Sensing antimonite and arsenite at the subattomole level with genetically engineered bioluminescent bacteria

Development of a set of simple bacterial biosensors for quantitative and rapid measurements of arsenite and arsenate in potable water

Testing for arsenic pollution is commonly performed with chemical test kits of unsatisfying accuracy. Bacterial biosensors are an interesting alternative as they are easily produced, simple, and highly accurate devices. Here, we describe the development of a set of bacterial biosensors based on a nonpathogenic laboratory strain of Escherichia coli, the natural resistance mechanism of E. coli against arsenite and arsenate, and three reporter proteins: bacterial luciferase, beta-galactosidase and Green Fluorescent Protein (GFP). The biosensors were genetically optimized to reduce background expression in the absence of arsenic. In calibration experiments with the biosensors and arsenite-amended potable water, arsenite concentrations at 4 microg of As/L (0.05 microM) were routinely and accurately measured. The currently most quantitative system expressed the bacterial luciferase as reporter protein, responding proportional with a concentration range between 8 and 80 microg of As/L. Sensor cells could be stored as frozen batches, resuspended in plain media, and exposed to the aqueous test sample, and light emission was measured after 30-min incubation. Field testing for arsenite was achieved with a system that contained beta-galactosidase, producing a visible blue color at arsenite concentrations above 8 microg/L. For this sensor, a protocol was developed in which the sensor cells were dried on a paper strip and placed in the aqueous test solution for 30 min after which time color development was allowed to take place. The GFP sensor showed good potential for continuous rather than end point measurements. In all cases, growth of the biosensors and production of the strip test was achieved by very simple means with common growth media, and quality control of the sensors was performed by isolating the respective plasmids with the genetic constructs according to simple standard genetic technologies. Therefore, the biosensor cells and protocols may offer a realistic alternative for measuring arsenic contamination in potable water.

Luminescence-based whole-cell-sensing systems for cadmium and lead using genetically engineered bacteria

Whole-cell-based sensing systems that respond to cadmium and lead ions have been designed and developed using genetically engineered bacteria. These systems take advantage of the ability of certain bacteria to survive in environments polluted with cadmium and lead ions. The bacteria used in this investigation have been genetically engineered to produce reporter proteins in response to the toxic ions. This was achieved by modifying a strain of Escherichia colito harbor plasmids pYSC1 and pYS2/pYSG1. In these dual-plasmid-based sensing systems, the expression of the reporters beta-galactosidase and red-shifted green fluorescent protein (rs-GFP) was controlled by CadC, the regulatory protein of the cad operon. Regulation of the expression of the reporter proteins is related to the amount of cadmium and lead ions employed to induce the bacteria. The bacterial sensing systems were found to respond to cadmium, lead, and zinc ions, and had no significant response to nickel, copper, manganese, and cobalt.

Exposing culprit organic pollutants: a review

There is a continuing need for monitoring the health of the environment due to the presence of pollutants. Here, we review the development and attributes of biosensors by which bacteria have been genetically modified to express the luminescence genes, i.e. to glow, in a quantified manner, in response to pollutants. We have concentrated on the detection of organic hydrocarbon pollutants and discussed the molecular mechanisms by which some of these chemicals act as effector molecules on the respective regulatory systems. The future of environmental biosensors is predictably bright. As more knowledge is gathered on the sensing regulatory component, the possibility of developing targeted or pollutant-specific biosensors is promising. Moreover, the repertoire of biosensors for culprit organic pollutants is expected to be enlarged through advances in genomics technology and identification of new sensory or receptor molecules. The need for pollutant detection at concentrations in the parts per trillion range or biosensors configured in a nanoscale is anticipated.

A luminescent Escherichia coli biosensor for the high throughput detection of beta-lactams

A group-specific bioluminescent Escherichia coli strain for studying the action of beta-lactam antibiotics is described. The strain contains a plasmid, pBlaLux1, in which the luciferase genes from Photorhabdus luminescens are inserted under the control of the beta-lactam-responsive element ampR/ampC from Citrobacter freundii. In the presence of beta-lactams, the bacterial cells are induced to express the luciferase enzyme and three additional enzymes generating the substrate for the luciferase reaction. This biosensor for beta-lactams does not need any substrate or cofactor additions, and the bioluminescence can be measured very sensitively in real time by using a luminometer. Basic parameters affecting the light production and induction in the gram-negative model organism E. coli SNO301/pBlaLux1 by various beta-lactams were studied. The dose-response curves were bell shaped, indicating toxic effects for the sensor strain at high concentrations of beta-lactams. Various beta-lactams had fairly different assay ranges: ampicillin, 0.05-1.0 microg/ml; piperacillin, 0.0025-25 microg/ml; imipenem, 0.0025-0.25 microg/ml; cephapirin, 0.025-2.5 microg/ml; cefoxitin, 0.0025-1.5 microg/ml; and oxacillin, 25-500 microg/ml. Also, the induction coefficients (signal over background noninduced control) varied considerably from 3 to 158 in a 2-hour assay. Different non-beta-lactam antibiotics did not cause induction. Because the assay can be automated using microplate technologies, the approach may be suitable for higher throughput analysis of beta-lactam action.

Microbial biosensors

A microbial biosensor consists of a transducer in conjunction with immobilised viable or non-viable microbial cells. Non-viable cells obtained after permeabilisation or whole cells containing periplasmic enzymes have mostly been used as an economical substitute for enzymes. Viable cells make use of the respiratory and metabolic functions of the cell, the analyte to be monitored being either a substrate or an inhibitor of these processes. Bioluminescence-based microbial biosensors have also been developed using genetically engineered microorganisms constructed by fusing the lux gene with an inducible gene promoter for toxicity and bioavailability testing. In this review, some of the recent trends in microbial biosensors with reference to the advantages and limitations are been discussed. Some of the recent applications of microbial biosensors in environmental monitoring and for use in food, fermentation and allied fields have been reviewed. Prospective future microbial biosensor designs have also been identified.

Detection of tetracyclines with luminescent bacterial strains

The performance of two bioluminescent Escherichia coli K-12 strains for the specific detection of the tetracycline family of antimicrobial agents was compared, and the analytical applicability of one of the strains was preliminarily evaluated. One sensor plasmid contained the bacterial luciferase operon of Photorhabdus luminescens under the control of the tetracycline-responsive element from transposon Tn10 (15). An analogous plasmid construction with firefly (Photinus pyralis) luciferase reporter gene was constructed, and these two divergent tetracycline-inducible light-emitting systems were compared for their suitability for the qualitative detection of tetracyclines. Both sensor strains behaved in a similar manner kinetically, and the most sensitive tetracycline response for both sensor strains was achieved in 90-120 min by performing the assay at 37 degrees C. The sensor strain containing the bacterial luciferase operon responded slightly more sensitively to different tetracyclines than the strain containing firefly luciferase gene. The sensor bacteria retained their inducibility in lyophilization, and freeze-dried cells detected tetracyclines as sensitively as freshly cultivated sensor cells. The preliminary results from the analysis of tetracycline-spiked pork serum samples indicated that these sensor bacteria could be used to screen veterinary samples for tetracycline residues in real-time.

Application of high-density optical microwell arrays in a live-cell biosensing system

In this paper, we use optical imaging fibers to fabricate a chemical and biochemical sensor that utilizes the ability of living cells to respond to biologically significant compounds. The sensor is created by randomly dispersing single NIH 3T3 mouse fibroblast cells into an optically addressable fiber-optic microwell array such that each microwell accommodates a single cell. The cells are encoded to identify their location within the array and to correlate changes or manipulations in the local environment to responses of specific cell types. The entire array can be simultaneously measured, yielding a rapid, repetitive, and high-density analysis method.

Measurement of firefly luciferase reporter gene activity from cells and lysates using Escherichia coli arsenite and mercury sensors

The structural gene encoding firefly luciferase from Photinus pyralis is a widely used reporter both in traditional monitoring of gene expression and in bacterial sensors. Its activity can be detected from living cells (in vivo) without disruption or from cell-free lysate (in vitro). We compared the two measurement methods by using an overall toxicity detecting strain Escherichia coli MC1061(pCSS810), a mercury-sensing strain E. coli MC1061(pTOO11), and two new arsenic sensor strains MC1061(pTOO31) and AW3110(pTOO31) which were constructed for this study. Plasmid pTOO31 was constructed by inserting the ars promoter and the arsR gene from plasmid R773 to control firefly luciferase gene expression. Both in vivo and in vitro methods correlated well with the strains tested [correlation coefficients R = 0.99484 and 0.99834] and gave highly comparable results with standard solutions of arsenite or mercury ions and from six environmental water samples spiked with the ions. Use of the in vivo method resulted in lower variation between replicates of the same sample (CVs ranging from 3.9 to 7.2%) and also between different samples (from 8.6 to 25.9%) compared to the in vitro method (CVs ranging from 8.6 to 17.8% for replicates and from 13.1 to 36.3% for different samples).

Identification and quantification of toxic chemicals by use of Escherichia coli carrying lux genes fused to stress promoters

The luxCDABE bioluminescence genes of the Vibrio fischeri lux system have been used as a reporter system for different stress and regulatory promoters of Escherichia coli. Selected E. coli strains carrying lux genes fused to different promoters were exposed to various toxic chemicals, and the recorded luminescence was used for the characterization of the biologic signature of each compound. Analysis of these data with the aid of a proper algorithm allowed quantitative and qualitative assessment of toxic chemicals. Of the 25 tested chemicals, 23 were identified by this novel strategy in a 3-h procedure. This system can also be adapted for the identification of simple mixtures of toxic agents when the biologic signatures of the individual compounds are known. This biologic recognition strategy also provides a tool for evaluating the degree of similarity between the modes of action of different toxic agents.