Copper resistance of biofilm cells of the plant pathogen Xylella fastidiosa

PII-like signaling proteins: a new paradigm in orchestrating cellular homeostasis

Members of the PII superfamily are versatile, multitasking signaling proteins ubiquitously found in all domains of life. They adeptly monitor and synchronize the cell's carbon, nitrogen, energy, redox, and diurnal states, primarily by binding interdependently to adenyl-nucleotides, including charged nucleotides (ATP, ADP, and AMP) and second messengers such as cyclic adenosine monophosphate (cAMP), cyclic di-adenosine monophosphate (c-di-AMP), and S-adenosylmethionine-AMP (SAM-AMP). These proteins also undergo a variety of posttranslational modifications, such as phosphorylation, adenylation, uridylation, carboxylation, and disulfide bond formation, which further provide cues on the metabolic state of the cell. Serving as precise metabolic sensors, PII superfamily proteins transmit this information to diverse cellular targets, establishing dynamic regulatory assemblies that fine-tune cellular homeostasis. Recently discovered, PII-like proteins are emerging families of signaling proteins that, while related to canonical PII proteins, have evolved to fulfill a diverse range of cellular functions, many of which remain elusive. In this review, we focus on the evolution of PII-like proteins and summarize the molecular mechanisms governing the assembly dynamics of PII complexes, with a special emphasis on the PII-like protein SbtB.

Persistence and viable but non-culturable state induced by streptomycin in Erwinia amylovora

Persister cell and viable but non-culturable (VBNC) state of bacteria are survival strategies against antibiotics and various environmental stresses, respectively, but they tend to be ignored in agriculture fields, even though bacteria can regain their abilities to survive and produce disease once those stresses disappear. This study was carried out to determine whether persister cell and VBNC state in Erwinia amylovora are present after exposures to streptomycin, the length of their persistence, and the steps needed to decrease the inoculum. Persister cells were observed using biphasic killed growth curve for 4-8 h when the late stationary phase cells of E. amylovora were cultured in liquid medium containing streptomycin. This state was maintained for up to 12 h based on the colony forming units (CFUs) of the colonies that grew on the mannitol glutamate yeast extract (MGY) medium after streptomycin was removed. The CFUs on the MGY medium were lower than the total count determined using the LIVE/DEAD Kit, suggesting that persister cells and VBNC state might co-exist for up to 12 h after exposure to streptomycin. However, after 12 h, E. amylovora cells did not continue to grow on the medium for 9 days, suggesting that they entered a VBNC state at that time and remained in a persistent state. In addition, based on the Redox Sensor Green staining method, the presence of both states was confirmed for up to 12 h, and only then did the VBNC state became apparent. Furthermore, persister cells were observed for up to 24 h, and damaged cells reduced when E. amylovora cells were culture in distilled water with streptomycin, indicating that the uptake of lower nutrients in E. amylovora led to prolonged persister cells and VBNC state, which are more likely to survive after streptomycin treatments. The addition of sucrose and oxytetracycline to distilled water containing streptomycin reduced persister cells than other sources did. Thus, to inhibit the spread of fire blight, management techniques must consider the hazards of using streptomycin treatments that induce dormancy, such as persister cells and VBNC state, beyond the development of resistant strain.

1H-NMR Metabolomics Study after Foliar and Endo-Therapy Treatments of Xylella fastidiosa subsp. pauca Infected Olive Trees: Medium Time Monitoring of Field Experiments

Here we report the medium-term effects of foliar spray and endo-therapy treatments with different doses of a Cu/Zn citric acid biocomplex (Dentamet®) in Xylella fastidiosa infected olive trees of Salento, Apulia region (South-east Italy). Leaf extract samples from field-treated 150 years old olive trees cvs Ogliarola salentina and Cellina di Nardò were studied by 1H NMR-based metabolomics. The result of different applications of Dentamet® endo-therapy after 60, 120 and 180 days in comparison with traditional foliar spray treatment and water injection as a control have been investigated. The metabolic profile analyses, performed by 1H NMR-based metabolomic approach, indicated plant metabolites variations connected to the disease progression such as mannitol, quinic acid, and oleuropein related compounds. The best results, in terms of discrimination of the metabolic profiles with respect to water injection, were found for monthly endo-therapy treatments. Dentamet® foliar application demonstrated more specific time related progressive effectiveness with respect to intravascular treatments. Therefore, besides a possible more effective performance of endo-therapy with respect to foliar treatments, the need of further doses/frequencies trimming to obtain long-term results was also assessed. The present field studies confirmed the indication of Dentamet® effectiveness in metabolic variation induction, potentially linked with reducing the X. fastidiosa subspecies pauca related Olive Quick Decline Syndrome (OQDS) symptoms development.

Molecular regulation of conditioning film formation and quorum quenching in sulfate reducing bacteria

Sensing surface topography, an upsurge of signaling biomolecules, and upholding cellular homeostasis are the rate-limiting spatio-temporal events in microbial attachment and biofilm formation. Initially, a set of highly specialized proteins, viz. conditioning protein, directs the irreversible attachment of the microbes. Later signaling molecules, viz. autoinducer, take over the cellular communication phenomenon, resulting in a mature microbial biofilm. The mandatory release of conditioning proteins and autoinducers corroborated the existence of two independent mechanisms operating sequentially for biofilm development. However, both these mechanisms are significantly affected by the availability of the cofactor, e.g., Copper (Cu). Generally, the Cu concentration beyond threshold levels is detrimental to the anaerobes except for a few species of sulfate-reducing bacteria (SRB). Remarkably SRB has developed intricate ways to resist and thrive in the presence of Cu by activating numerous genes responsible for modifying the presence of more toxic Cu(I) to Cu(II) within the periplasm, followed by their export through the outer membrane. Therefore, the determinants of Cu toxicity, sequestration, and transportation are reconnoitered for their contribution towards microbial adaptations and biofilm formation. The mechanistic details revealing Cu as a quorum quencher (QQ) are provided in addition to the three pathways involved in the dissolution of cellular communications. This review articulates the Machine Learning based data curing and data processing for designing novel anti-biofilm peptides and for an in-depth understanding of QQ mechanisms. A pioneering data set has been mined and presented on the functional properties of the QQ homolog in Oleidesulfovibrio alaskensis G20 and residues regulating the multicopper oxidase properties in SRB.

Occurrence and dissemination of antibiotic resistance genes in mine soil ecosystems

Metal(loid) selection contributes to selection pressure on antibiotic resistance, but to our knowledge, evidence of the dissemination of antibiotic resistance genes (ARGs) induced by metal(loid)s in mine soil ecosystems is rare. In the current study, using a high-throughput sequencing (HTS)-based metagenomic approach, 819 ARG subtypes were identified in a mine soil ecosystem, indicating that these environmental habitats are important reservoirs of ARGs. The results showed that metal(loid)-induced coselection has an important role in the distribution of soil ARGs. Furthermore, metal(loid) selection-induced ARGs were mainly associated with resistance-nodulation-division (RND) antibiotic efflux, which is distinct from what is observed in agricultural soil ecosystems. By using independent genome binning, metal(loid)s were shown impose coselection pressure on multiple ARGs residing on mobile genetic elements (MGEs), which promotes the dissemination of the antibiotic resistome. Interestingly, the current results showed that the density of several MGEs conferring ARGs was considerably higher in organisms most closely related to the priority pathogens Pseudomonas aeruginosa and Escherichia coli. Together, the results of this study indicate that mine soil ecosystems are important reservoirs of ARGs and that metal(loid)-induced coselection plays critical roles in the dissemination of ARGs in this type of soil habitat. KEY POINTS: • Mining soil ecosystem is a reservoir of antibiotic resistance genes (ARGs). • ARGs distribute via bacterial resistance-nodulation-division efflux systems. • Metal(loid)s coselected ARGs residing on mobile genetic elements in P. aeruginosa and E. coli.

The Copper-Binding Protein CutC Is Involved in Copper Homeostasis and Affects Virulence in the Xylem-Limited Pathogen Xylella fastidiosa

Copper (Cu) is an essential element that can be toxic if homeostasis is disrupted. Xylella fastidiosa, a xylem-limited plant pathogenic bacterium that causes disease in many economically important crops worldwide, has been exposed to Cu stress caused by wide application of Cu-containing antimicrobials used to control other diseases. However, X. fastidiosa Cu homeostasis mechanisms are still poorly understood. The potentially Cu-related protein CutC, which is involved in Cu tolerance in Escherichia coli and humans, has not been analyzed functionally in plant pathogenic bacteria. We demonstrate that recombinantly expressed X. fastidiosa CutC binds Cu and deletion of cutC gene (PD0586) in X. fastidiosa showed increased sensitivity to Cu-shock compared with wild type (WT) strain TemeculaL. When infecting plants in the greenhouse, cutC mutant showed decreased disease incidence and severity compared with WT but adding Cu exaggerated severity. Interestingly, the inoculation of cutC mutant caused reduced symptoms in the acropetal regions of plants. We hypothesize that X. fastidiosa cutC is involved in Cu homeostasis by binding Cu in cells, leading to Cu detoxification, which is crucial to withstand Cu-shock stress. Unveiling the role of cutC gene in X. fastidiosa facilitates further understanding of Cu homeostasis in bacterial pathogens.

Photodynamic control of citrus crop diseases

Citrus are economically important fruit crops to which infectious diseases like citrus canker caused by Xanthomonas citri subs. citri, citrus variegated chlorosis caused by Xylella fastidiosa, "huanglongbing" associated with the presence of Candidatus liberibacter species, anthracnose caused by Colletotrichum gloeosporioides and citrus black spot caused by Phyllosticta citricarpa, impose significant losses. Control measures involve chemical treatment of orchards but often, eradication of infected plants is unavoidable. To circumvent the environmental impacts of pesticides and the socio-economic impacts of eradication, innovative antimicrobial approaches like photodynamic inactivation are being tested. There is evidence of the susceptibility of Xanthomonas citri subs. citri and C. gloeosporioides to photodynamic damage. However, the realistic assessment of perspectives for widespread application of photodynamic inactivation in the control of citrus diseases, necessarily implies that other microorganisms are also considered. This review intends to provide a critical summary of the current state of research on photodynamic inactivation of citrus pathogens and to identify some of the current limitations to the widespread use of photodynamic treatments in citrus crops.

Overexpression of mqsR in Xylella fastidiosa Leads to a Priming Effect of Cells to Copper Stress Tolerance

Copper-based compounds are widely used in agriculture as a chemical strategy to limit the spread of multiple plant diseases; however, the continuous use of this heavy metal has caused environmental damage as well as the development of copper-resistant strains. Thus, it is important to understand how the bacterial phytopathogens evolve to manage with this metal in the field. The MqsRA Toxin-Antitoxin system has been recently described for its function in biofilm formation and copper tolerance in Xylella fastidiosa, a plant-pathogen bacterium responsible for economic damage in several crops worldwide. Here we identified differentially regulated genes by X. fastidiosa MqsRA by assessing changes in global gene expression with and without copper. Results show that mqsR overexpression led to changes in the pattern of cell aggregation, culminating in a global phenotypic heterogeneity, indicative of persister cell formation. This phenotype was also observed in wild-type cells but only in the presence of copper. This suggests that MqsR regulates genes that alter cell behavior in order to prime them to respond to copper stress, which is supported by RNA-Seq analysis. To increase cellular tolerance, proteolysis and efflux pumps and regulator related to multidrug resistance are induced in the presence of copper, in an MqsR-independent response. In this study we show a network of genes modulated by MqsR that is associated with induction of persistence in X. fastidiosa. Persistence in plant-pathogenic bacteria is an important genetic tolerance mechanism still neglected for management of phytopathogens in agriculture, for which this work expands the current knowledge and opens new perspectives for studies aiming for a more efficient control in the field.

The Influence of Copper Homeostasis Genes copA and copB on Xylella fastidiosa Virulence Is Affected by Sap Copper Concentration

Xylella fastidiosa is a xylem-limited plant pathogenic bacterium that causes diseases worldwide in crops such as grape, citrus, and olive. Although copper (Cu)-containing compounds are not used for management of X. fastidiosa-caused diseases, they are widely used in X. fastidiosa hosts in vineyards and orchards. The accumulation of Cu in soils and, therefore, plant saps, could be a challenge for X. fastidiosa survival. Here, the molecular basis of Cu homeostasis was studied in relation to virulence. Although homologous Cu-related genes copA (X. fastidiosa loci PD0100) and copB (PD0101) have been characterized in other bacteria, their functions differ among bacterial species. In vitro, both copA and copB mutants were more sensitive to Cu than the wild-type (WT) strain. Interestingly, the copA mutant was more sensitive to Cu shock, while the copB mutant was more sensitive to chronic Cu treatments. In tobacco greenhouse experiments with normal watering, both mutants reduced virulence compared with WT. But when Cu was added as a drench treatment, both copA and copB mutants had increased disease severity approximately 20 and 50% compared with mutants without Cu added, respectively, which were significantly higher than the approximately 5% observed for WT under the same conditions. These results indicate that the pathogen's Cu homeostasis affects virulence and is influenced by Cu concentration in the environment. Understanding Cu homeostasis in X. fastidiosa will help discern the outcome of Cu treatments and the adaptation of this pathogen to the xylem of plants that have been exposed to high Cu concentrations because of agricultural practices.

GC-TOF/MS-based metabolomics analysis to investigate the changes driven by N-Acetylcysteine in the plant-pathogen Xanthomonas citri subsp. citri

N-Acetylcysteine (NAC) is an antioxidant, anti-adhesive, and antimicrobial compound. Even though there is much information regarding the role of NAC as an antioxidant and anti-adhesive agent, little is known about its antimicrobial activity. In order to assess its mode of action in bacterial cells, we investigated the metabolic responses triggered by NAC at neutral pH. As a model organism, we chose the Gram-negative plant pathogen Xanthomonas citri subsp. citri (X. citri), the causal agent of citrus canker disease, due to the potential use of NAC as a sustainable molecule against phytopathogens dissemination in citrus cultivated areas. In presence of NAC, cell proliferation was affected after 4 h, but damages to the cell membrane were observed only after 24 h. Targeted metabolite profiling analysis using GC-MS/TOF unravelled that NAC seems to be metabolized by the cells affecting cysteine metabolism. Intriguingly, glutamine, a marker for nitrogen status, was not detected among the cells treated with NAC. The absence of glutamine was followed by a decrease in the levels of the majority of the proteinogenic amino acids, suggesting that the reduced availability of amino acids affect protein synthesis and consequently cell proliferation.

Persister Cells Form in the Plant Pathogen Xanthomonas citri subsp. citri under Different Stress Conditions

Citrus canker disease, caused by the bacterium Xanthomonas citri subsp. citri is a constant threat to citrus-producing areas. Since it has no cure, agricultural practices to restrain its dissemination are essential to reduce the economic damage. Hence, increased knowledge of the basic aspects of X. citri biology could lead to more efficient management practices that can eliminate dormant bacteria in the field. The dormant cells, also referred to as persisters, are phenotypic variants with lowered metabolism, which in turn leads to tolerance to antimicrobials and undermines existing control approaches. We show here that X. citri forms persisters, identifying triggers for this phenotype, including antibiotics, high temperature, and metals (copper and zinc), which increase persistence rates by 10–100 times. The antioxidant N-acetylcysteine reduced copper and zinc-induced persisters, but not those induced by tetracycline, indicating that oxidative stress may be an important inducer of X. citri persistence. In addition, we found that metabolism-independent drugs like cisplatin and mitomycin C are able to eliminate X. citri persistent cells, as well as copper, at high concentrations. Specific amino acids like proline and isoleucine interfered with the physiological balance of the dormancy in X. citri, stimulating or preventing persister resuscitation. Taken together, we discover chemicals that can induce, wake, and kill X. citri persister cells; these results provide insights that should be considered for more efficient integrated control management in the field.

Functional and structural characterization of PII-like protein CutA does not support involvement in heavy metal tolerance and hints at a small-molecule carrying/signaling role

The PII-like protein CutA is annotated as being involved in Cu²⁺ tolerance, based on analysis of Escherichia coli mutants. However, the precise cellular function of CutA remains unclear. Our bioinformatic analysis reveals that CutA proteins are universally distributed across all domains of life. Based on sequence-based clustering, we chose representative cyanobacterial CutA proteins for physiological, biochemical, and structural characterization and examined their involvement in heavy metal tolerance, by generating CutA mutants in filamentous Nostoc sp. and in unicellular Synechococcus elongatus. However, we were unable to find any involvement of cyanobacterial CutA in metal tolerance under various conditions. This prompted us to re-examine experimentally the role of CutA in protecting E. coli from Cu²⁺ . Since we found no effect on copper tolerance, we conclude that CutA plays a different role that is not involved in metal protection. We resolved high-resolution CutA structures from Nostoc and S. elongatus. Similarly to their counterpart from E. coli and to canonical PII proteins, cyanobacterial CutA proteins are trimeric in solution and in crystal structure; however, no binding affinity for small signaling molecules or for Cu²⁺ could be detected. The clefts between the CutA subunits, corresponding to the binding pockets of PII proteins, are formed by conserved aromatic and charged residues, suggesting a conserved binding/signaling function for CutA. In fact, we find binding of organic Bis-Tris/MES molecules in CutA crystal structures, revealing a strong tendency of these pockets to accommodate cargo. This highlights the need to search for the potential physiological ligands and for their signaling functions upon binding to CutA. DATABASES: Structural data are available in Protein Data Bank (PDB) under the accession numbers 6GDU, 6GDV, 6GDW, 6GDX, 6T76, and 6T7E.

Strain variability in biofilm formation: A food safety and quality perspective

The inherent differences in microbial behavior among identically treated strains of the same microbial species, referred to as "strain variability", are regarded as an important source of variability in microbiological studies. Biofilms are defined as the structured multicellular communities with complex architecture that enable microorganisms to grow adhered to abiotic or living surfaces and constitute a fundamental aspect of microbial ecology. The research studies assessing the strain variability in biofilm formation are relatively few compared to the ones evaluating other aspects of microbial behavior such as virulence, growth and stress resistance. Among the available research data on intra-species variability in biofilm formation, compiled and discussed in the present review, most of them refer to foodborne pathogens as compared to spoilage microorganisms. Molecular and physiological aspects of biofilm formation potentially related to strain-specific responses, as well as information on the characterization and quantitative description of this type of biological variability are presented and discussed. Despite the considerable amount of available information on the strain variability in biofilm formation, there are certain data gaps and still-existing challenges that future research should cover and address. Current and future advances in systems biology and omics technologies are expected to aid significantly in the explanation of phenotypic strain variability, including biofilm formation variability, allowing for its integration in microbiological risk assessment.

Co-contaminant effects on 1,4-dioxane biodegradation in packed soil column flow-through systems

Biodegradation of 1,4-dioxane was examined in packed quartz and soil column flow-through systems. The inhibitory effects of co-contaminants, specifically trichloroethene (TCE), 1,1-dichloroethene (1,1-DCE), and copper (Cu²⁺) ions, were investigated in the columns either with or without bioaugmentation with a 1,4-dioxane degrading bacterium Pseudonocardia dioxanivorans CB1190. Results indicate that CB1190 cells readily grew and colonized in the columns, leading to significant degradation of 1,4-dioxane under oxic conditions. Degradation of 1,4-dioxane was also observed in the native soil (without bioaugmentation), which had been previously subjected to enhanced reductive dechlorination treatment for co-contaminants TCE and 1,1-DCE. Bioaugmentation of the soil with CB1190 resulted in nearly complete degradation at influent concentrations of 3-10 mg L^-1 1,4-dioxane and a residence reaction time of 40-80 h, but the presence of co-contaminants, 1,1-DCE and Cu²⁺ ions (up to 10 mg L^-1), partially inhibited 1,4-dioxane degradation in the untreated and bioaugmented soil columns. However, the inhibitory effects were much less severe in the column flow-through systems than those previously observed in planktonic cultures, which showed near complete inhibition at the same co-contaminant concentrations. These observations demonstrate a low susceptibility of soil microbes to the toxicity of 1,1-DCE and Cu²⁺ in packed soil flow-through systems, and thus have important implications for predicting biodegradation potential and developing sustainable, cost-effective technologies for in situ remediation of 1,4-dioxane contaminated soils and groundwater.

In Vitro Formation of Dickeya zeae MS1 Biofilm

Bacterial soft rot caused by Dickeya zeae MS1 (Erwinia chrysanthemi) is one of the most devastating banana diseases worldwide. However, knowledge of the development and ecological interactions of D. zeae MS1 biofilm is limited. Here, we visualized the development and architecture of D. zeae MS1 biofilm using confocal laser scanning microscopy, and we evaluated the ability of D. zeae MS1 to form biofilms under different environmental conditions (carbon sources, temperatures, pH levels and mineral elements) using a microtiter plate assay. We found that the development of D. zeae MS1 biofilm could be categorized into four phases and that mature biofilm consisted of a highly organized architecture of both bacterial cells and a self-produced matrix of extracellular polysaccharides. Furthermore, sucrose was the most suitable carbon source for supporting the growth of biofilm cells and that 32 °C and pH 7.0 were the most favorable of the temperatures and pH levels examined. Meanwhile, the addition of Ca²⁺, Fe²⁺, K⁺ and Na⁺ enhanced the formation of biofilm in minimal medium cultures, whereas 2.5 mM Cu²⁺ and Mn²⁺ was inhibitory. A better understanding of biofilm formation under different environmental parameters will improve our knowledge of the growth kinetics of D. zeae MS1 biofilm.

Persistence in Phytopathogenic Bacteria: Do We Know Enough?

Phytopathogenic bacteria affect a wide range of crops worldwide and have a negative impact in agriculture due to their associated economic losses and environmental impacts. Together with other biotic and abiotic stress factors, they pose a threat to global food production. Therefore, understanding bacterial survival strategies is an essential step toward the development of new strategies to control plant diseases. One mechanism used by bacteria to survive under stress conditions is the formation of persister cells. Persisters are a small fraction of phenotypic variants within an isogenic population that exhibits multidrug tolerance without undergoing genetic changes. They are dormant cells that survive treatment with antimicrobials by inactivating the metabolic functions that are disrupted by these compounds. They are thus responsible for the recalcitrance of many human diseases, and in the same way, they are thought to contribute to the survival of bacterial phytopathogens under a range of stresses they face in the environment. It is believed that persister cells of bacterial phytopathogens may lead to the reoccurrence of disease by recovering growth and recolonizing the host plant after the end of stress. However, compared to human pathogens, little is known about persister cells in phytopathogens, especially about their genetic regulation. In this review, we describe the overall knowledge on persister cells and their regulation in bacterial phytopathogens, focusing on their ability to survive stress conditions, to recover from dormancy and to maintain virulence.

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Bacteriophages and Bacterial Plant Diseases

Losses in crop yields due to disease need to be reduced in order to meet increasing global food demands associated with growth in the human population. There is a well-recognized need to develop new environmentally friendly control strategies to combat bacterial crop disease. Current control measures involving the use of traditional chemicals or antibiotics are losing their efficacy due to the natural development of bacterial resistance to these agents. In addition, there is an increasing awareness that their use is environmentally unfriendly. Bacteriophages, the viruses of bacteria, have received increased research interest in recent years as a realistic environmentally friendly means of controlling bacterial diseases. Their use presents a viable control measure for a number of destructive bacterial crop diseases, with some phage-based products already becoming available on the market. Phage biocontrol possesses advantages over chemical controls in that tailor-made phage cocktails can be adapted to target specific disease-causing bacteria. Unlike chemical control measures, phage mixtures can be easily adapted for bacterial resistance which may develop over time. In this review, we will examine the progress and challenges for phage-based disease biocontrol in food crops.

Bacterial disease management: challenges, experience, innovation and future prospects: Challenges in Bacterial Molecular Plant Pathology

Plant diseases caused by bacterial pathogens place major constraints on crop production and cause significant annual losses on a global scale. The attainment of consistent effective management of these diseases can be extremely difficult, and management potential is often affected by grower reliance on highly disease-susceptible cultivars because of consumer preferences, and by environmental conditions favouring pathogen development. New and emerging bacterial disease problems (e.g. zebra chip of potato) and established problems in new geographical regions (e.g. bacterial canker of kiwifruit in New Zealand) grab the headlines, but the list of bacterial disease problems with few effective management options is long. The ever-increasing global human population requires the continued stable production of a safe food supply with greater yields because of the shrinking areas of arable land. One major facet in the maintenance of the sustainability of crop production systems with predictable yields involves the identification and deployment of sustainable disease management solutions for bacterial diseases. In addition, the identification of novel management tactics has also come to the fore because of the increasing evolution of resistance to existing bactericides. A number of central research foci, involving basic research to identify critical pathogen targets for control, novel methodologies and methods of delivery, are emerging that will provide a strong basis for bacterial disease management into the future. Near-term solutions are desperately needed. Are there replacement materials for existing bactericides that can provide effective disease management under field conditions? Experience should inform the future. With prior knowledge of bactericide resistance issues evolving in pathogens, how will this affect the deployment of newer compounds and biological controls? Knowledge is critical. A comprehensive understanding of bacterial pathosystems is required to not only identify optimal targets in the pathogens, but also optimal seasonal timings for deployment. Host resistance to effectors must be exploited, carefully and correctly. Are there other candidate genes that could be targeted in transgenic approaches? How can new technologies (CRISPR, TALEN, etc.) be most effectively used to add sustainable disease resistance to existing commercially desirable plant cultivars? We need an insider's perspective on the management of systemic pathogens. In addition to host resistance or reduced sensitivity, are there other methods that can be used to target these pathogen groups? Biological systems are variable. Can biological control strategies be improved for bacterial disease management and be made more predictable in function? The answers to the research foci outlined above are not all available, as will become apparent in this article, but we are heading in the right direction. In this article, we summarize the contributions from past experiences in bacterial disease management, and also describe how advances in bacterial genetics, genomics and host-pathogen interactions are informing novel strategies in virulence inhibition and in host resistance. We also outline potential innovations that could be exploited as the pressures to maximize a safe and productive food supply continue to become more numerous and more complex.

The MqsRA Toxin-Antitoxin System from Xylella fastidiosa Plays a Key Role in Bacterial Fitness, Pathogenicity, and Persister Cell Formation

Through the formation of persister cells, bacteria exhibit tolerance to multidrug and other environmental stresses without undergoing genetic changes. The toxin-antitoxin (TA) systems are involved in the formation of persister cells because they are able to induce cell dormancy. Among the TA systems, the MqsRA system has been observed to be highly induced in persister cells of Xylella fastidiosa (causal agent of citrus variegated chlorosis-CVC) activated by copper stress, and has been described in Escherichia coli as related to the formation of persister cells and biofilms. Thus, we evaluated the role of this TA system in X. fastidiosa by overexpressing the MqsR toxin, and verified that the toxin positively regulated biofilm formation and negatively cell movement, resulting in reduced pathogenicity in citrus plants. The overexpression of MqsR also increased the formation of persister cells under copper stress. Analysis of the gene and protein expression showed that this system likely has an autoregulation mechanism to express the toxin and antitoxin in the most beneficial ratio for the cell to oppose stress. Our results suggest that this TA system plays a key role in the adaptation and survival of X. fastidiosa and reveal new insights into the physiology of phytopathogen-host interactions.

Proteomics dedicated to biofilmology: What have we learned from a decade of research?

Advances in proteomics techniques over the past decade, closely integrated with genomic and physicochemical approach, have played a great role in developing knowledge of the biofilm lifestyle of bacteria. Despite bacterial proteome versatility, many studies have demonstrated the ability of proteomics approaches to elucidating the biofilm phenotype. Though these investigations have been largely used for biofilm studies in the last decades, they represent, however, a very low percentage of proteomics works performed up to now. Such approaches have offered new targets for combating microbial biofilms by providing a comprehensive quantitative and qualitative overview of their protein cell content. Herein, we summarized the state of the art in knowledge about biofilm physiology after one decade of proteomic analysis. In a second part, we highlighted missing research tracks for the next decade, emphasizing the emergence of posttranslational modifications in proteomic studies stemming from recent advances in mass spectrometry-based proteomics.

Toxic metal resistance in biofilms: diversity of microbial responses and their evolution

Since biofilms are an important issue in the fields of medicine and health, several recent microbiological studies have focused on their formation and their contribution to toxic compound resistance mechanisms. In this review, we describe how metals impact biofilm formation and resistance, and how biofilms can help cells resist toxic metals. First, the organic matrix acts as a barrier isolating the cells from many environmental stresses. Secondly, the metabolism of the cells changes, and a slowly-growing or non-growing sub-population of cells known as persisters emerges. Thirdly, in the case of multispecies biofilms, metabolic interactions are developed, allowing cells to be more persistent or to have greater capacity to survive than a single species biofilm. Finally, we discuss how the high density of the cells may promote horizontal gene transfer processes, resulting in the acquisition of new features. All these crucial mechanisms enable microorganisms to survive and colonize toxic environments, and probably accelerate ongoing evolutionary processes.

Zinc Detoxification Is Required for Full Virulence and Modification of the Host Leaf Ionome by Xylella fastidiosa

Zinc (Zn) is an essential element for all forms of life because it is a structural or catalytic cofactor of many proteins, but it can have toxic effects at high concentrations; thus, microorganisms must tightly regulate its levels. Here, we evaluated the role of Zn homeostasis proteins in the virulence of the xylem-limited bacterium Xylella fastidiosa, causal agent of Pierce's disease of grapevine, among other diseases. Two mutants of X. fastidiosa 'Temecula' affected in genes which regulate Zn homeostasis (zur) and Zn detoxification (czcD) were constructed. Both knockouts showed increased sensitivity to Zn at physiologically relevant concentrations and increased intracellular accumulation of this metal compared with the wild type. Increased Zn sensitivity was correlated with decreased growth in grapevine xylem sap, reduced twitching motility, and downregulation of exopolysaccharide biosynthetic genes. Tobacco plants inoculated with either knockout mutant showed reduced foliar symptoms and a much reduced (czcD) or absent (zur) modification of the leaf ionome (i.e., the mineral nutrient and trace element composition), as well as reduced bacterial populations. The results show that detoxification of Zn is crucial for the virulence of X. fastidiosa and verifies our previous findings that modification of the host leaf ionome correlates with bacterial virulence.

Metal ion homeostasis in Listeria monocytogenes and importance in host-pathogen interactions

Listeria monocytogenes is responsible for one of the most life-threatening food-borne infections and the leading cause of food-poisoning associated deaths in the UK. Infection may be of the unborn/newly born infant where disease may manifest as listeric abortion, stillbirth or late-onset neonatal listeriosis, while in adults, infection usually affects the central nervous system causing meningitis. Crucial to the survival of L. monocytogenes, both inside and outside the host, is its ability to acquire metals which act as cofactors for a broad range of its cellular proteins. However, L. monocytogenes must also protect itself against the innate toxicity of metals. The importance of metals in host-pathogen interactions is illustrated by the restriction of metals (including zinc and iron) in vertebrates in response to infection and the use of high levels of metals (copper and zinc) as part of the antimicrobial defences within host phagocytes. As such, L. monocytogenes is equipped with various mechanisms to tightly control its cellular metal pools and avoid metal poisoning. These include multiple DNA-binding metal-responsive transcription factors, metal-acquisition, metal-detoxification and metal-storage systems, some of which represent key L. monocytogenes virulence determinants. This review discusses current knowledge of the role of metals in L. monocytogenes infections, with a focus on the mechanisms that contribute to zinc and copper homeostasis in this organism. The requirement to precisely control cellular metal levels may impose a vulnerability to L. monocytogenes which can be exploited in antimicrobials and therapeutics.

Distinct diversity of the czcA gene in two sedimentary horizons from a contaminated estuarine core

In estuarine ecosystems, trace metals are mainly associated with fine grain sediments which settle on mudflats. Over time, the layers of sediments accumulate and are then transformed by diagenetic processes, recording the history of the estuary's chemical contamination. In such a specific environment, we investigated to what extent a chronic exposure to contaminants could affect metal-resistant sedimentary bacteria in subsurface sediments. The occurrence and diversity of cadmium resistance genes (cadA, czcA) was investigated in 5- and 33-year-old sediments from a highly contaminated estuary (Seine France). Primers were designed to detect a 252-bp fragment of the czcA gene, specifically targeting a transmembrane helice domain (TMH IV) involved in the proton substrate antiport of this efflux pump. Although the cadA gene was not detected, the highest diversity of the sequence of the czcA gene was observed in the 5-year-old sediment. According to the percentage of identity at the amino acid level, the closest CzcA relatives were identified among Proteobacteria (α, β, γ, and δ), Verrucomicrobia, Nitrospirae, and Bacteroidetes. The most abundant sequences were affiliated with Stenotrophomonas. In contrast, in the 33-year-old sediment, CzcA sequences were mainly related to Rhodanobacter thiooxydans and Stenotrophomonas, suggesting a shaping of the metal-resistant microbial communities over time by both diagenetic processes and trace metal contamination.

Response to copper of Acidithiobacillus ferrooxidans ATCC 23270 grown in elemental sulfur

The response of Acidithiobacillus ferrooxidans ATCC 23270 to copper was analyzed in sulfur-grown cells by using quantitative proteomics. Forty-seven proteins showed altered levels in cells grown in the presence of 50 mM copper sulfate. Of these proteins, 24 were up-regulated and 23 down-regulated. As seen before in ferrous iron-grown cells, there was a notorious up-regulation of RND-type Cus systems and different RND-type efflux pumps, indicating that these proteins are very important in copper resistance. Copper also triggered the down-regulation of the major outer membrane porin of A. ferrooxidans in sulfur-grown bacteria, suggesting they respond to the metal by decreasing the influx of cations into the cell. On the contrary, copper in sulfur-grown cells caused an overexpression of putative TadA and TadB proteins known to be essential for biofilm formation in bacteria. Surprisingly, sulfur-grown microorganisms showed increased levels of proteins related with energy generation (rus and petII operons) in the presence of copper. Although rus operon is overexpressed mainly in cells grown in ferrous iron, the up-regulation of rusticyanin in sulfur indicates a possible role for this protein in copper resistance as well. Finally, copper response in A. ferrooxidans appears to be influenced by the substrate being oxidized by the microorganism.

Foliar application of biofilm formation-inhibiting compounds enhances control of citrus canker caused by Xanthomonas citri subsp. citri

Citrus canker caused by the bacterium Xanthomonas citri subsp. citri is an economically important disease of citrus worldwide. Biofilm formation plays an important role in early infection of X. citri subsp. citri on host leaves. In this study, we assessed the hypothesis that small molecules inhibiting biofilm formation reduce X. citri subsp. citri infection and enhance the control of citrus canker disease. D-leucine and 3-indolylacetonitrile (IAN) were found to prevent biofilm formation by X. citri subsp. citri on different abiotic surfaces and host leaves at a concentration lower than the minimum inhibitory concentration (MIC). Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis indicated that IAN repressed expression of chemotaxis/motility-related genes in X. citri subsp. citri. In laboratory experiments, planktonic and biofilm cells of X. citri subsp. citri treated with D-leucine and IAN, either alone or in combination, were more susceptible to copper (CuSO4) than those untreated. In greenhouse assays, D-leucine and IAN applied alone or combined with copper reduced both the number of canker lesions and bacterial populations of X. citri subsp. citri on citrus host leaves. This study provides the basis for the use of foliar-applied biofilm inhibitors for the control of citrus canker alone or combined with copper-based bactericides.

The exopolysaccharide of Xylella fastidiosa is essential for biofilm formation, plant virulence, and vector transmission

Exopolysaccharides (EPS) synthesized by plant-pathogenic bacteria are generally essential for virulence. The role of EPS produced by the vector-transmitted bacterium Xylella fastidiosa was investigated by knocking out two genes implicated in the EPS biosynthesis, gumD and gumH. Mutant strains were affected in growth characteristics in vitro, including adhesion to surfaces and biofilm formation. In addition, different assays were used to demonstrate that the mutant strains produced significantly less EPS compared with the wild type. Furthermore, gas chromatography-mass spectrometry showed that both mutant strains did not produce oligosaccharides. Biologically, the mutants were deficient in movement within plants, resulting in an avirulent phenotype. Additionally, mutant strains were affected in transmission by insects: they were very poorly transmitted by and retained within vectors. The gene expression profile indicated upregulation of genes implicated in cell-to-cell signaling and adhesins while downregulation in genes was required for within-plant movement in EPS-deficient strains. These results suggest an essential role for EPS in X. fastidiosa interactions with both plants and insects.

Bacteriocins active against plant pathogenic bacteria

Gram-negative phytopathogens cause significant losses in a diverse range of economically important crop plants. The effectiveness of traditional countermeasures, such as the breeding and introduction of resistant cultivars, is often limited by the dearth of available sources of genetic resistance. An alternative strategy to reduce loss to specific bacterial phytopathogens is to use narrow-spectrum protein antibiotics such as colicin-like bacteriocins as biocontrol agents. A number of colicin-like bacteriocins active against phytopathogenic bacteria have been described previously as have strategies for their application to biocontrol. In the present paper, we discuss these strategies and our own recent work on the identification and characterization of candidate bacteriocins and how these potent and selective antimicrobial agents can be effectively applied to the control of economically important plant disease.

Xylella fastidiosa differentially accumulates mineral elements in biofilm and planktonic cells

Xylella fastidiosa is a bacterial plant pathogen that infects numerous plant hosts. Disease develops when the bacterium colonizes the xylem vessels and forms a biofilm. Inductively coupled plasma optical emission spectroscopy was used to examine the mineral element content of this pathogen in biofilm and planktonic states. Significant accumulations of copper (30-fold), manganese (6-fold), zinc (5-fold), calcium (2-fold) and potassium (2-fold) in the biofilm compared to planktonic cells were observed. Other mineral elements such as sodium, magnesium and iron did not significantly differ between biofilm and planktonic cells. The distribution of mineral elements in the planktonic cells loosely mirrors the media composition; however the unique mineral element distribution in biofilm suggests specific mechanisms of accumulation from the media. A cell-to-surface attachment assay shows that addition of 50 to 100 µM Cu to standard X. fastidiosa media increases biofilm, while higher concentrations (>200 µM) slow cell growth and prevent biofilm formation. Moreover cell-to-surface attachment was blocked by specific chelation of copper. Growth of X. fastidiosa in microfluidic chambers under flow conditions showed that addition of 50 µM Cu to the media accelerated attachment and aggregation, while 400 µM prevented this process. Supplementation of standard media with Mn showed increased biofilm formation and cell-to-cell attachment. In contrast, while the biofilm accumulated Zn, supplementation to the media with this element caused inhibited growth of planktonic cells and impaired biofilm formation. Collectively these data suggest roles for these minerals in attachment and biofilm formation and therefore the virulence of this pathogen.

Foliar-Applied Small Molecule that Suppresses Biofilm Formation and Enhances Control of Copper-Resistant Xanthomonas euvesicatoria on Pepper

We report a small molecule additive, a member of the 2-aminoimidazole (2AI) group that is an analogue of the marine sponge natural product oroidin that suppresses resistance of Xanthomonas euvesicatoria to copper and decreases biofilm formation in an in vitro system. In laboratory experiments, 2AI combined with copper reduced both bacterial multiplication in broth and bacterial recovery on pepper leaf discs of a copper-resistant strain of X. euvesicatoria to a level close to that of a copper-sensitive strain. Compound 2AI used alone exhibited minimal bactericidal activity. In 3 years of field experiments, when combined with a copper-containing material, copper hydroxide (Kocide 3000), and other antibacterial materials, these spray mixtures resulted in decreased bacterial spot foliar disease and increased fruit yields using hybrid bell pepper (Capsicum annuum) cultivars and copper-resistant strains of X. euvesicatoria. This study demonstrates the concept for using small molecules as additives to antibacterial compounds at nonbactericidal concentrations under field conditions that, in the laboratory, were demonstrated to suppress bacterial biofilms and copper-resistant strains.

Global expression profile of biofilm resistance to antimicrobial compounds in the plant-pathogenic bacterium Xylella fastidiosa reveals evidence of persister cells

Investigations of biofilm resistance response rarely focus on plant-pathogenic bacteria. Since Xylella fastidiosa is a multihost plant-pathogenic bacterium that forms biofilm in the xylem, the behavior of its biofilm in response to antimicrobial compounds needs to be better investigated. We analyzed here the transcriptional profile of X. fastidiosa subsp. pauca in response to inhibitory and subinhibitory concentrations of copper and tetracycline. Copper-based products are routinely used to control citrus diseases in the field, while antibiotics are more widely used for bacterial control in mammals. The use of antimicrobial compounds triggers specific responses to each compound, such as biofilm formation and phage activity for copper. Common changes in expression responses comprise the repression of genes associated with metabolic functions and movement and the induction of toxin-antitoxin systems, which have been associated with the formation of persister cells. Our results also show that these cells were found in the population at a ca. 0.05% density under inhibitory conditions for both antimicrobial compounds and that pretreatment with subinhibitory concentration of copper increases this number. No previous report has detected the presence of these cells in X. fastidiosa population, suggesting that this could lead to a multidrug tolerance response in the biofilm under a stressed environment. This is a mechanism that has recently become the focus of studies on resistance of human-pathogenic bacteria to antibiotics and, based on our data, it seems to be more broadly applicable.

Calcium increases Xylella fastidiosa surface attachment, biofilm formation, and twitching motility

Xylella fastidiosa is a plant-pathogenic bacterium that forms biofilms inside xylem vessels, a process thought to be influenced by the chemical composition of xylem sap. In this work, the effect of calcium on the production of X. fastidiosa biofilm and movement was analyzed under in vitro conditions. After a dose-response study with 96-well plates using eight metals, the strongest increase of biofilm formation was observed when medium was supplemented with at least 1.0 mM CaCl(2). The removal of Ca by extracellular (EGTA, 1.5 mM) and intracellular [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM), 75 μM] chelators reduced biofilm formation without compromising planktonic growth. The concentration of Ca influenced the force of adhesion to the substrate, biofilm thickness, cell-to-cell aggregation, and twitching motility, as shown by assays with microfluidic chambers and other assays. The effect of Ca on attachment was lost when cells were treated with tetracycline, suggesting that Ca has a metabolic or regulatory role in cell adhesion. A double mutant (fimA pilO) lacking type I and type IV pili did not improve biofilm formation or attachment when Ca was added to the medium, while single mutants of type I (fimA) or type IV (pilB) pili formed more biofilm under conditions of higher Ca concentrations. The concentration of Ca in the medium did not significantly influence the levels of exopolysaccharide produced. Our findings indicate that the role of Ca in biofilm formation may be related to the initial surface and cell-to-cell attachment and colonization stages of biofilm establishment, which rely on critical functions by fimbrial structures.

Differentiation of Xylella fastidiosa strains via multilocus sequence analysis of environmentally mediated genes (MLSA-E)

Isolates of the plant pathogen Xylella fastidiosa are genetically very similar, but studies on their biological traits have indicated differences in virulence and infection symptomatology. Taxonomic analyses have identified several subspecies, and phylogenetic analyses of housekeeping genes have shown broad host-based genetic differences; however, results are still inconclusive for genetic differentiation of isolates within subspecies. This study employs multilocus sequence analysis of environmentally mediated genes (MLSA-E; genes influenced by environmental factors) to investigate X. fastidiosa relationships and differentiate isolates with low genetic variability. Potential environmentally mediated genes, including host colonization and survival genes related to infection establishment, were identified a priori. The ratio of the rate of nonsynonymous substitutions to the rate of synonymous substitutions (dN/dS) was calculated to select genes that may be under increased positive selection compared to previously studied housekeeping genes. Nine genes were sequenced from 54 X. fastidiosa isolates infecting different host plants across the United States. Results of maximum likelihood (ML) and Bayesian phylogenetic (BP) analyses are in agreement with known X. fastidiosa subspecies clades but show novel within-subspecies differentiation, including geographic differentiation, and provide additional information regarding host-based isolate variation and specificity. dN/dS ratios of environmentally mediated genes, though <1 due to high sequence similarity, are significantly greater than housekeeping gene dN/dS ratios and correlate with increased sequence variability. MLSA-E can more precisely resolve relationships between closely related bacterial strains with low genetic variability, such as X. fastidiosa isolates. Discovering the genetic relationships between X. fastidiosa isolates will provide new insights into the epidemiology of populations of X. fastidiosa, allowing improved disease management in economically important crops.

Analysis of the biofilm proteome of Xylella fastidiosa

BACKGROUND

Xylella fastidiosa is limited to the xylem of the plant host and the foregut of insect vectors (sharpshooters). The mechanism of pathogenicity of this bacterium differs from other plant pathogens, since it does not present typical genes that confer specific interactions between plant and pathogens (avr and/or hrp). The bacterium is injected directly into the xylem vessels where it adheres and colonizes. The whole process leads to the formation of biofilms, which are considered the main mechanism of pathogenicity. Cells in biofilms are metabolically and phenotypically different from their planktonic condition. The mature biofilm stage (phase of higher cell density) presents high virulence and resistance to toxic substances such as antibiotics and detergents. Here we performed proteomic analysis of proteins expressed exclusively in the mature biofilm of X. fastidiosa strain 9a5c, in comparison to planktonic growth condition.

RESULTS

We found a total of 456 proteins expressed in the biofilm condition, which correspond to approximately 10% of total protein in the genome. The biofilm showed 37% (or 144 proteins) different protein than we found in the planktonic growth condition. The large difference in protein pattern in the biofilm condition may be responsible for the physiological changes of the cells in the biofilm of X. fastidiosa. Mass spectrometry was used to identify these proteins, while real-time quantitative polymerase chain reaction monitored expression of genes encoding them. Most of proteins expressed in the mature biofilm growth were associated with metabolism, adhesion, pathogenicity and stress conditions. Even though the biofilm cells in this work were not submitted to any stress condition, some stress related proteins were expressed only in the biofilm condition, suggesting that the biofilm cells would constitutively express proteins in different adverse environments.

CONCLUSIONS

We observed overexpression of proteins related to quorum sensing, proving the existence of communication between cells, and thus the development of structuring the biofilm (mature biofilm) leading to obstruction of vessels and development of disease. This paper reports a first proteomic analysis of mature biofilm of X. fastidiosa, opening new perspectives for understanding the biochemistry of mature biofilm growth in a plant pathogen.

CutC is induced late during copper exposure and can modify intracellular copper content in Enterococcus faecalis

Copper is a micronutrient that is required for proper metabolic functioning of most prokaryotic and eukaryotic organisms. To sustain an adequate supply of copper, a cell requires molecular mechanisms that control the metal content to avoid copper toxicity. This toxicity comes primarily from the reactivity of copper, which can lead to the generation of free radicals. In bacteria, two independent systems are responsible for maintaining the balance of copper within the cells (Cop and Cut family proteins). Previous studies describe CutC as a member of the Cut family that is probably involved in copper homeostasis. However, the role of CutC in copper homeostasis is still unclear. In this work, a homolog of CutC was studied in Enterococcus faecalis, a bacterial model for copper homeostasis. The molecular 3D model of efCutC shows the presence of triose phosphate isomerase (TIM) barrel motifs, previously described in CutC crystals from other organisms, which illustrates the conservation of amino acids with the potential ability to coordinate copper. Through quantitative real-time PCR (qPCR), it was demonstrated that efcutC expression is induced late by copper stimulus, Interestingly this transcriptional response directly correlates with a significant increase in the intracellular copper concentration when the protein is absent in the bacteria, suggesting its participation in mechanisms related to efflux of the metal. Our results describe efCutC as a protein able to respond transcriptionally to copper and to participate in the control of copper homeostasis in E. faecalis. This bacterium is the first reported organism containing a cop operon and an active member of the Cut protein family.

Localization and characterization of Xylella fastidiosa haemagglutinin adhesins

Xylella fastidiosa is a gram-negative, xylem-inhabiting, plant-pathogenic bacterium responsible for several important diseases including Pierce's disease (PD) of grapevines. The bacteria form biofilms in grapevine xylem that contribute to the occlusion of the xylem vessels. X. fastidiosa haemagglutinin (HA) proteins are large afimbrial adhesins that have been shown to be crucial for biofilm formation. Little is known about the mechanism of X. fastidiosa HA-mediated cell-cell aggregation or the localization of the adhesins on the cell. We generated anti-HA antibodies and show that X. fastidiosa HAs are present in the outer membrane and secreted both as soluble proteins and in membrane vesicles. Furthermore, the HA pre-proteins are processed from the predicted molecular mass of 360 kDa to a mature 220 kDa protein. Based on this information, we are evaluating a novel form of potential resistance against PD by generating HA-expressing transgenic grapevines.

Knock down of Caenorhabditis elegans cutc-1 exacerbates the sensitivity toward high levels of copper

Copper, though toxic in excess, is an essential trace element that serves as a cofactor in many critical biological processes such as respiration, iron transport, and oxidative stress protection. To maintain this balance between requirement and toxicity, biological systems have developed intricate systems allowing the preservation of homeostasis while ensuring delivery of copper to the appropriate cellular component. The nematode Caenorhabditis elegans was exploited to assess the effects of copper toxicity at the population level to identify key changes in life cycle traits including, lethality, brood size, generation time, growth, and life span. To enhance our understanding of the complexities of copper homeostasis at the genetic level, the expression profile and functional significance of a putative copper cytoplasmic metallochaperone cutc-1 were analyzed. Using quantitative PCR technology, cutc-1 was found to be downregulated with increasing CuSO(4) concentrations. However, although total (whole body) copper levels increased in nematodes exposed to elevated levels of copper, wild-type and knock down of cutc-1 by RNA-mediated interference (RNAi) were statistically indistinguishable. Nevertheless, RNAi of cutc-1 affected brood size, growth and induced a marked increase in protruding vulva and bagging phenotypes at higher copper exposures. This indicates that cutc-1 plays a crucial role in the protection from excess copper.