Plasmid-mediated resistance to tellurite: expressed and cryptic

Antimicrobial efficacy and amino acid substitutions associated with susceptibility to the tellurium compound AS101 against Haemophilus influenzae and Haemophilus parainfluenzae

The tellurite toxicity in Haemophilus influenzae and H. parainfluenzae remains unclear. To understand the potential of tellurite as a therapeutic option for these bacteria, we investigated the antimicrobial efficacy of AS101, a tellurium compound, against H. influenzae and H. parainfluenzae and the molecular basis of their differences in AS101 susceptibility. Through broth microdilution, we examined the minimum inhibitory concentration (MIC) of AS101 in 51 H. influenzae and 28 H. parainfluenzae isolates. Whole-genome sequencing was performed on the H. influenzae isolates to identify genetic variations associated with AS101 susceptibility. The MICs of AS101 were ≦ 4, 16-32, and ≧ 64 μg/mL in 9 (17.6%), 12 (23.5%), and 30 (58.8%) H. influenzae isolates, respectively, whereas ≦ 0.5 μg/mL in all H. parainfluenzae isolates, including multidrug-resistant isolates. Time-killing kinetic assay and scanning electron microscopy revealed the in vitro bactericidal activity of AS101 against H. parainfluenzae. Forty variations in nine tellurite resistance-related genes were associated with AS101 susceptibility. Logistic regression, receiver operator characteristic curve analysis, Venn diagram, and protein sequence alignment indicated that Val195Ile substitution in TerC, Ser93Gly in Gor (glutathione reductase), Pro44Ala/Ala50Pro in NapB (nitrate reductase), Val307Leu in TehA (tellurite resistance protein), Cys105Arg in CysK (cysteine synthase), and Thr364Ser in Csd (Cysteine desulfurase) were strongly associated with reduced AS101 susceptibility, whereas Ser155Pro in TehA with increased AS101 susceptibility. In conclusions, the antimicrobial efficacy of AS101 is high against H. parainfluenzae but low against H. influenzae. Genetic variations and corresponding protein changes relevant to AS101 non-susceptibility in H. influenzae were identified.

The tellurite resistance gene cluster of pathogenic bacteria and its effect on oxidative stress response

Tellurite resistance gene clusters have been identified in numerous pathogenic bacteria, including clinical isolates of Escherichia coli. The rareness of tellurium in host organisms and the noncontaminated environment raises a question about the true functionality of tellurite resistance gene clusters in pathogenesis and their possible contribution to bacterial fitness. The study aims to point out the beneficial effects of the tellurite resistance gene cluster of pathogenic bacteria to survive in ROS-rich environments. Here, we analysed the bacterial response to oxidative stress conditions with and without tellurite resistance gene clusters, which are composed of terWY1XY2Y3 and terZABCDEF genes. By measuring the levels of protein carbonylation, lipid peroxidation, and expression changes of oxidative stress genes upon oxidative stress, we propose a tellurite resistance gene cluster contribution to the elimination of oxidative damage, potentially increasing fitness and resistance to reactive oxygen species during macrophage attack. We have shown a different beneficial effect of various truncated versions of the tellurite resistance gene cluster on cell survival. The terBCDEF genes increased the survival of E. coli strain MC4100 by 13.21%, terW and terZABCDEF by 10.09%, and terWY1XY2Y3 and terZABCDEF by 25.57%, respectively. The ability to survive tellurite treatment is the most significant at 44.8% in wild clinical strain KL53 compared to laboratory strain E. coli MC4100 due to a complete wild-type plasmid presence.

Tellurite and Selenite: how can these two oxyanions be chemically different yet so similar in the way they are transformed to their metal forms by bacteria?

This opinion review explores the microbiology of tellurite, TeO₃²⁻ and selenite, SeO₃²⁻ oxyanions, two similar Group 16 chalcogen elements, but with slightly different physicochemical properties that lead to intriguing biological differences. Selenium, Se, is a required trace element compared to tellurium, Te, which is not. Here, the challenges around understanding the uptake transport mechanisms of these anions, as reflected in the model organisms used by different groups, are described. This leads to a discussion around how these oxyanions are subsequently reduced to nanomaterials, which mechanistically, has controversies between ideas around the molecule chemistry, chemical reactions involving reduced glutathione and reactive oxygen species (ROS) production along with the bioenergetics at the membrane versus the cytoplasm. Of particular interest is the linkage of glutathione and thioredoxin chemistry from the cytoplasm through the membrane electron transport chain (ETC) system/quinones to the periplasm. Throughout the opinion review we identify open and unanswered questions about the microbial physiology under selenite and tellurite exposure. Thus, demonstrating how far we have come, yet the exciting research directions that are still possible. The review is written in a conversational manner from three long-term researchers in the field, through which to play homage to the late Professor Claudio Vásquez.

Tellurite Promotes Stress Granules and Nuclear SG-Like Assembly in Response to Oxidative Stress and DNA Damage

Tellurium oxyanion, tellurite (TeO₃^–2), is a highly toxic compound for many organisms. Its presence in the environment has increased over the past years due to industrial manufacturing processes and has been associated with adverse effects on human health. Although tellurite induces the phosphorylation of eIF2α, DNA damage and oxidative stress, the molecular mechanisms related to the cellular responses to tellurite-induced stress are poorly understood. In this work, we evaluated the ability of tellurite to induce phosphorylation of eIF2α, stress granules (SGs) assembly and their relationship with DNA damage in U2OS cells. We demonstrate that tellurite promotes the assembly of bona fide cytoplasmic SGs. Unexpectedly, tellurite also induces the assembly of nuclear SGs. Interestingly, we observed that the presence of tellurite-induced nuclear SGs correlates with γH2AX foci. However, although H₂O₂ also induce DNA damage, no nuclear SGs were observed. Our data show that tellurite promotes the assembly of cytoplasmic and nuclear SGs in response to oxidative stress and DNA damage, revealing a new aspect of cellular stress response mediated by the assembly of nuclear stress granules.

Isolation and characterization of Corynebacterium spp. from bulk tank raw cow's milk of different dairy farms in Germany

We detected Corynebacterium spp. in raw milk samples of three farms by means of a selective, tellurite-containing medium. The isolated strains were identified based on full 16S rRNA gene sequences and partial rpoB gene sequences as C. xerosis, C. variabile, C. lactis, C. callunae, C. confusum, C. glutamicum and C. crudilactis. The identification based on 16S rRNA and rpoB sequences was not reliable for isolates of C. xerosis. Chemotaxonomic markers of the isolates, fatty acids, acyl type of peptidoglycan, presence and length of mycolic acids, quinone patterns, and polar lipids, were in accord with the known characteristics of these species. Biochemical profiles, analyzed with the API Coryne system, were able to differentiate all groups, but were unable to identify the strains due to an inappropriate database for raw-milk associated corynebacteria. Most of the tested isolates showed a single-substance resistance against oxacillin, but three single isolates were classified as multidrug resistant.

Proteomic comparison of three clinical diarrhoeagenic drug-resistant Escherichia coli isolates grown on CHROMagar™STEC media

Shiga-toxin-producing Escherichia coli (STEC) and enteropathogenic Escherichia coli (EPEC) are key diarrhoea-causing foodborne pathogens. We used proteomics to characterize the virulence and antimicrobial resistance protein profiles of three clinical pathogenic E. coli isolates (two EPEC [one resistant to ciprofloxacin] and one STEC) cultured on CHROMagar™STEC solid media after minimal laboratory passage. We identified 4767 unique peptides from 1630 protein group across all three clinical E. coli strains. Label-free proteomic analysis allowed the identification of virulence and drug resistance proteins that were unique to each of the clinical isolates compared in this study. The B subunit of Shiga toxin, ToxB, was uniquely detected in the STEC strain while several other virulence factors including SheA, OmpF, OmpC and OmpX were significantly more abundant in the STEC strain. The ciprofloxacin resistant EPEC isolate possessed reduced levels of key virulence proteins compared to the ciprofloxacin susceptible EPEC and STEC strains. Parallel reaction monitoring assays validated the presence of biologically relevant proteins across biologically-replicated cultures. Propagation of clinical isolates on a relevant solid medium followed by mass spectrometry analysis represents a convenient means to quantify virulence factors and drug resistance determinants that might otherwise be lost through extensive in vitro passage in enteropathogenic bacteria.

SIGNIFICANCE

Through the use of quantitative proteomics, we have characterized the virulence and antimicrobial resistance attributes of three clinically isolated, pathogenic E. coli strains cultured on solid media. Our results provide new, quantitative data on the expressed proteomes of these tellurite-resistant, diarrhoeagenic E. coli strains and reveal a subset of antimicrobial resistance and virulence proteins that are differentially abundant between these clinical strains. Our quantitative proteomics-based approach should thus have applicability in microbiological diagnostic labs for the identification of pathogenic/drug resistant E. coli in the future.

Evaluation of CHROMagar STEC and STEC O104 chromogenic agar media for detection of Shiga Toxin-producing Escherichia coli in stool specimens

The performance of CHROMagar STEC and CHROMagar STEC O104 (CHROMagar Microbiology, Paris, France) media for the detection of Shiga toxin-producing Escherichia coli (STEC) was assessed with 329 stool specimens collected over 14 months from patients with suspected STEC infections (June 2011 to August 2012). The CHROMagar STEC medium, after an enrichment broth step, allowed the recovery of the STEC strain from 32 of the 39 (82.1%) Shiga toxin-positive stool specimens, whereas the standard procedure involving Drigalski agar allowed the recovery of only three additional STEC strains. The isolates that grew on CHROMagar STEC medium belonged to 15 serotypes, including the prevalent non-sorbitol-fermenting (NSF) O157:H7, O26:H11, and O104:H4 serotypes. The sensitivity, specificity, and positive and negative predictive values for the CHROMagar STEC medium were between 89.1% and 91.4%, 83.7% and 86.7%, 40% and 51.3%, and 98% and 98.8%, respectively, depending on whether or not stx-negative eae-positive E. coli was considered atypical enteropathogenic E. coli (EPEC) or STEC that had lost Shiga toxin genes during infection. In conclusion, the good performance of CHROMagar STEC agar medium, in particular, the high negative predictive value, and its capacity to identify NSF O157:H7 as well as common non-O157 STEC may be useful for clinical bacteriology, public health, and reference laboratories; it could be used in addition to a method targeting Shiga toxins (detection of stx genes by PCR, immunodetection of Shiga toxins in stool specimens, or Vero cell cytotoxicity assay) as an alternative to O157 culture medium. This combined approach should allow rapid visualization of both putative O157 and non-O157 STEC colonies for subsequent characterization, essential for real-time surveillance of STEC infections and investigations of outbreaks.

The bacterial thiopurine methyltransferase tellurite resistance process is highly dependent upon aggregation properties and oxidative stress response

Bacterial thiopurine methyltransferases (bTPMTs) can favour resistance towards toxic tellurite oxyanions through a pathway leading to the emission of a garlic-like smell. Gene expression profiling completed by genetic, physiological and electron microscopy analyses was performed to identify key bacterial activities contributing to this resistance process. Escherichia coli strain MG1655 expressing the bTPMT was used as a cell model in these experiments. This strain produced a garlic-like smell which was found to be due to dimethyl telluride, and cell aggregates in culture media supplemented with tellurite. Properties involved in aggregation were correlated with cell attachment to polystyrene, which increased with tellurite concentrations. Gene expression profiling supported a role of adhesins in the resistance process with 14% of the tellurite-regulated genes involved in cell envelope, flagella and fimbriae biogenesis. Other tellurite-regulated genes were, at 27%, involved in energy, carbohydrate and amino acid metabolism including the synthesis of antioxidant proteins, and at 12% in the synthesis of transcriptional regulators and signal transduction systems. Escherichia coli mutants impaired in tellurite-regulated genes showed ubiquinone and adhesins synthesis, oxidative stress response, and efflux to be essential in the bTPMT resistance process. High tellurite resistance required a synergistic expression of these functions and an efficient tellurium volatilization by the bTPMT.

tdd8: a TerD domain-encoding gene involved in Streptomyces coelicolor differentiation

The Streptomyces coelicolor genome contains 17 TerD domain-encoding genes (tdd genes) of unknown function. The proteins encoded by these genes have been presumed to be involved in tellurite resistance on the basis of their homology with the protein TerD of Serratia marcescens. To elucidate the role of a Tdd protein (Tdd8), both a deletion mutant for the corresponding gene tdd8 (SCO2368) and a recombinant strain over-expressing tdd8 were produced in S. coelicolor M145. The deletion mutant (Δtdd8), like the wild strain, was not resistant to potassium tellurite. The deletion was not lethal but had a marked effect on differentiation. The deletion strain showed more rapid growth in liquid medium and produced long chains of short spores with a dense and non-spherical spore wall on agar plates. The strain over-expressing tdd8 had a growth delay in liquid medium and produced very few spores of irregular shapes and sizes on solid medium. The results of this study demonstrated that Tdd proteins might have a function other than tellurite resistance and this function seems to be of crucial importance for the proper development of the actinomycete S. coelicolor.

Sulfate assimilation mediates tellurite reduction and toxicity in Saccharomyces cerevisiae

Despite a century of research and increasing environmental and human health concerns, the mechanistic basis of the toxicity of derivatives of the metalloid tellurium, Te, in particular the oxyanion tellurite, Te(IV), remains unsolved. Here, we provide an unbiased view of the mechanisms of tellurium metabolism in the yeast Saccharomyces cerevisiae by measuring deviations in Te-related traits of a complete collection of gene knockout mutants. Reduction of Te(IV) and intracellular accumulation as metallic tellurium strongly correlated with loss of cellular fitness, suggesting that Te(IV) reduction and toxicity are causally linked. The sulfate assimilation pathway upstream of Met17, in particular, the sulfite reductase and its cofactor siroheme, was shown to be central to tellurite toxicity and its reduction to elemental tellurium. Gene knockout mutants with altered Te(IV) tolerance also showed a similar deviation in tolerance to both selenite and, interestingly, selenomethionine, suggesting that the toxicity of these agents stems from a common mechanism. We also show that Te(IV) reduction and toxicity in yeast is partially mediated via a mitochondrial respiratory mechanism that does not encompass the generation of substantial oxidative stress. The results reported here represent a robust base from which to attack the mechanistic details of Te(IV) toxicity and reduction in a eukaryotic organism.

Tellurite resistance and reduction by obligately aerobic photosynthetic bacteria

Seven species of obligately aerobic photosynthetic bacteria of the genera Erythromicrobium, Erythrobacter, and Roseococcus demonstrated high-level resistance to tellurite and accumulation of metallic tellurium crystals. High-level resistance without tellurite reduction was observed for Roseococcus thiosulfatophilus and Erythromicrobium ezovicum grown with certain organic carbon sources, implying that tellurite reduction is not essential to confer tellurite resistance.

Contributions of O island 48 to adherence of enterohemorrhagic Escherichia coli O157:H7 to epithelial cells in vitro and in ligated pig ileal loops

O island 48 (OI-48) of Escherichia coli consists of three functional gene clusters that encode urease, tellurite resistance (Te(r)), and putative adhesins Iha and AIDA-1. The functions of these clusters in enterohemorrhagic E. coli (EHEC) O157:H7 infection are unknown. Deletion mutants for these three regions were constructed and evaluated for their ability to adhere to epithelial cells in vitro and in ligated pig ileal loops. Deletion of the Te(r) gene cluster reduced the ability of the organism to adhere to and form large clusters on IPEC-J2 and HEp-2 cells. Complementation of the mutation by introducing the wild-type ter genes restored adherence and large-cluster formation. Tests in ligated pig ileal loops showed a decrease in colonization by the Te(r)-negative mutant, but the difference was not significant compared to colonization by the wild type (26.4% +/- 21.2% versus 40.1% +/- 19.1%; P = 0.168). The OI-48 aidA gene deletion had no effect on adherence in vitro or in vivo. Deletion of the iha and ureC genes had no effect on adherence in vitro but significantly reduced the colonization of EHEC O157:H7 in the ligated pig intestine. These data suggest that Te(r), Iha, and urease may contribute to EHEC O157:H7 pathogenesis by promoting adherence of the pathogen to the host intestinal epithelium.

NMR solution structure of KP-TerB, a tellurite-resistance protein from Klebsiella pneumoniae

Klebsiella pneumoniae (KP), a Gram-negative bacterium, is a common cause of hospital-acquired bacterial infections worldwide. Tellurium (Te) compounds, although relatively rare in the environment, have a long history as antimicrobial and therapeutic agents. In bacteria, tellurite (TeO(3) (-2)) resistance is conferred by the ter (Te(r)) operon (terZABCDEF). Here, on the basis of 2593 restraints derived from NMR analysis, we report the NMR structure of TerB protein (151 amino acids) of KP (KP-TerB), which is mainly composed of seven alpha-helices and a 3(10) helix, with helices II to V apparently forming a four-helix bundle. The ensemble of 20 NMR structures was well-defined, with a RMSD of 0.32 +/- 0.06 A for backbone atoms and 1.11 +/- 0.07 A for heavy atoms, respectively. A unique property of the KP-TerB structure is that the positively and negatively charged clusters are formed by the N-terminal positively and C-terminal negatively charged residues, respectively. To the best of our knowledge, the protein sequence and structures of KP-TerB are unique.

The bacterial response to the chalcogen metalloids Se and Te

Microbial metabolism of inorganics has been the subject of interest since the 1970s when it was recognized that bacteria are involved in the transformation of metal compounds in the environment. This area of research is generally referred to as bioinorganic chemistry or microbial biogeochemistry. Here, we overview the way the chalcogen metalloids Se and Te interact with bacteria. As a topic of considerable interest for basic and applied research, bacterial processing of tellurium and selenium oxyanions has been reviewed a few times over the past 15 years. Oddly, this is the first time these compounds have been considered together and their similarities and differences highlighted. Another aspect touched on for the first time by this review is the bacterial response in cell-cell or cell-surface aggregates (biofilms) against the metalloid oxyanions. Finally, in this review we have attempted to rationalize the considerable amount of literature available on bacterial resistance to the toxic metalloids tellurite and selenite.

The highly toxic oxyanion tellurite (TeO (3) (2-) ) enters the phototrophic bacterium Rhodobacter capsulatus via an as yet uncharacterized monocarboxylate transport system

The facultative phototroph Rhodobacter capsulatus takes up the highly toxic oxyanion tellurite when grown under both photosynthetic and respiratory growth conditions. Previous works on Escherichia coli and R. capsulatus suggested that tellurite uptake occurred through a phosphate transporter. Here we present evidences indicating that tellurite enters R. capsulatus cells via a monocarboxylate transport system. Indeed, intracellular accumulation of tellurite was inhibited by the addition of monocarboxylates such as pyruvate, lactate and acetate, but not by dicarboxylates like malate or succinate. Acetate was the strongest tellurite uptake antagonist and this effect was concentration dependent, being already evident at 1 microM acetate. Conversely, tellurite at 100 microM was able to restrict the acetate entry into the cells. Both tellurite and acetate uptakes were energy dependent processes, since they were abolished by the protonophore FCCP and by the respiratory electron transport inhibitor KCN. Interestingly, cells grown on acetate, lactate or pyruvate showed a high level resistance to tellurite, whereas cells grown on malate or succinate proved to be very sensitive to the oxyanion. Taking these data together, we propose that: (a) tellurite enters R. capsulatus cells via an as yet uncharacterized monocarboxylate(s) transporter, (b) competition between acetate and tellurite results in a much higher level of tolerance against the oxyanion and (c) the toxic action of tellurite at the cytosolic level is significantly restricted by preventing tellurite uptake.

Catalases are NAD(P)H-dependent tellurite reductases

Reactive oxygen species damage intracellular targets and are implicated in cancer, genetic disease, mutagenesis, and aging. Catalases are among the key enzymatic defenses against one of the most physiologically abundant reactive oxygen species, hydrogen peroxide. The well-studied, heme-dependent catalases accelerate the rate of the dismutation of peroxide to molecular oxygen and water with near kinetic perfection. Many catalases also bind the cofactors NADPH and NADH tenaciously, but, surprisingly, NAD(P)H is not required for their dismutase activity. Although NAD(P)H protects bovine catalase against oxidative damage by its peroxide substrate, the catalytic role of the nicotinamide cofactor in the function of this enzyme has remained a biochemical mystery to date. Anions formed by heavy metal oxides are among the most highly reactive, natural oxidizing agents. Here, we show that a natural isolate of Staphylococcus epidermidis resistant to tellurite detoxifies this anion thanks to a novel activity of its catalase, and that a subset of both bacterial and mammalian catalases carry out the NAD(P)H-dependent reduction of soluble tellurite ion (TeO₃²⁻) to the less toxic, insoluble metal, tellurium (Te°), in vitro. An Escherichia coli mutant defective in the KatG catalase/peroxidase is sensitive to tellurite, and expression of the S. epidermidis catalase gene in a heterologous E. coli host confers increased resistance to tellurite as well as to hydrogen peroxide in vivo, arguing that S. epidermidis catalase provides a physiological line of defense against both of these strong oxidizing agents. Kinetic studies reveal that bovine catalase reduces tellurite with a low Michaelis-Menten constant, a result suggesting that tellurite is among the natural substrates of this enzyme. The reduction of tellurite by bovine catalase occurs at the expense of producing the highly reactive superoxide radical.

The thiol:disulfide oxidoreductase DsbB mediates the oxidizing effects of the toxic metalloid tellurite (TeO32-) on the plasma membrane redox system of the facultative phototroph Rhodobacter capsulatus

The highly toxic oxyanion tellurite (TeO3(2-)) is a well known pro-oxidant in mammalian and bacterial cells. This work examines the effects of tellurite on the redox state of the electron transport chain of the facultative phototroph Rhodobacter capsulatus, in relation to the role of the thiol:disulfide oxidoreductase DsbB. Under steady-state respiration, the addition of tellurite (2.5 mM) to membrane fragments generated an extrareduction of the cytochrome pool (c- and b-type hemes); further, in plasma membranes exposed to tellurite (0.25 to 2.5 mM) and subjected to a series of flashes of light, the rate of the QH2:cytochrome c (Cyt c) oxidoreductase activity was enhanced. The effect of tellurite was blocked by the antibiotics antimycin A and/or myxothiazol, specific inhibitors of the QH2:Cyt c oxidoreductase, and, most interestingly, the membrane-associated thiol:disulfide oxidoreductase DsbB was required to mediate the redox unbalance produced by the oxyanion. Indeed, this phenomenon was absent from R. capsulatus MD22, a DsbB-deficient mutant, whereas the tellurite effect was present in membranes from MD22/pDsbB(WT), in which the mutant gene was complemented to regain the wild-type DsbB phenotype. These findings were taken as evidence that the membrane-bound thiol:disulfide oxidoreductase DsbB acts as an "electron conduit" between the hydrophilic metalloid and the lipid-embedded Q pool, so that in habitats contaminated with subinhibitory amounts of Te(IV), the metalloid is likely to function as a disposal for the excess reducing power at the Q-pool level of facultative phototrophic bacteria.

Effects of the metalloid oxyanion tellurite (TeO32-) on growth characteristics of the phototrophic bacterium Rhodobacter capsulatus

This work examines the effects of potassium tellurite (K2TeO3) on the cell viability of the facultative phototroph Rhodobacter capsulatus. There was a growth mode-dependent response in which cultures anaerobically grown in the light tolerate the presence of up to 250 to 300 microg of tellurite (TeO3(2-)) per ml, while dark-grown aerobic cells were inhibited at tellurite levels as low as 2 microg/ml. The tellurite sensitivity of aerobic cultures was evident only for growth on minimal salt medium, whereas it was not seen during growth on complex medium. Notably, through the use of flow cytometry, we show that the cell membrane integrity was strongly affected by tellurite during the early growth phase (< or =50% viable cells); however, at the end of the growth period and in parallel with massive tellurite intracellular accumulation as elemental Te0 crystallites, recovery of cytoplasmic membrane integrity was apparent (> or =90% viable cells), which was supported by the development of a significant membrane potential (Deltapsi = 120 mV). These data are taken as evidence that in anaerobic aquatic habitats, the facultative phototroph R. capsulatus might act as a natural scavenger of the highly soluble and toxic oxyanion tellurite.

New genes implicated in the protection of anaerobically grown Escherichia coli against nitric oxide

Nitric oxide produced by activated macrophages plays a key role as one of the immune system's weapons against pathogens. Because the lifetime of nitric oxide is short in aerobic conditions, whereas in anaerobic conditions the cytotoxic effects of nitric oxide are greatly increased as in the infection/inflammation processes, it is important to establish which systems are able to detoxify nitric oxide under anaerobic conditions. In the present work a new set of Escherichia coli K-12 genes conferring anaerobic resistance to nitric oxide is presented, namely the gene product of YtfE and a potential transcriptional regulator of the helix-turn-helix LysR-type (YidZ). The crucial role of flavohemoglobin for anaerobic nitric oxide protection is also demonstrated. Furthermore, nitric oxide is shown to cause a significant alteration of the global E. coli gene transcription profile that includes the increase of the transcript level of genes encoding for detoxification enzymes, iron-sulfur cluster assembly systems, DNA-repairing enzymes, and stress response regulators.

Field and soil microcosm studies on the survival and conjugation of aPseudomonas putidastrain bearing a recombinant plasmid, pADPTel

Pseudomonas putida CR30RNS (pADPTel) is an antibiotic-resistant strain with a recombinant plasmid that confers resistance to tellurite and the ability to catabolize atrazine. The survival of this strain as well as its ability to transfer genes for atrazine degradation and tellurite resistance to indigenous soil bacteria were tested in both fallow soil and canola (Brassica napus) rhizosphere by the use of parallel field and laboratory releases. Culturable CR30RNS (pADPTel) were enumerated in field and microcosm soils at 7- to 14-day intervals over 49 d. Strain CR30RNS (pADPTel) survived for up to 7 weeks in microcosm soils at a density of 104CFU/g soil, whereas in field soils the population declined to 103CFU/g soil by the fourth week. In contrast, when CR30RNS (pADPTel) was introduced into the soil as a seed coating of canola (B. napus 'Karoo'), the bacterium established at higher cell densities in the rhizosphere (106–105CFU/g fresh root mass), with no subsequent decrease in numbers. The presence of selective pressure (i.e., atrazine) had no significant effect on the survival of CR30RNS (pADPTel) in either field or microcosm soils. One year postinoculation field sites were examined for the presence of CR30RNS (pADPTel) and no evidence of culturable parental cells was observed when samples were plated onto selective media. However, the atzC and telAB gene segments were amplified from the field soils at that time. Under laboratory conditions, indigenous soil bacteria were capable of receiving and expressing the engineered plasmid construct at frequencies ranging from 1 to 10-3transconjugants per donor. However, no plasmid transfer to indigenous soil bacteria was detected in the field or microcosm soils regardless of the presence of canola rhizosphere and (or) the application of atrazine. Our results show that the survival and population size of P. putida CR30RNS (pADPTel) might be sufficient for degradation of environmental pollutants but that the transfer frequency was too low to be detected under the conditions of this study.Key words: Pseudomonas putida CR30RNS (pADPTel), survival, gene transfer, field, microcosm.

Geobacillus stearothermophilus V ubiE gene product is involved in the evolution of dimethyl telluride in Escherichia coli K-12 cultures amended with potassium tellurate but not with potassium tellurite

A 3.8-kb fragment of chromosomal DNA of Geobacillus stearothermophilus V cloned in pSP72 (p1VH) confers resistance to potassium tellurite (K(2)TeO(3)) and to potassium tellurate (K(2)TeO(4)) when the encoded genes are expressed in Escherichia coli K-12. The nt sequence of the cloned fragment predicts three ORFs of 780, 399, and 600 bp, whose encoded protein products exhibit about 80% similarity with the SUMT methyltransferase and the BtuR protein of Bacillus megaterium, and with the UbiE methyltransferase of Bacillus anthracis A2012, respectively. In addition, E. coli/p1VH cells evolved dimethyl telluride, which was released into the headspace gas above liquid cultures when amended with K(2)TeO(3) or with K(2)TeO(4). After 48 h of growth in the presence of these compounds, a protein of about 25 kDa was found at a significantly higher level when crude extracts were analyzed by SDS-PAGE. The N-terminal amino acid (aa) sequence of this protein, obtained by Edman degradation, matched the deduced aa sequence predicted by the G. stearothermophilus V ubiE gene. This gene was amplified by PCR, subcloned in pET21b, and transformed into E. coli JM109(DE3). Interestingly, DMTe evolution occurred when these modified cells were grown in K(2)TeO(4) - but not in K(2)TeO(3) - amended media. These results may be indicative that the two Te oxyanions could be detoxified in the cell by different metabolic pathways.

Optimization of enrichment and plating procedures for the recovery of Escherichia coli O111 and O26 from minced beef

AIM

Optimization of enrichment media and selective agars for the detection of Escherichia coli O26 and O111 from minced beef.

METHODS AND RESULTS

This study compared a number of different enrichment conditions and plating media for the recovery of E. coli O26 and E. coli O111 from minced beef. The optimum enrichment conditions for E. coli O26 was observed in beef samples enriched at 41.5 degrees C in tryptone soya broth supplemented with cefixime (50 microg l(-1)), vancomycin (40 mg l(-1)) and potassium tellurite (2.5 mg l(-1)). Similar enrichment conditions were optimal for E. coli O111 with the omission of potassium tellurite. The optimum agar for recovery of E. coli O26 and giving the most effective suppression of contaminants was MacConkey agar [lactose replaced by rhamnose (20 g l(-1))] and supplemented with cefixime (50 microg ml(-1)) and potassium tellurite (2.5 mg l(-1)). Optimum recovery of E. coli O111 was on chromocult agar, supplemented with cefixime (50 microg ml(-1)), cefsulodin (5 mg l(-1)) and vancomycin (8 mg l(-1)). Minced beef samples were inoculated with a number of strains of E. coli O26 (n=9) and O111 (n=8), and the developed enrichment and plating methods, used in combination with immunomagnetic separation, were shown to be an effective method for the recovery of all strains.

CONCLUSIONS

Routine cultural methods for the recovery of E. coli O26 and O111 from minced beef are described.

SIGNIFICANCE AND IMPACT OF THE STUDY

The optimized enrichment and plating procedure described for the recovery of E. coli O111 and O26 from meat can be used to extend research on these emerging pathogens in beef.

An inducible tellurite-resistance operon in Proteus mirabilis

Tellurite resistance (Te(r)) is widespread in nature and it is shown here that the natural resistance of Proteus mirabilis to tellurite is due to a chromosomally located orthologue of plasmid-borne ter genes found in enteric bacteria. The P. mirabilis ter locus (terZABCDE) was identified in a screen of Tn5lacZ-generated mutants of which one contained an insertion in terC. The P. mirabilis terC mutant displayed increased susceptibility to tellurite (Te(s)) and complementation with terC carried on a multicopy plasmid restored high-level Te(r). Primer extension analysis revealed a single transcriptional start site upstream of terZ, but only with RNA harvested from bacteria grown in the presence of tellurite. Northern blotting and reverse transcriptase-PCR (RT-PCR) analyses confirmed that the ter operon was inducible by tellurite and to a lesser extent by oxidative stress inducers such as hydrogen peroxide and methyl viologen (paraquat). Direct and inverted repeat sequences were identified in the ter promoter region as well as motifs upstream of the -35 hexamer that resembled OxyR-binding sequences. Finally, the 390 bp intergenic promoter region located between orf3 and terZ showed no DNA sequence identity with any other published ter sequences, whereas terZABCDE genes exhibited 73-85 % DNA sequence identity. The ter operon was present in all clinical isolates of P. mirabilis and Proteus vulgaris tested and is inferred for Morganella and Providencia spp. based on screening for high level Te(r) and preliminary PCR analysis. Thus, a chromosomally located inducible tellurite resistance operon appears to be a common feature of the genus Proteus.

Genomic variability of O islands encoding tellurite resistance in enterohemorrhagic Escherichia coli O157:H7 isolates

Strains of Escherichia coli causing enterohemorrhagic colitis belonging to the O157:H7 lineage are reported to be highly related. Fifteen strains of E. coli O157:H7 and 1 strain of E. coli O46:H(-) (nonflagellated) were examined for the presence of potassium tellurite resistance (Te(r)). Te(r) genes comprising terABCDEF were shown previously to be part of a pathogenicity island also containing integrase, phage, and urease genes. PCR analysis, both conventional and light cycler based, demonstrated that about one-half of the Te(r) E. coli O157:H7 strains (6 of 15), including the Sakai strain, which has been sequenced, carried a single copy of the Te(r) genes. Five of the strains, including EDL933, which has also been sequenced, contained two copies. Three other O157:H7 strains and the O46:H(-) strain did not contain the Te(r) genes. In strains containing two copies, the Te(r) genes were associated with the serW and serX tRNA genes. Five O157:H7 strains resembled the O157 Sakai strain whose sequence contained one copy, close to serX, whereas in one isolate the single copy was associated with serW. There was no correlation between Te(r) and the ability to produce Shiga toxin ST1 or ST2. The Te(r) MIC for most strains, containing either one or two copies, was 1,024 micro g/ml, although for a few the MIC was intermediate, 64 to 128 micro g/ml, which could be increased to 512 micro g/ml by pregrowth of strains in subinhibitory concentrations of potassium tellurite. Reverse transcriptase PCR analysis confirmed that in most strains Te(r) was constitutive but that in the rest it was inducible and involved induction of terB and terC genes. Only the terB, -C, -D, and -E genes are required for Te(r). The considerable degree of homology between the ter genes on IncH12 plasmid R478, which originated in Serratia marcescens, and pTE53, from an E. coli clinical isolate, suggests that the pathogenicity island was acquired from a plasmid. This work demonstrates diversity among E. coli O157:H7 isolates, at least as far as the presence of Te(r) genes is concerned.

Use of potassium tellurite for testing the survival and viability of Pseudomonas pseudoalcaligenes KF707 in soil microcosms contaminated with polychlorinated biphenyls

This study shows that the oxyanion tellurite TeO3(2-) can be used as a tool to detect and quantify the release in soil microcosms of Pseudomonas pseudoalcaligenes KF707, a strain spontaneously resistant to tellurite with a minimal inhibitory concentration (MIC) of 150 microg ml(-1). KF707 cells which carry the genes for degradation of a wide range of polychlorinated biphenyl congeners (PCBs) were used for inoculation of laboratory microcosms prepared with two different PCB-contaminated soils (Ci/s and Di/s) in the presence or absence of biphenyl as carbon source. In all microcosms supplemented with biphenyl, significant survival of strain KF707 was noted over a time period of 35 days; conversely, in microcosms containing Ci/s soil without biphenyl addition a rapid decrease in KF707 inoculated cells was observed. By comparing the number of inoculated KF707 cells with the number of indigenous bacteria growing on biphenyl (IBGB) of both Ci/s and Di/s microcosms, it could be concluded that the KF707/IBGB ratio is a relevant parameter in determining the fate of the added strain. The efficacy of potassium tellurite as a selective marker to monitor strain KF707 in laboratory microcosms was confirmed by ARDRA analyses of the 16S rDNA, while the isolated indigenous bacteria growing on biphenyl were identified as members of three different species of the genus Pseudomonas. We also report that in microcosms inoculated with KF707 cells in the absence of biphenyl, only low chlorinated biphenyls were degraded.

A novel selenite- and tellurite-inducible gene in Escherichia coli

Selenium is both an essential and a toxic trace element, and the range of concentrations between the two is extremely narrow. Although tellurium is not essential and is only rarely found in the environment, it is considered to be extremely toxic. Several hypotheses have been proposed to account for the toxic effects of selenite and tellurite. However, these potential mechanisms have yet to be fully substantiated. Through screening of an Escherichia coli luxAB transcriptional gene fusion library, we identified a clone whose luminescence increased in the presence of increasing concentrations of sodium selenite or sodium tellurite. Cloning and sequencing of the luxAB junction revealed that the fusion had occurred in a previously uncharacterized open reading frame, termed o393 or yhfC, which we have now designated gutS, for gene up-regulated by tellurite and selenite. Transcription from gutS in the presence of selenite or tellurite was confirmed by RNA dot blot analysis. In vivo expression of the GutS polypeptide, using the pET expression system, revealed a polypeptide of approximately 43 kDa, in good agreement with its predicted molecular mass. Although the function of GutS remains to be elucidated, homology searches as well as protein motif and secondary-structure analyses have provided clues which may implicate GutS in transport in response to selenite and tellurite.

Selective isolation of eae-positive strains of shiga toxin-producing Escherichia coli

Culture on cefixime, tellurite, and sorbitol-MacConkey agar after HCl treatment facilitated the growth of 410 (94%) of 436 eae-positive Shiga toxin-producing Escherichia coli (STEC) strains and 17 (16%) of 107 eae-negative STEC strains. This selectivity was closely related to acid resistance in E. coli and tellurite resistance in eae-positive STEC strains.

Low iron concentration and aconitase deficiency in a yeast frataxin homologue deficient strain

Deletion of the yeast frataxin homologue, YFH1, elicits accumulation of iron in mitochondria and mitochondrial defects. We report here that in the presence of an iron chelator in the culture medium, the concentration of iron in mitochondria is the same in wild-type and YFH1 deletant strains. Under these conditions, the activity of the respiratory complexes is restored. However, the activity of the mitochondrial aconitase, a 4Fe-4S cluster-containing protein, remains low. The frataxin family bears homology to a bacterial protein family which confers resistance to tellurium, a metal closely related to sulfur. Yfh1p might control the synthesis of iron-sulfur clusters in mitochondria.

Bacterial tellurite resistance

Tellurium compounds are used in several industrial processes, although they are relatively rare in the environment. Genes associated with tellurite resistance (TeR) are found in many pathogenic bacteria. Tellurite can be detoxified through interactions with cellular thiols, such as glutathione, or a methyltransferase-catalyzed reaction, although neither process appears involved in plasmid-mediated TeR.

Resistance to tellurite as a selection marker for genetic manipulations of Pseudomonas strains

Resistance to the toxic compound potassium tellurite (Telr) has been employed as a selection marker built into a set of transposon vectors and broad-host-range plasmids tailored for genetic manipulations of Pseudomonas strains potentially destined for environmental release. In this study, the activated Telr determinants encoded by the cryptic telAB genes of plasmid RK2 were produced, along with the associated kilA gene, as DNA cassettes compatible with cognate vectors. In one case, the Telr determinants were assembled between the I and O ends of a suicide delivery vector for mini-Tn5 transposons. In another case, the kilA and telAB genes were combined with a minimal replicon derived from a variant of Pseudomonas plasmid pPS10, which is able to replicate in a variety of gram-negative hosts and is endowed with a modular collection of cloning and expression assets. Either in the plasmid or in the transposon vector, the Telr marker was combined with a 12-kb DNA segment of plasmid pWW0 of Pseudomonas putida mt-2 encoding the upper TOL pathway enzymes. This allowed construction of antibiotic resistance-free but selectable P. putida strains with the ability to grow on toluene as the sole carbon source through an ortho-cleavage catabolic pathway.

Aerobic anoxygenic phototrophic bacteria

The aerobic anoxygenic phototrophic bacteria are a relatively recently discovered bacterial group. Although taxonomically and phylogenetically heterogeneous, these bacteria share the following distinguishing features: the presence of bacteriochlorophyll a incorporated into reaction center and light-harvesting complexes, low levels of the photosynthetic unit in cells, an abundance of carotenoids, a strong inhibition by light of bacteriochlorophyll synthesis, and the inability to grow photosynthetically under anaerobic conditions. Aerobic anoxygenic phototrophic bacteria are classified in two marine (Erythrobacter and Roseobacter) and six freshwater (Acidiphilium, Erythromicrobium, Erythromonas, Porphyrobacter, Roseococcus, and Sandaracinobacter) genera, which phylogenetically belong to the alpha-1, alpha-3, and alpha-4 subclasses of the class Proteobacteria. Despite this phylogenetic information, the evolution and ancestry of their photosynthetic properties are unclear. We discuss several current proposals for the evolutionary origin of aerobic phototrophic bacteria. The closest phylogenetic relatives of aerobic phototrophic bacteria include facultatively anaerobic purple nonsulfur phototrophic bacteria. Since these two bacterial groups share many properties, yet have significant differences, we compare and contrast their physiology, with an emphasis on morphology and photosynthetic and other metabolic processes.

Biochemical characterization of tellurite-reducing activities of Bacillus stearothermophilus V

Bacillus stearothermophilus V is a naturally occurring Gram-positive rod which exhibits resistance to potassium tellurite. Crude extracts of this bacterium catalyse the NADH-dependent, protease-sensitive reduction of K2TeO3 in vitro. Two fractions which showed the ability to reduce potassium tellurite (H1 and H2) were obtained. Fraction H1 behaved as a macroaggregate exhibiting a very high molecular mass that could not be estimated accurately. Upon electrophoresis in polyacrylamide gels in the presence of SDS, however, it was resolved into three distinct bands of 60, 41 and 37.5 kDa. On the other hand, an M(r) of 121 was determined for fraction H2 by means of gel filtration and high-pressure liquid chromatography. In SDS-PAGE a unique protein band of 60 kDa was observed, suggesting that it is actually a dimer. Both fractions showed pH and temperature optima of 7.5 and 57 degrees C, respectively. Concentrations of 2.5 M NaCl or 0.35 mM SDS inhibited fraction H2 almost completely, while fraction H1 retained 20% of its activity under the same conditions. Concentrations of 5 mM EDTA caused the activity of both fractions to increase 2-fold. In addition to reducing tellurite, they were also able to reduce Na2SeO3 and Na2SO3 in vitro.

In vivo and in vitro cloning and phenotype characterization of tellurite resistance determinant conferred by plasmid pTE53 of a clinical isolate of Escherichia coli

A determinant encoding resistance against potassium tellurite (Te(r)) was discovered in a clinical isolate of Escherichia coli strain KL53. The strain formed typical black colonies on solid LB medium with tellurite. The determinant was located on a large conjugative plasmid designated pTE53. Electron-dense particles were observed in cells harboring pTE53 by electron microscopy. X-Ray identification analysis identified these deposits as elemental tellurium and X-ray diffraction analysis showed patterns typical of crystalline structures. Comparison with JCPDS 4-0554 (Joint Committee on Powder Diffraction Standards) reference data confirmed that these crystals were pure tellurium crystals. In common with other characterized Te(r) determinants, accumulation studies with radioactively labeled tellurite showed that reduced uptake of tellurite did not contribute to the resistance mechanism. Tellurite accumulation rates for E. coli strain AB1157 harboring pTE53 were twice higher than for the plasmid-free host strain. In addition, no efflux mechanism was detected. The potassium tellurite resistance determinant of plasmid pTE53 was cloned using both in vitro and in vivo techniques in low-copy-number vectors pACYC184 and mini-Mu derivative pPR46. Cloning of the functional Te(r) determinant into high-copy cloning vectors pTZ19R and mini-Mu derivatives pBEf and pJT2 was not successful. During in vivo cloning experiments, clones with unusual "white colony" phenotypes were found on solid LB with tellurite. All these clones were Mucts62 lysogens. Their tellurite resistance levels were in the same order as the wild type strains. Clones with the "white" phenotype had a 3.6 times lower content of tellurium than the tellurite-reducing strain. Transformation of a "white" mutant with a recombinant pACYC184 based Te(r) plasmid did not change the phenotype. However, when one clone was cured from Mucts62 the "white" phenotype reverted to the wild-type "black" phenotype. It was suggested that the "white" phenotype was the result of an insertional inactivation of an unknown chromosomal gene by Mucts62, which reduced the tellurite uptake.

Microbial solubilization and immobilization of toxic metals: key biogeochemical processes for treatment of contamination

Microorganisms play important roles in the environmental fate of toxic metals with a multiplicity of physico-chemical and biological mechanisms effecting transformations between soluble and insoluble phases. Such mechanisms are important components of natural biogeochemical cycles for metals and metalloids with some processes being of potential application to the treatment of contaminated materials. This paper will concentrate on three selected aspects which illustrate the key importance of microorganisms in effecting changes in metal(loid) solubility, namely toxic metal sulfide precipitation by sulfate-reducing bacteria, heterotrophic leaching by fungi, and microbial transformations of metalloids, which includes reduction and methylation. The basic microbiology of these processes is described as well as their environmental significance and use in bioremediation.

Distribution, diversity and evolution of the bacterial mercury resistance (mer) operon

Mercury and its compounds are distributed widely across the earth. Many of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cycling of mercury in the natural environment. Five mechanisms of resistance to mercury compounds have been identified, of which resistance to inorganic mercury (HgR) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistance to heavy metals in general. Resistance to inorganic mercury is encoded by the genes of the mer operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical distribution, and furthermore, are found across a wide range of both Gram-negative and Gram-positive bacteria from both natural and clinical environments. The presence of mer genes in bacteria from sediment cores suggest that mer is an ancient system. Analysis of DNA sequences from mer operons and genes has revealed genetic variation both in operon structure and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has identified a number of vestigial non-functional operons. It is hypothesised that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response to increased levels of mercury in natural environments, perhaps resulting from volcanic activity. Models for the evolution of both a basic mer operon and for the Tn21-related family of mer operons and transposons are suggested. The study of evolution in bacteria has recently become dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which incorporate the effects of horizontal gene transfer. In addition, the mer operon provides a model system in the study of environmental microbiology which is useful both as an example of a genotype which is responsive to environmental pressures and as a generic tool for the development of new methodology for the analysis of bacterial communities in natural environments.

Tellurite reductase activity of nitrate reductase is responsible for the basal resistance of Escherichia coli to tellurite

Tellurite and selenate reductase activities were identified in extracts of Escherichia coli. These activities were detected on non-denaturing polyacrylamide gels using an in situ methyl viologen activity-staining technique. The activity bands produced from membrane-protein extracts had the same RF values as those of nitrate reductases (NRs) A and Z. Tellurite and selenate reductase activities were absent from membranes obtained from mutants deleted in NRs A and Z. Further evidence of the tellurite and selenate reductase activities of NR was demonstrated using rocket immunoelectrophoresis analysis, where the tellurite and selenate reductase activities corresponded to the precipitation arc of NR. Additionally, hypersensitivity to potassium tellurite was observed under aerobic growth conditions in nar mutants. The tac promoter expression of NR A resulted in elevated tellurite resistance. The data obtained also imply that a minimal threshold level of NR A is required to increase resistance. Under anaerobic growth conditions additional tellurite reductase activity was identified in the soluble fraction on non-denaturing gels. Nitrate reductase mutants were not hypersensitive under anaerobic conditions, possibly due to the presence of this additional reductase activity.

Expression of Escherichia coli TehA gives resistance to antiseptics and disinfectants similar to that conferred by multidrug resistance efflux pumps

The genes tehAB located at 32.3 min on the Escherichia coli chromosome were initially identified by their ability to mediate resistance to potassium tellurite (128 micrograms of K2TeO3 per ml) when overexpressed with a high-copy-number plasmid. The genes encode an integral membrane protein (TehA) of 36 kDa with 10 putative transmembrane segments and a second protein (TehB) of 23 kDa. Overexpression of TehAB results in hypersensitivity to dequalinium CI and methyl viologen (paraquat). Expression of TehA alone gives similar hypersensitivity. Overexpression of TehA gave resistance to tetraphenylarsonium CI, ethidium bromide, crystal violet and proflavin. The efflux of ethidium, measured by fluorescence quenching, revealed that TehA transported ethidium at twice the control rate and 10% of the rate of the highly resistant efflux transporter Emr Eco. Addition of tellurite had no effect on ethidium transport. In addition to the ethidium transport assay, a proflavin fluorescence assay which was approximately 200-fold more sensitive was also used. TehA was also found to have proflavin efflux activity. The addition of TeO32- to the proflavin transport assay on TehA caused a 20% increase in transport rate. Both ethidium and proflavin transport were found to be energy dependent.

Bacterial resistances to toxic metal ions--a review

Bacterial plasmids encode resistance systems for toxic metal ions, including Ag+, AsO2-, AsO4(3-), Cd2+, Co2+, CrO4(2-), Cu2+, Hg2+, Ni2+, Pb2+, Sb3+, TeO3(2-), Tl+ and Zn2+. The function of most resistance systems is based on the energy-dependent efflux of toxic ions. Some of the efflux systems are ATPases and others are chemiosmotic cation/proton antiporters. The Cd(2+)-resistance ATPase of Gram-positive bacteria (CadA) is membrane cation pump homologous with other bacterial, animal and plant P-type ATPases. CadA has been labeled with 32P from [alpha-32P] ATP and drives ATP-dependent Cd2+ (and Zn2+) uptake by inside-out membrane vesicles (equivalent to efflux from whole cells). Recently, isolated genes defective in the human hereditary diseases of copper metabolism, namely Menkes syndrome and Wilson's disease, encode P-type ATPases that are more similar to bacterial CadA than to other ATPases from eukaryotes. The arsenic resistance efflux system transports arsenite [As(III)], alternatively using either a double-polypeptide (ArsA and ArsB) ATPase or a single-polypeptide (ArsB) functioning as a chemiosmotic transporter. The third gene in the arsenic resistance system, arsC, encodes an enzyme that converts intracellular arsenate [As(V)] to arsenite [As(III)], the substrate of the efflux system. The triple-polypeptide Czc (Cd2+, Zn2+ and Co2+) chemiosmotic efflux pump consists of inner membrane (CzcA), outer membrane (CzcC) and membrane-spanning (CzcB) proteins that together transport cations from the cytoplasm across the periplasmic space to the outside of the cell.

Bacterial heavy metal resistance: new surprises

Bacterial plasmids encode resistance systems for toxic metal ions including Ag+, AsO2-, AsO4(3-), Cd2+, CO2+, CrO4(2-), Cu2+, Hg2+, Ni2+, Pb2+, Sb3+, TeO3(2-), Tl+, and Zn2+. In addition to understanding of the molecular genetics and environmental roles of these resistances, studies during the last few years have provided surprises and new biochemical mechanisms. Chromosomal determinants of toxic metal resistances are known, and the distinction between plasmid resistances and those from chromosomal genes has blurred, because for some metals (notably mercury and arsenic), the plasmid and chromosomal determinants are basically the same. Other systems, such as copper transport ATPases and metallothionein cation-binding proteins, are only known from chromosomal genes. The largest group of metal resistance systems function by energy-dependent efflux of toxic ions. Some of the efflux systems are ATPases and others are chemiosmotic cation/proton antiporters. The CadA cadmium resistance ATPase of gram-positive bacteria and the CopB copper efflux system of Enterococcus hirae are homologous to P-type ATPases of animals and plants. The CadA ATPase protein has been labeled with 32P from gamma-32P-ATP and drives ATP-dependent Cd2+ uptake by inside-out membrane vesicles. Recently isolated genes defective in the human hereditary diseases of copper metabolism, Menkes syndrome and Wilson's disease, encode P-type ATPases that are more similar to the bacterial CadA and CopB ATPases than to eukaryote ATPases that pump different cations. The arsenic resistance efflux system transports arsenite, using alternatively either a two-component (ArsA and ArsB) ATPase or a single polypeptide (ArsB) functioning as a chemiosmotic transporter. The third gene in the arsenic resistance system, arsC, encodes an enzyme that converts intracellular arsenate [As (V)] to arsenite [As (III)], the substrate of the efflux system. The three-component Czc (Cd2+, Zn2+, and CO2+) chemiosmotic efflux pump of soil microbes consists of inner membrane (CzcA), outer membrane (CzcC), and membrane-spanning (CzcB) proteins that together transport cations from the cytoplasm across the periplasmic space to the outside of the cell. Finally, the first bacterial metallothionein (which by definition is a small protein that binds metal cations by means of numerous cysteine thiolates) has been characterized in cyanobacteria.

Phage inhibition, colicin resistance, and tellurite resistance are encoded by a single cluster of genes on the IncHI2 plasmid R478

A region of the IncHI2 plasmid R478, encoding the phenotypes of tellurite resistance (Ter), phage inhibition (Phi), and colicin resistance (PacB), was cloned and sequenced. Analysis indicated seven open reading frames (ORFs), whose genes were designated terZ, -A, -B, -C, -D, -E, and -F. Five of these predicted ORFs (A to E) had extensive amino acid homology with the previously reported ORFs of the IncHI2 Ter operon from plasmid pMER610. There were domains of highly conserved amino acid residues within the group TerA, -D, -E, and -F and within TerD, -E, and -Z, but no consensus could be found among all five putative polypeptides. There were also regions of good identity and similarity between individual pairs of ORFs which was not reflected in the multiple alignments. The three phenotypes were expressed in Escherichia coli DH5 alpha by an 8.4-kb EcoRI insert subcloned from a cosmid of R478. The latter insert was clonable only as a double insertion with a 4.5-kb fragment, and forced deletion of the smaller fragment was lethal to cells. This lethality was not dependent on the cloned orientation of either fragment, suggesting that there is a trans-acting element in the 4.5-kb fragment. Tn1000 mutagenesis of one of the double-insert clones, pDT2575, showed that the phenotypes, including multiple colicin resistance, were genetically linked. Transpositions into terD, terC, and terZ reduced or abolished all phenotypes, while inserts into terE and terF had no effect on the phenotypes. Insertions in terA reduced phage inhibition levels only. The presence of the terZ and terF ORFs in pMER610 was confirmed, and derivatives of this plasmid mediated Phi, PacB, and Ter.

Cytological aspects of resistance to potassium tellurite conferred on Pseudomonas cells by plasmids

The ultrastructure of strains Pseudomonas putida BS228 and Pseudomonas aeruginosa ML4262 harboring plasmids pBS10, pBS31, and pBS221, which determine resistance to potassium tellurite, was studied. Bacteria were grown in media containing increasing concentrations of potassium tellurite. Crystalline structures containing tellurium appeared in their periplasmic space. The dynamics of crystal growth was studied. Crystals were released into the medium by pinching off of the outer membrane vesicles containing growing crystals. A possible mechanism of this process was described; cytobiochemical peculiarities were discussed.

Understanding cellular responses to toxic agents: a model for mechanism-choice in bacterial metal resistance

Bacterial resistances to metals are heterogeneous in both their genetic and biochemical bases. Metal resistance may be chromosomally-, plasmid- or transposon-encoded, and one or more genes may be involved: at the biochemical level at least six different mechanisms are responsible for resistance. Various types of resistance mechanisms can occur singly or in combination and for a particular metal different mechanisms of resistance can occur in the same species. To understand better the diverse responses of bacteria to metal ion challenge we have constructed a qualitative model for the selection of metal resistance in bacteria. How a bacterium becomes resistant to a particular metal depends on the number and location of cellular components sensitive to the specific metal ion. Other important selective factors include the nature of the uptake systems for the metal, the role and interactions of the metal in the normal metabolism of the cell and the availability of plasmid (or transposon) encoded resistance mechanisms. The selection model presented is based on the interaction of these factors and allows predictions to be made about the evolution of metal resistance in bacterial populations. It also allows prediction of the genetic basis and of mechanisms of resistance which are in substantial agreement with those in well-documented populations. The interaction of, and selection for resistance to, toxic substances in addition to metals, such as antibiotics and toxic analogues, involve similar principles to those concerning metals. Potentially, models for selection of resistance to any substance can be derived using this approach.

Bacterial resistance mechanisms for heavy metals of environmental concern

Bacterial species have genetically-determined systems for resistances to toxic heavy metals. Those for metals of environmental concern including mercury cadmium, arsenic and others are briefly summarized, considering the genes of the systems and the biochemical mechanisms by which the resistance proteins function.

Neither reduced uptake nor increased efflux is encoded by tellurite resistance determinants expressed in Escherichia coli

Rates of uptake of the TeO3(2-) oxyanion were investigated in Escherichia coli cells containing tellurite resistance determinants from both plasmid (RK2Ter, R478, pMER610, MIP233, pHH1508a, pMUR) and chromosomal (tehAB) sources. The uptake was investigated to determine whether or not reduced uptake or increased efflux is involved in the tellurite resistance mechanism. Reduced TeO3(2-) uptake generated by cultures harboring arsABC from the plasmid R773, which has been previously shown to be an oxyanion efflux transporter, was used as the standard. Uptake curves were found to be essentially identical among E. coli cultures harboring the tellurite resistance plasmids RK2Ter, pMER610, pHH1508a, and pMUR and cultures harboring tellurite-sensitive control plasmids. Cultures harboring clones of the tehAB operon from E. coli showed no change in the TeO3(2-) accumulation. Cultures harboring R478 demonstrated reduced uptake. However, a subclone containing only the tellurite resistance determinant displayed no reduced uptake. This suggests that there may be another determinant on R478 other than the primary tellurite resistance determinant that gives rise to TeO3(2-) efflux. These results demonstrate that neither reduced uptake nor increased efflux is responsible for the tellurite resistance in the resistance determinants investigated here.