, these two enzymes were inhibited significantly only in the aorta. $\text{In vitro}$, cadmium was found to inhibit noradrenaline binding to membranes from the heart, lung, and kidney, while stimulating binding to aortic membranes, which suggests that the effects may be specific. These results suggest that, in the aorta, cadmium may inhibit the two catabolic enzymes of noradrenaline, while at the same time stimulating noradrenaline-binding. Thus the effects of noradrenaline on vascular smooth muscle would be increased as well as prolonged.     

Inhibition of L-threonine intestinal absorption in rabbits by cadmium

Cadmium compounds are widely spread in the environment. Animal exposure to cadmium compounds occurs mainly through foods or drinks contaminated by this metal. Cadmium has been shown to produce several negative effects on the gastrointestinal tract such as inhibition on sugars and amino acids absorption. The aim of the present work was to study the inhibitory characteristics of cadmium on L-threonine intestinal absorption in rabbits in order to understand about this malabsorption of nutrients. Our results show that L-thereonine tissue accumulation as well as mucosal to serosal transepithelial fluxes are decreased in a dose-dependent manner in rabbit jejunum. Amino acid diffusion across the intestinal epithelium was not affected by cadmium. A noncompetitive mechanism and a partial reversion by dithioerythritol (thiol groups protector) is described for this inhibition.     

土壤中镉、铅、锌及其相互作用对作物的影响

通过作物盆栽模拟试验(砂壤质褐土、pH值8.2)揭示：土壤中分别施入镉(CdCl2)、铅[Pb(CH3COO)2]或锌(ZnSO4)其影响表现为,植物各器官镉的含量超过对照植物的数倍至500倍。土壤镉浓度<5ppm和<10ppm分别造成某些蔬菜和水稻的污染。铅主要积累在植物根部,土壤铅污染对作物的影响较小。锌主要积累在植物叶片和根部,对水稻产生生长抑制的土壤锌浓度临界值不大于200ppm,此浓度对旱作无影响。土壤中同时施入镉和铅,植物对镉的吸收增加。而土壤中镉的增加却减少了植物体内铅的含量。土壤中由于镉、锌或铅、锌相互作用的结果,水稻对它们的吸收都有增加。在旱地土壤锌浓度的增高,降低了植物对镉、铅的吸收。镉、铅、锌同时施入土壤由于相互作用的结果,除锌之外,植物对镉、铅的吸收有明显下降。评价土壤重金属污染,不仅要看它们的含量及其存在形态,而且要分析它们之间的相互作用(促进或拮抗)特点。     

Plant screening of halophyte species for cadmium phytoremediation

Chloride (Cl-) has been related to increased phytoavailability of cadmium (Cd) in soil. A glasshouse experiment using a historically contaminated soil was undertaken to evaluate the effect of chloride on Cd uptake by salt-tolerant plants and possibly quantify the uptake of discrete Cd species (e.g. Cd2+, CdCl+, CdCl(0)2). Chloride treatments were applied as 100 mM NaCl and compared with equivalent Na2SO4 treatments. Activities of Cd species in soil pore water were calculated using the WHAM-VI speciation model. Cadmium solubility and uptake by plants was generally enhanced by addition of chloride to soil. Good correlations were found between Cd uptake and concentrations of Cd chloride complexes in soil pore water.     

Cadmium(II) Removal by a Hyperaccumulator Fungus <Emphasis Type="Italic">Phoma</Emphasis> sp. F2 Isolated from Blende Soil

A cadmium(II)-resistant fungus, strain F2, isolated from blende soil was identified as Phoma sp. by morphological study and internal transcribed spacer sequencing. This strain could accumulate 280 mg of Cd(II)/g dry weight mycelium. In liquid medium containing 163.8 mg Cd(II)/L, 96% of Cd(II) was removed by the actively growing mycelium. In addition, both oven-dried and lyophilized mycelium could effectively adsorb Cd(II). There were removed 91% and 46.2% of Cd(II) from 51.6 mg Cd(II)/L solution by lyophilized biomass and oven-dried biomass respectively. Transmission electron microscopy and energy-dispersive X-ray analysis showed the accumulation of Cd(II) in the mycelium cell walls. Our results demonstrated that Phoma sp. F2 was a hyperaccumulator for the removal of Cd(II) from contaminated soil and water.     

Inhibition of Nitrate Transporter 1.1-Controlled Nitrate Uptake Reduces Cadmium Uptake in Arabidopsis

Identification of mechanisms that decrease cadmium accumulation in plants is a prerequisite for minimizing dietary uptake of cadmium from contaminated crops. Here, we show that cadmium inhibits nitrate transporter 1.1 (NRT1.1)-mediated nitrate (NO₃⁻) uptake in Arabidopsis (Arabidopsis thaliana) and impairs NO₃⁻ homeostasis in roots. In NO₃⁻-containing medium, loss of NRT1.1 function in nrt1.1 mutants leads to decreased levels of cadmium and several other metals in both roots and shoots and results in better biomass production in the presence of cadmium, whereas in NO₃⁻-free medium, no difference is seen between nrt1.1 mutants and wild-type plants. These results suggest that inhibition of NRT1.1 activity reduces cadmium uptake, thus enhancing cadmium tolerance in an NO₃⁻ uptake-dependent manner. Furthermore, using a treatment rotation system allowing synchronous uptake of NO₃⁻ and nutrient cations and asynchronous uptake of cadmium, the nrt1.1 mutants had similar cadmium levels to wild-type plants but lower levels of nutrient metals, whereas the opposite effect was seen using treatment rotation allowing synchronous uptake of NO₃⁻ and cadmium and asynchronous uptake of nutrient cations. We conclude that, although inhibition of NRT1.1-mediated NO₃⁻ uptake by cadmium might have negative effects on nitrogen nutrition in plants, it has a positive effect on cadmium detoxification by reducing cadmium entry into roots. NRT1.1 may regulate the uptake of cadmium and other cations by a common mechanism.Cadmium is highly toxic to humans (Nicholson et al., 1983), and its primary route of entry into the body is through crops grown in cadmium-contaminated soil (Clemens et al., 2013). A recent survey indicated that vegetables and rice (Oryza sativa) account for approximately 40% and 38%, respectively, of total cadmium exposure in residents of Shanghai, China’s largest city (He et al., 2013). However, cadmium contamination of agricultural soils as a result of rapid industrial development and release of agrochemicals into the environment is an increasingly serious problem. Many strategies have been proposed for remediating cadmium-contaminated soil to prevent cadmium uptake by crops. These strategies include the dig-and-dump method or encapsulation of the contaminated soil, chemical immobilization or extraction of cadmium, and phytoremediation by cadmium-hyperaccumulating plants (Pulford and Watson, 2003). However, the dig-and-dump and chemical methods are expensive, whereas phytoremediation requires several growing seasons to be effective, making it impractical in regions where farmland is limited and food supply insufficient.The shortfalls of these strategies have prompted researchers to develop alternative techniques that are cost-effective and interfere less with crop production. Use of nitrogen fertilizers is one of the most important agronomic practices and it has been suggested that their appropriate use might provide a relatively inexpensive, time-saving, and effective strategy for reducing cadmium entry into, and accumulation in, crops because NO₃⁻ facilitates cadmium uptake in hydroponically grown plants (Sarwar et al., 2010; Luo et al., 2012). However, in a preliminary study, we found that, in plants grown in soil, the effect of the nitrogen form on cadmium accumulation was strongly associated with the pH-buffering capacity of the soil. In soil with a lower pH-buffering capacity, application of ammonium (NH₄⁺) resulted in higher cadmium levels in plants than application of NO₃⁻, probably as a result of soil acidification by NH₄⁺, and the opposite effect was seen when plants were grown in soil with higher pH-buffering capacity (S.K. Fan, S.T. Du, and C.W. Jin, unpublished data). This suggests that management of the use of nitrogen fertilizers to prevent cadmium entry into crops might be difficult because of the wide variation in soil pH-buffering capacity.Because NO₃⁻ facilitates cadmium uptake in hydroponically grown plants as described above, modification of NO₃⁻ uptake pathways in plants might also affect cadmium uptake, in which case modifying these pathways to reduce cadmium entry into crops could circumvent the risks and the difficulties involved in nitrogen fertilizer management. Exposure to cadmium has been shown to reduce NO₃⁻ uptake by roots (Hernández et al., 1997; Gouia et al., 2000; Rizzardo et al., 2012), but this has been assumed to be deleterious to plant growth (Finkemeier et al., 2003; Rizzardo et al., 2012). The process by which NO₃⁻ is taken up across the root plasma membrane is complex, and several nitrate transporters (NRTs) involved in NO₃⁻ uptake from the growth medium have been characterized. In Arabidopsis (Arabidopsis thaliana), NRT1.1 is a dual-affinity transporter involved in both high- and low-affinity uptake, NRT1.2 is involved only in low-affinity NO₃⁻ uptake, whereas NRT2.1, NRT2.2, and NRT2.4 are only involved in high-affinity NO₃⁻ uptake (Wang et al., 2012; Léran et al., 2014). However, the transporter responsible for the cadmium-induced decrease in NO₃⁻ uptake remains unknown. Given the presumed association between NO₃⁻ uptake and cadmium uptake, it is important to identify the molecular mechanism involved in this process, and it is particularly important to determine whether the modulation of relevant NO₃⁻ transporters affects cadmium entry into plants.In this study, we investigated the relationship between NO₃⁻ uptake and cadmium uptake in Arabidopsis roots. To our knowledge, our results reveal a new mechanism for resisting cadmium toxicity: Cadmium reduces NO₃⁻ uptake by inhibiting NRT1.1 activity, which in turn reduces cadmium entry into root cells. As a result, cadmium levels in plants are lower and plant growth is improved. Our findings may provide a strategy for minimizing cadmium accumulation in crops grown in contaminated soil using biotechnological pathways to decrease NO₃⁻ uptake.     

A possible mechanism for cadmium-induced hypertension in rats

The mechanism of cadmium-induced hypertension was explored by measuring noradrenaline metabolism. Cadmium $\text{in vitro}$ was shown to inhibit both monoamine oxidase and catechol-$\text{O}$-methyltransferase, the two enzymes which inactivate the neurotransmitters noradrenaline and adrenaline. However, rats which were injected or fed (via the drinking water) with cadmium showed that, among the tissues surveyed, these two enzymes were inhibited significantly only in the aorta. $\text{In vitro}$, cadmium was found to inhibit noradrenaline binding to membranes from the heart, lung, and kidney, while stimulating binding to aortic membranes, which suggests that the effects may be specific. These results suggest that, in the aorta, cadmium may inhibit the two catabolic enzymes of noradrenaline, while at the same time stimulating noradrenaline-binding. Thus the effects of noradrenaline on vascular smooth muscle would be increased as well as prolonged.     

Deregulation of transition metals homeostasis is a key feature of cadmium toxicity in Salmonella

Cadmium is a highly toxic metal whose presence in the environment represents a challenge for all forms of life. To improve our knowledge on cadmium toxicity, we have explored Salmonella Typhimurium responses to this metal. We have found that cadmium induces the concomitant expression of the cation efflux pump ZntA and of the high affinity zinc import system ZnuABC. This observation suggests that cadmium accumulation within the cell induces a condition of apparent zinc starvation, possibly due to the ability of this metal to compete with zinc for the metal binding site of proteins. This hypothesis is supported by the finding that strains lacking ZntA or ZnuABC are hyper-susceptible to cadmium and that the cadmium-induced growth defect of a znuABC mutant strain is largely relieved by zinc supplementation. A similar growth defect was observed for a mutant with impaired ability to acquire iron, whereas cadmium does not affect growth of a strain defective in manganese import. Cadmium also influences the expression and activity of the two cytoplasmic superoxide dismutases FeSOD and MnSOD, which are required to control cadmium-mediate oxidative stress. Exposure to cadmium causes a reduction of FeSOD activity in Salmonella wild type and the complete abrogation of its expression in the strain defective in iron import. In contrast, although MnSOD intracellular levels increase in response to cadmium, we observed discrepancies between protein levels and enzymatic activity which are suggestive of incorporation of non-catalytic metals in the active site or to cadmium-mediated inhibition of manganese import. Our results indicate that cadmium interferes with the ability of cells to manage transition metals and highlight the close interconnections between the homeostatic mechanisms regulating the intracellular levels of different metals.