A suite of recombinant luminescent bacterial strains for the quantification of bioavailable heavy metals and toxicity testing

Background  

Recombinant whole-cell sensors have already proven useful in the assessment of the bioavailability of environmental pollutants like heavy metals and organic compounds. In this work 19 recombinant bacterial strains representing various Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Escherichia coli, Pseudomonas fluorescens) bacteria were constructed to express the luminescence encoding genes luxCDABE (from Photorhabdus luminescens) as a response to bioavailable heavy metals ("lights-on" metal sensors containing metal-response elements, 13 strains) or in a constitutive manner ("lights-off" constructs, 6 strains).     

Axenic aerobic biofilms inhibit corrosion of SAE 1018 steel through oxygen depletion

Corrosion inhibition of SAE 1018 steel by pure-culture biofilms of Pseudomonas fragi and Escheri-chia coli DH5α has been evaluated in complex Luria-Bertani medium, seawater-mimicking medium, and modified Baar's medium at 30 °C. In batch cultures, both bacteria inhibited corrosion three to six fold compared to sterile controls, and the corrosion was comparable to that observed in anaerobic sterile media. To corroborate this result, a continuous reactor and electrochemical impedance spectroscopy were used to show that both P. fragi K and E. coli DH5α decreased the corrosion rate by 4- to 40-fold as compared to sterile controls; this matched the decrease in corrosion found with sterile medium in the absence of oxygen and with E. coli DH5α grown anaerobically. In addition, the requirement for live respiring cells was demonstrated by the increase in the corrosion rate that was observed upon killing the P. fragi K biofilm in continuous cultures, and it was shown that fermentation products do not cause an increase in corrosion. Hence, pure-culture biofilms inhibit corrosion of SAE 1018 steel by depleting oxygen at the metal surface. Received: 16 December 1996 / Received revision: 18 March 1997 / Accepted: 27 March 1997     

Microbial biodiversity and in situ bioremediation of endosulfan contaminated soil

Molecular characterization based on 16s rDNA gene sequence analysis of bacterial colonies isolated from endosulfan contaminated soil showed the presence of Ochrobacterum sp, Burkholderia sp, Pseudomonas alcaligenes, Pseudomonas sp and Arthrobacter sp which degraded 57–90% of α-endosulfan and 74–94% of β-endosulfan after 7days. Whole cells of Pseudomonas sp and Pseudomonas alcaligenes showed 94 and 89% uptake of α-isomer and 86 and 89% of β-endosulfan respectively in 120 min. In Pseudomonas sp, endosulfan sulfate was the major metabolite detected during the degradation of α-isomer, with minor amount of endosulfan diol while in Pseudomonas alcaligenes endosulfan diol was the only product during α-endosulfan degradation. Whole cells of Pseudomonas sp also utilized 83% of endosulfan sulfate in 120 min. In situ applications of the defined consortium consisting of Pseudomonas alcaligenes and Pseudomonas sp (1:1) in plots contaminated with endosulfan showed that 80% of α-endosulfan and 65% of β-endosulfan was degraded after 12 weeks of incubation. Endosulfan sulfate formed during endosulfan degradation was subsequently degraded to unknown metabolites. ERIC-PCR analysis indicated 80% survival of introduced population of Pseudomonas alcaligenes and Pseudomonas sp in treated plots.     

The Escherichia coli rhamnose promoter rhaP BAD is in Pseudomonas putida KT2440 independent of Crp–cAMP activation

We developed an expression vector system based on the broad host range plasmid pBBR1MCS2 with the Escherichia coli rhamnose-inducible expression system for applications in Pseudomonas. For validation and comparison to E. coli, enhanced green fluorescent protein (eGFP) was used as a reporter. For further characterization, we also constructed plasmids containing different modifications of the rhaP _BAD promoter. Induction experiments after the successful transfer of these plasmids into Pseudomonas putida KT2440 wild-type and different knockout strains revealed significant differences. In Pseudomonas, we observed no catabolite repression of the rhaP _BAD promoter, and in contrast to E. coli, the binding of cyclic adenosine monophosphate (cAMP) receptor protein (Crp)–cAMP to this promoter is not necessary for induction as shown by deletion of the Crp binding site. The crp ⁻ mutant of P. putida KT2440 lacked eGFP expression, but this is likely due to problems in rhamnose uptake, since this defect was complemented by the insertion of the l-rhamnose-specific transporter rhaT into its genome via transposon mutagenesis. Other global regulators like Crc, PtsN, and CyoB had no or minor effects on rhamnose-induced eGFP expression. Therefore, this expression system may also be generally useful for Pseudomonas and other γ-proteobacteria.     

Plasmid Transfer Detection in Soil using the Inducible lPR System Fused to Eukaryotic Luciferase Genes

We report a model system for plasmid transfer analysis using the regulated lambda phage right promoter, λPr, fused to luc and lucOR as repoter genes. We have demonstrated that the systems cI857-λPr::luc and cI857-λPr::lucOR are temperature-inducible in Escherichia coli but not in other Gram-negative bacteria analyzed, enabling detection of luminescence when plasmids were mobilized from E. coli to those Gram-negative backgrounds. Using light for the detection, we have observed plasmid transfer from E. coli harboring RK2 and R388 derived plasmids to Pseudomonas putida KT2440 (co-introduced with donors) and to indigenous microorganisms, in vitro and in nonsterile soil microcosms. The importance of nutrients for an efficient plasmid transfer in nonsterile soil microcosms has been confirmed. When plasmid transfer experiments were carried out into nonsterile soil microcosms, significant populations of indigenous transconjugants arose. This system provides efficient marker genes and avoids the use of antibiotics for the selection of transconjugants.     

Effects of Pseudomonas fluorescens F113 on Ecological Functions in the Pea Rhizosphere Are Dependent on pH

Abstract The aim of this microcosm study was to determine influence of the antibiotic 2,4-diacetylphloroglucinol (DAPG) on the effect of wild-type and functionally modified Pseudomonas fluorescens F113 strains in a sandy loam soil of pH 5.4 planted with pea (Pisum sativum var Montana). The functional modification of strain F113 was a repressed production of DAPG, useful in plant disease control, creating the DAPG negative strain F113 G22; both were marked with a lacZY gene cassette. Lowering the soil pH to 4.4 significantly reduced the plant shoot and root weights and the root length, whereas the bacterial inocula had no significant effect. Both inocula significantly reduced the shoot/root ratio at pH 5.4, but this effect was not evident at the lowered or elevated (6.4) pH levels. The decrease in pH significantly increased the fungal and yeast colony-forming units from the rhizosphere (root extract), but did not affect the total bacterial c.f.u.'s. Inoculatioin with strain F113 in the pH 4.4 soil resulted in a significantly greater total bacterial population. The fungal and yeast c.f.u.'s were not significantly affected by the inocula at any pH studied. Increasing the pH significantly increased the indigenous Pseudomonas population in comparison to the reduced pH treatment and significantly increased both the introduced and total Pseudomonas populations. The antibiotic producing strain significantly reduced the total bacterial population and the NAGase activity (related to fungal activity) at pH 6.4 where the inocula population was the greatest. Alkaline phosphatase, phosphodiesterase, aryl sulfatase, β-glucosidase, alkaline β-galactosidase, and NAGase activities significantly increased with increasing in pH. The F113 inocula reduced the acid phosphatase activity at pH 5.4 and increased the acid β-galactosidase activity over all the pH treatments. The results presented illustrate the variation in impact with soil pH, with implications for variability in efficacy of Pseudomonas fluorescens biocontrol agents with soil pH. Received: 26 June 1998; Accepted: 1 February 1999     

3-Ketoglycoside-mediated metabolism of sucrose in E. coli as conferred by genes from Agrobacterium tumefaciens

Escherichia coli strains that did not have the ability to use sucrose as a sole carbon source gained this ability after receiving a cloned fragment of DNA from Agrobacterium tumefaciens. No invertase was detected in the sucrose-metabolizing E. coli, but evidence for the activity of certain enzymes, known to be produced by biotype 1 strains of Agrobacterium, were found. Evidence was found for the presence of d-glucoside 3-dehydrogenase (G3DH) and α-3-ketoglucosidase. The activity of enzyme extracts on 3-ketosucrose also indicated that 3-ketoglucose reductase, or some enzyme that acts on 3-ketoglucose, was present in the Suc⁺ E. coli as well. The fragment was found to complement a G3DH mutant of A. tumefaciens and was also found to confer chemotaxis towards sucrose in E. coli. Received: 13 September 1996 / Received revision: 15 January 1997 / Accepted: 24 January 1997     

Quantification and Modeling of Plasmid Mobilization on Seeds and Roots

Mobilization frequencies of the nonconjugative plasmid pMON5003 were quantified using Escherichia coli TB1(pRK2013) as donor of a helper plasmid, E. coli M182 (pMON5003) as donor of the nonconjugative plasmid, and Pseudomonas fluorescens as recipient. Initial mating experiments were conducted in nutrient and minimal salts media and pea seed exudates. Mobilization rates were higher during early stationary growth of donors, helpers, and recipients. Numbers of transconjugants were higher in biparental matings when donors contained both conjugative and nonconjugative plasmids, versus tri-parental matings. A mathematical model was developed to predict a nonconjugative plasmid transfer rate parameter (δ), estimating the proportion of conjugative matings in which a plasmid is mobilized. Values of δ ranged from 8 × 10⁻³ to 7.9 × 10⁻¹. Transfer frequencies for pMON5003 from E. coli to P. fluorescens on pea seeds and roots were determined. Transconjugants (P. fluorescens 2-79 (pMON5003)) were isolated from seeds, roots, and soil, but mobilization frequencies were lower than in liquid media.     

Isoform-Specific Function and Distribution of Na/K Pumps in the Frog Lens Epithelium

Epithelial cells from the anterior and equatorial surfaces of the frog lens were isolated and used the same day for studies of the Na/K ATPase. RNase protection assays showed that all cells express α₁- and α₂-isoforms of the Na/K pump but not the α₃-isoform, however the α₂-isoform dominates in anterior cells whereas the α₁-isoform dominates in equatorial cells. The whole cell patch-clamp technique was used to record functional properties of the Na/K pump current (I _P ), defined as the current specifically inhibited by dihydro-ouabain (DHO). DHO-I _P blockade data indicate the α₁-isoform has a dissociation constant of 100 μm DHO whereas for the α₂-isoform it is 0.75 μm DHO. Both α₁- and α₂-isoforms are half maximally activated at an intracellular Na⁺-concentration of 9 mm. The α₁-isoform is half maximally activated at an extracellular K⁺-concentration of 3.9 mm whereas for the α₂-isoform, half maximal activation occurs at 0.4 mm. Lastly, transport by the α₁-isoform is inhibited by a drop in extracellular pH, which does not affect transport by the α₂-isoform. Under normal physiological conditions, I _P in equatorial cells is approximately 0.23 μA/μF, and in anterior cells it is about 0.14 μA/μF. These current densities refer to the area of cell membrane assuming a capacitance of around 1 μF/cm². Because cell size and geometry are different at the equatorial vs. anterior surface of the intact lens, we estimate Na/K pump current density per area of lens surface to be around 10 μA/cm² at the equator vs. 0.5 μA/cm² at the anterior pole. Received: 17 May 2000/Revised: 11 August 2000     

Effects of 5-fluoro-2′-deoxyuridine on bacterial growth its use for DNA labelling

Experiments on the action of 5-fluoro-2′-deoxyuridine on growth ofEscherichia coli B, CECT 101;Pseudomonas fluorescens, CECT 318;Pseudomonas savastanoi, CECT 93;Micrococcus luteus, ATCC 4698;Bacillus cereus, CIP 52.58;Bacillus macerans, ClP 52.58 andBacillus subtilis, ATCC 6633, are described. The inhibition of growth is reversed by thymine plus uracil in all cases except inPseudomonas strains in which uracil alone is active, and in which no exogenous thymine is taken up, not even in the presece of 2′-deoxyguanosine. Growth conditions for improved labelling of bacterial DNA are discussed in the light of the results.     

Isolation of Lightning-Competent Soil Bacteria

Artificial transformation is typically performed in the laboratory by using either a chemical (CaCl₂) or an electrical (electroporation) method. However, laboratory-scale lightning has been shown recently to electrotransform Escherichia coli strain DH10B in soil. In this paper, we report on the isolation of two “lightning-competent” soil bacteria after direct electroporation of the Nycodenz bacterial ring extracted from prairie soil in the presence of the pBHCRec plasmid (Tc^r, Sp^r, Sm^r). The electrotransformability of the isolated bacteria was measured both in vitro (by electroporation cuvette) and in situ (by lightning in soil microcosm) and then compared to those of E. coli DH10B and Pseudomonas fluorescens C7R12. The electrotransformation frequencies measured reached 10⁻³ to 10⁻⁴ by electroporation and 10⁻⁴ to 10⁻⁵ by simulated lightning, while no transformation was observed in the absence of electrical current. Two of the isolated lightning-competent soil bacteria were identified as Pseudomonas sp. strains.     

Corrosion inhibition by aerobic biofilms on SAE 1018 steel

Carbon steel (SAE 1018) samples were exposed to complex liquid media containing either the aerobic bacterium Pseudomonas fragi or the facultative anaerobe Escherichia coli DH5α. Compared to sterile controls, mass loss was consistently 2- to 10-fold lower in the presence of these bacteria which produce a protective biofilm. Increasing the temperature from 23 °C to 30 °C resulted in a 2- to 5-fold decrease in corrosion inhibition with P. fragi whereas the same shift in temperature resulted in a 2-fold increase in corrosion inhibition with E. coli DH5α. Corrosion observed with non-biofilm-forming Streptomyces lividans TK24 was similar to that observed in sterile media. A dead biofilm, generated in situ by adding kanamycin to an established biofilm, did not protect the metal (corrosion rates were comparable to those in the sterile control), and mass loss in cell-free, spent Luria-Bertani (LB) medium was similar to that in sterile medium. Confocal laser scanning microscopy analysis confirmed the presence of a biofilm consisting of live and dead cells embedded in a sparse glycocalyx matrix. Mass-loss measurements were consistent with microscopic observations of the metal surface after 2 weeks of exposure, indicating that uniform corrosion occurred. The biofilm was also able to withstand mild agitation (60 rpm), provided that sufficient time was given for its development. Received: 3 May 1996 / Received revision: 8 August 1996 / Accepted: 24 August 1996     

Effects of Rhamnolipids from Pseudomonas aeruginosa DS10-129 on Luminescent Bacteria: Toxicity and Modulation of Cadmium Bioavailability

In this study, the mixture of mono- and di-rhamnolipids produced by Pseudomonas aeruginosa DS10-129 was characterized for its toxicity and modulatory effects on Cd availability to different bacteria. Gram-negative naturally bioluminescent Vibrio fischeri and recombinant bioluminescent Pseudomonas fluorescens, P. aeruginosa, Escherichia coli, and Gram-positive Bacillus subtilis were used as model organisms. Rhamnolipids reduced the bioluminescence of these bacteria in less than a second of exposure even in relatively low concentrations (30-min EC₅₀ 45–167 mg l⁻¹). Toxicity of Cd to Gram-negative bacteria (30-min EC₅₀ values 0.16 mg l⁻¹ for E. coli, 0.96 mg l⁻¹ for P. fluorescens, and 4.4 mg l⁻¹ for V. fischeri) was remarkably (up to 10-fold) reduced in the presence of 50 mg l⁻¹ rhamnolipids. Interestingly, the toxicity of Cd to Gram-positive B. subtilis (30-min EC₅₀ value 0.49 mg l⁻¹) was not affected by rhamnolipids. Rhamnolipids had an effect on desorption of Cd from soil: 40 mg l⁻¹ rhamnolipids increased the water-extracted fraction of Cd twice compared with untreated control. However, this additionally desorbed fraction of Cd remained bound with rhamnolipids and was not available to bacteria. Hence, in carefully chosen concentrations (still effectively complexing heavy metals but not yet toxic to soil bacteria), rhamnolipids could be applied in remediation of polluted areas.     

The use of Escherichia coli bearing a phoN gene for the removal of uranium and nickel from aqueous flows

A Citrobacter sp. originally isolated from metal-polluted soil accumulates heavy metals via metal-phosphate deposition utilizing inorganic phosphate liberated via PhoN phosphatase activity. Further strain development was limited by the non-transformability of this environmental isolate. Recombinant Escherichia coli DH5α bearing cloned phoN or the related phoC acquired metal-accumulating ability, which was compared with that of the Citrobacter sp. with respect to removal of uranyl ion (UO₂ ²⁺) from dilute aqueous flows and its deposition in the form of polycrystalline hydrogen uranyl phosphate (HUO₂PO₄). Subsequently, HUO₂PO₄-laden cells removed Ni²⁺ from dilute aqueous flows via intercalation of Ni²⁺ into the HUO₂PO₄ lattice. Despite comparable acid phosphatase activity in all three strains, the E. coli DH5α (phoN) construct was superior to Citrobacter N14 in both uranyl and nickel accumulation, while the E. coli DH5α (phoC) construct was greatly inferior in both respects. Expression of phosphatase activity alone is not the only factor that permits efficient and prolonged metal phosphate accumulation, and the data highlight possible differences in the PhoN and PhoC phosphatases, which are otherwise considered to be related in many respects. Received: 30 December 1997 / Received revision: 25 March 1998 / Accepted: 26 March 1998     

Cloning,functional expression and biochemical characterization of a stereoselective alcohol dehydrogenase from Pseudomonas fluorescens DSM50106

Sequencing of a genomic library prepared from Pseudomonas fluorescens DSM 50106 identified an orf showing 29% identity to a C α-dehydrogenase of Pseudomonas paucimobilis and high homology to several sequences with unknown functions derived from genome projects. The corresponding gene adhF1 encodes a dehydrogenase of 296 amino acids with a calculated molecular mass of 31.997 kDa. The gene was functionally expressed in E. coli using a rhamnose inducible expression system. The resulting recombinant enzyme was active in the pH range 6–10 (best pH 8) and at 5–25 °C. This dehydrogenase converts cyclic ketones to the corresponding alcohols utilizing the cofactor NADH. The highest activity was found for cyclohexanone. The enzyme also exhibits high stereoselectivity in the desymmetrization of the prochiral ketone acetophenone, producing optically pure (R)-α-phenyl ethanol (>99%ee) at high conversion (95%). Electronic Publication     

Characterization of a novel ginsenoside-hydrolyzing α-l-arabinofuranosidase, AbfA, from Rhodanobacter ginsenosidimutans Gsoil 3054T

The gene encoding an α-l-arabinofuranosidase that could biotransform ginsenoside Rc {3-O-[β-d-glucopyranosyl-(1–2)-β-d-glucopyranosyl]-20-O-[α-l-arabinofuranosyl-(1–6)-β-d-glucopyranosyl]-20(S)-protopanaxadiol} to ginsenoside Rd {3-O-[β-d-glucopyranosyl-(1–2)-β-d-glucopyranosyl]-20-O-β-d-glucopyranosyl-20(S)-protopanaxadiol} was cloned from a soil bacterium, Rhodanobacter ginsenosidimutans strain Gsoil 3054^T, and the recombinant enzyme was characterized. The enzyme (AbfA) hydrolyzed the arabinofuranosyl moiety from ginsenoside Rc and was classified as a family 51 glycoside hydrolase based on amino acid sequence analysis. Recombinant AbfA expressed in Escherichia coli hydrolyzed non-reducing arabinofuranoside moieties with apparent K _m values of 0.53 ± 0.07 and 0.30 ± 0.07 mM and V _max values of 27.1 ± 1.7 and 49.6 ± 4.1 μmol min⁻¹ mg⁻¹ of protein for p-nitrophenyl-α-l-arabinofuranoside and ginsenoside Rc, respectively. The enzyme exhibited preferential substrate specificity of the exo-type mode of action towards polyarabinosides or oligoarabinosides. AbfA demonstrated substrate-specific activity for the bioconversion of ginsenosides, as it hydrolyzed only arabinofuranoside moieties from ginsenoside Rc and its derivatives, and not other sugar groups. These results are the first report of a glycoside hydrolase family 51 α-l-arabinofuranosidase that can transform ginsenoside Rc to Rd.     

High-level Expression of an α-l-arabinofuranosidase from Thermotoga maritima in Escherichia coli for the Production of Xylobiose from Xylan

To efficiently produce xylobiose from xylan, high-level expression of an α-l-arabinofuranosidase gene from Thermotoga maritima was carried out in Escherichia coli. A 1.5-kb DNA fragment, coding for an α-l-arabinofuranosidase of T. maritima, was inserted into plasmid pET-20b without the pelB signal sequence leader, and produced pET-20b-araA1 with 8 nt spacing between ATG and Shine–Dalgarno sequence. A maximum activity of 12 U mg⁻¹ was obtained from cellular extract of E. coli BL21-CodonPlus (DE3)-RIL harboring pET-20b-araA1. The over-expressed α-l-arabinofuranosidase was purified 13-fold with a 94% yield from the cellular extract of E. coli by a simple heat treatment. Production of xylooligosaccharides from corncob xylan by endoxylanase and α-l-arabinofuranosidase was examined by TLC and HPLC: xylobiose was the major product from xylan at 90 °C and its proportion in the xylan hydrolyzates increased with the reaction time. Hydrolysis with in the xylanase absence of α-l-arabinofuranosidase gave only half this yield. Revisions requested 27 October 2005; Revisions received 5 September 2005     

The BapF protein from Pseudomonas aeruginosa is a β-peptidyl aminopeptidase

A gene encoding a so far uncharacterized β-peptidyl aminopeptidase from the opportunistic human pathogen Pseudomonas aeruginosa PAO1 was cloned and actively expressed in the heterologue host Escherichia coli. The gene was identified in the genome sequence by its homology to the S58 family of peptidases. The sequence revealed an open reading frame of 1,101 bp with a deduced amino acid sequence of 366 amino acids. The gene was amplified by PCR, ligated into pET22b(+) and was successfully expressed in E. coli BL21 (DE3). It was shown that the enzyme consists of two polypeptides (α- and β-subunit), which are processed from the precursor. The enzyme is specific for N-terminal β-alanyl dipeptides (β-Ala-Xaa). BapF hydrolyses efficiently β-alanine at the N-terminal position, including H-β³hAla-pNA, H–D-β³hAla-pNA and β-Ala-l-His (l-carnosine). d- and l-alaninamide were also hydrolysed by the enzyme.     

Inhibitors of N α-acetyl-l-ornithine deacetylase: synthesis, characterization and analysis of their inhibitory potency

A series of N ^α-acyl (alkyl)- and N ^α-alkoxycarbonyl-derivatives of l- and d-ornithine were prepared, characterized, and analyzed for their potency toward the bacterial enzyme N ^α-acetyl-l-ornithine deacetylase (ArgE). ArgE catalyzes the conversion of N ^α-acetyl-l-ornithine to l-ornithine in the fifth step of the biosynthetic pathway for arginine, a necessary step for bacterial growth. Most of the compounds tested provided IC₅₀ values in the μM range toward ArgE, indicating that they are moderately strong inhibitors. N ^α-chloroacetyl-l-ornithine (1g) was the best inhibitor tested toward ArgE providing an IC₅₀ value of 85 μM while N ^α-trifluoroacetyl-l-ornithine (1f), N ^α-ethoxycarbonyl-l-ornithine (2b), and N ^α-acetyl-d-ornithine (1a) weakly inhibited ArgE activity providing IC₅₀ values between 200 and 410 μM. Weak inhibitory potency toward Bacillus subtilis-168 for N ^α-acetyl-d-ornithine (1a) and N ^α-fluoro- (1f), N ^α-chloro- (1g), N ^α-dichloro- (1h), and N ^α-trichloroacetyl-ornithine (1i) was also observed. These data correlate well with the IC₅₀ values determined for ArgE, suggesting that these compounds might be capable of getting across the cell membrane and that ArgE is likely the bacterial enzymatic target.     

Novel detection of <Emphasis Type="Italic">Escherichia coli</Emphasis> β-<Emphasis Type="SmallCaps">d</Emphasis>-glucuronidase activity using a microbially-modified glassy carbon electrode and its potential for faecal pollution monitoring

The electrochemical detection of Escherichia coli β-d-glucuronidase activity as a means of monitoring water pollution by faecal material was investigated using separate Moraxella- and Pseudomonas putida-modified glassy carbon electrodes. The former was more sensitive and selective. The Moraxella-modified biosensor was 100 times more rapid and sensitive than the spectrophotometric detection of β-d-glucuronidase activity. The experimental limit of detection of the biosensor was two c.f.u. per 100 ml polluted water sample within 20 min. The biosensor gave a linear response to commercial β-d-glucuronidase concentration between 0.2 ng and 2 μg ml⁻¹. The biosensor detected activity of β-d-glucuronidase from viable but non-culturable (VBNC) cells and can therefore serve as a presence or absence device for rapid water quality monitoring.