Development of a novel bacterial surface display system using truncated OmpT as an anchoring motif

The Trimeric Autotransporter Adhesin SadA from Salmonella spp. as a Novel Bacterial Surface Display System

Bacterial surface display platforms have been developed for applications such as vaccine delivery and peptide library screening. The type V secretion system is an attractive anchoring motif for the surface expression of foreign proteins in gram-negative bacteria. SadA belongs to subtype C of the type V secretion system derived from Salmonella spp. and promotes biofilm formation and host cell adherence. The inner membrane lipoprotein SadB is important for SadA translocation. In this study, SadA was used as an anchoring motif to expose heterologous proteins in Salmonella typhimurium using SadB. The ability of SadA to display heterologous proteins on the S. typhimurium surface in the presence of SadB was approximately three-fold higher than that in its absence of SadB. Compared to full-length SadA, truncated SadAs (SadA877 and SadA269) showed similar display capacities when exposing the B-cell epitopes of urease B from Helicobacter pylori (UreB158-172aa and UreB349-363aa). We grafted different protein domains, including mScarlet (red fluorescent protein), the urease B fragment (UreBm) from H. pylori SS1, and/or protective antigen domain 4 from Bacillus anthracis A16R (PAD4), onto SadA877 or SadA1292. Whole-cell dot blotting, immunofluorescence, and flow cytometric analyses confirmed the localization of Flag×3-mScarlet (~30 kDa) and Flag×3-UreBm-mScarlet (~58 kDa) to the S. typhimurium surface using truncated SadA877 or SadA1292 as an anchoring motif. However, Flag×3-UreBm-PAD4-mScarlet (~75 kDa) was displayed on S. typhimurium using SadA1292. The oral administrated pSadBA1292-FUM/StmΔygeAΔmurI and pSadBA877-FUM/StmΔygeAΔmurI could elicit a significant mucosal and humoral immunity response. SadA could thus be used as an anchoring motif for the surface expression of large heterologous proteins as a potential strategy for attenuated bacterial vaccine development.

Construction of bacterial laccase displayed on the microbial surface for ultrasensitive biosensing of phenolic pollutants with nanohybrids-enhanced performance

Although bacterial laccase (BLac) has many advantages including short fermentation period and adaptable activity to wide temperature and pH ranges, it is of challenge and significance to apply BLac to the biosensors, due to the intracellular secretion and poor electron transfer efficiency of BLac. Here, cell surface-displayed BLac (CSDBLac) was successfully constructed as whole-cell biocatalyst through microbial surface display technology, eliminating the mass transfer restriction and laborious purification steps. Meanwhile, MXenes/polyetherimide-multiwalled carbon nanotubes (MXenes/PEI-MWCNTs) nanohybrids were designed to immobilize CSDBLac and improve their electrochemical activity. Then, an electrochemical biosensor was successfully constructed to detect common phenolic pollutants (catechol and hydroquinone) by the co-immobilization of CSDBLac and MXenes/PEI-MWCNTs nanohybrids onto a glassy carbon electrode. Subsequently, it was successfully applied to the water samples assay with good reliability and repeatability. This work innovatively used BLac and nanohybrid as the core elements of biosensor, which not only effectively solved the application bottleneck of BLac on biosensors, but also dramatically promote the electro transfer efficiency between whole-cell biocatalyst and electrode. This method is of profound meanings for significantly improving the performance of phenolic biosensors and other biosensors from the origin.

Recent advances in bacterial biosensing and bioremediation of cadmium pollution: a mini-review

Cadmium (Cd) pollution has become a global environmental issue because Cd gets easily accumulated and translocated in the food chain, threatening human health. Considering the detrimental effects and non-biodegradability of environmental Cd, this is an urgent issue that needs to be addressed through the development of robust, cost-effective, and eco-friendly green routes for monitoring and remediating toxic levels of Cd. This article attempts to review various bacterial approaches toward biosensing and bioremediation of Cd in the environment. This review focuses on the recent development of bacterial cell-based biosensors for the detection of bioavailable Cd and the bioremediation of toxic Cd by natural or genetically-engineered bacteria. The present limitations and future perspectives of these available bacterial approaches are outlined. New trends for integrating synthetic biology and metabolic engineering into the design of bacterial biosensors and bioadsorbers are additionally highlighted.

Development of bacterial biosensor for sensitive and selective detection of acetaldehyde

Acetaldehyde is a human carcinogen and widely existed in alcoholic beverages and polluted air. In this study, a simple, fast, convenient and sensitive acetaldehyde biosensor was developed based on an acetaldehyde dehydrogenase (AldDH) bacteria surface display system. The whole-cell catalyst facilitated the dehydrogenation of acetaldehyde, while coenzyme NAD⁺ was reduced and the resultant NADH can be detected spectrometrically at 340 nm. The correct location of AldDH on the bacteria surface was confirmed by the subcellular fraction and immunofluorescence analysis. By comparing the fusion protein expression level and whole-cell activity, the proper display system for anchoring AldDH on the cell surface was obtained. The results of kinetics analysis towards both surface-displayed AldDH and intracellular expressed AldDH demonstrated that the mass-transport resistance was dramatically alleviated by cell-surface display strategy. Under optimal conditions, AldDH-surface display strain with the highest whole-cell activity (3.41 ± 0.3 mU/OD₆₀₀) was applied to spectrophotometry acetaldehyde detection system. An excellent linear relationship between the increases of absorbance at 340 nm and acetaldehyde concentration over the range from 1 μM to 300 μM was reached. The proposed approach offered adequate sensitivity for the detection of acetaldehyde at 0.33 μM. Most importantly, the developed biosensor showed the narrowest substrate specificity towards acetaldehyde, which has been employed for quick determination of acetaldehyde in real samples with good accuracy. The total detection time was within 20 min. The method reported here provided a simple, rapid, and low-cost strategy for the sensitive and selective measurement of acetaldehyde. Therefore, genetically engineered cells may find broad application in biosensors and biocatalysts.

Detection of Bioavailable Cadmium by Double-Color Fluorescence Based on a Dual-Sensing Bioreporter System

Cadmium (Cd) is carcinogenic to humans and can accumulate in the liver, kidneys, and bones. There is widespread presence of cadmium in the environment as a consequence of anthropogenic activities. It is important to detect cadmium in the environment to prevent further exposure to humans. Previous whole-cell biosensor designs were focused on single-sensing constructs but have had difficulty in distinguishing cadmium from other metal ions such as lead (Pb) and mercury (Hg). We developed a dual-sensing bacterial bioreporter system to detect bioavailable cadmium by employing CadC and CadR as separate metal sensory elements and eGFP and mCherry as fluorescent reporters in one genetic construct. The capability of this dual-sensing biosensor was proved to simultaneously detect bioavailable cadmium and its toxic effects using two sets of sensing systems while still maintaining similar specificity and sensitivity of respective signal-sensing biosensors. The productions of double-color fluorescence were directly proportional to the exposure concentration of cadmium, thereby serving as an effective quantitative biosensor to detect bioavailable cadmium. This novel dual-sensing biosensor was then validated to respond to Cd(II) spiked in environmental water samples. This is the first report of the development of a novel dual-sensing, whole-cell biosensor for simultaneous detection of bioavailable cadmium. The application of two biosensing modules provides versatile biosensing signals and improved performance that can make a significant impact on monitoring high concentration of bioavailable Cd(II) in environmental water to reduce human exposure to the harmful effects of cadmium.

Indigoidine biosynthesis triggered by the heavy metal-responsive transcription regulator: a visual whole-cell biosensor

During the last few decades, whole-cell biosensors have attracted increasing attention for their enormous potential in monitoring bioavailable heavy metal contaminations in the ecosystem. Visual and measurable output signals by employing natural pigments have been demonstrated to offer another potential choice to indicate the existence of bioavailable heavy metals in recent years. The biosynthesis of the blue pigment indigoidine has been achieved in E. coli following heterologous expression of both BpsA (a single-module non-ribosomal peptide synthetase) and PcpS (a PPTase to activate apo-BpsA). Moreover, we demonstrated herein the development of the indigoidine-based whole-cell biosensors to detect bioavailable Hg(II) and Pb(II) in water samples by employing metal-responsive transcriptional regulator MerR and PbrR as the sensory elements, and the indigoidine biosynthesis gene cluster as a reporter element. The resulting indigoidine-based biosensors presented a good selectivity and high sensitivity to target metal ions. High concentration of target metal exposure could be clearly recognized by the naked eye due to the color change by the secretion of indigoidine, and quantified by measuring the absorbance of the culture supernatants at 600 nm. Dose-response relationships existed between the exposure concentrations of target heavy metals and the production of indigoidine. Although fairly good linear relationships were obtained in a relatively limited concentration range of the concentrations of heavy metal ions, these findings suggest that genetically controlled indigoidine biosynthesis triggered by the MerR family transcriptional regulator can enable a sensitive, visual, and qualitative whole-cell biosensor for bioindicating the presence of bioaccessible heavy metal in environmental water samples. KEY POINTS: • Biosynthesis pathway of indigoidine reconstructed in a high copy number plasmid in E. coli. • Visual and colorimetric detection of Hg(II) and Pb(II) by manipulation of indigoidine biosynthesis through MerR family metalloregulator. •Enhanced detection sensitivity toward Hg(II) and Pb(II) achieved using novel pigment-based whole-cell biosensors.

Versatile artificial mer operons in Escherichia coli towards whole cell biosensing and adsorption of mercury

Mercury exists naturally and mainly as a man-made pollutant in the environment, where it exerts adverse effects on local ecosystems and living organisms. It is important to develop an appropriate synthetic biological device that recognizes, detects and removes the bioavailable fraction of environmental mercury. Both single-signal and double-signal output mercury biosensors were assembled using a natural mer operon as a template. Selectivity and sensitivity of whole-cell biosensors based on artificial mer operons were determined. Three whole-cell biosensors were highly stable at very high concentrations of mercuric chloride, and could detect bioavailable Hg(II) in the concentration range of 6.25-200 μM HgCl2. A novel Hg(II) bioadsorption coupled with biosensing artificial mer operon was assembled. This would allow Hg(II)-induced Hg(II) binding protein cell surface display and green fluorescence emission to be achieved simultaneously while retaining the linear relationship between fluorescent signal and Hg(II) exposure concentration. The present study provides an innovative way to simultaneously detect, quantify, and remove bioavailable heavy metal ions using an artificially reconstructed heavy metal resistance operon.

Development of Cadmium Multiple-Signal Biosensing and Bioadsorption Systems Based on Artificial Cad Operons

The development of genetic engineering, especially synthetic biology, greatly contributes to the development of novel metal biosensors. The cad operon encoding cadmium resistance was previously characterized from Pseudomonas putida. In this study, single-, dual-, and triple-signal output Cd(II) biosensors were successfully developed using artificial translationally coupled cad operons. Sensitivity, selectivity, and response toward Cd(II) and Hg(II), of three biosensors were all determined. Reporter signals of three biosensors all increased within the range 0.1-3.125 μM Cd(II). Three biosensors responded strongly to Cd(II), and weakly to Hg(II). However, the detection ranges of Cd(II) and Hg(II) do not overlap in all three biosensors. Next, novel Cd(II) biosensing coupled with bioadsorptive artificial cad operons were assembled for the first time. Cd(II)-induced fluorescence emission, enzymatic indication, and Cd(II) binding protein surface display can be achieved simultaneously. This study provides an example of one way to realize multiple signal outputs and bioadsorption based on the redesigned heavy metal resistance operons, which may be a potential strategy for biodetection and removal of toxic metal in the environment, facilitating the study of the mechanism and dynamics of bioremediation.

Construction of a RFP-lacZα bicistronic reporter system and its application in lead biosensing

The combination of a fluorescent reporter and enzymatic reporter provides a flexible and versatile way for the study of diverse biological processes, such as the detection of transcription and translation. Thus, there is an urgent need to develop this novel bifunctional reporter system. This study reports the design, construction, and validation of a new dicistronic mCherry-lacZα reporter system by artificial lac operon and pbr operon models in lacZM15-producing E. coli. It allows two reporter genes to be co-transcribed into a dicistronic mRNA strand, followed by coupled expression of mCherry and lacZα. In artificial lac operons, expression of the downstream lacZα was demonstrated to be positively related to expression of the upstream ORF. In artificial pbr operons, compared with the insertion of downstream full-length lacZ, the insertion of downstream lacZα exerted a slight effect on the response from the upstream mCherry. Furthermore, the downstream lacZα reporter showed stronger response to Pb(II) than the downstream full-length lacZ. Importantly, the response sensitivity of downstream lacZα was still higher than that of upstream mCherry in a dual RFP-lacZα reporter construct. The highly efficient expression profile of the reporter lacZα peptide makes it a preferred downstream reporter in polycistronic constructs. This novel bifunctional reporter system offers a robust tool for biological studies.

Improved Monitoring of Low-Level Transcription in Escherichia coli by a β-Galactosidase α-Complementation System

Genetically encoded reporter proteins are important and widely used tools for the identification and capture of a promoter, tracking the dynamic behavior of transcription, and the quantification of promoter activity. The sensitivity of the reporter gene is a critical factor for an ideal reporter system because weak transcriptional signal has usually failed to be detected using classical reporters. In this study, we present a novel reporter system for improved monitoring of transcription in E. coli based on β-galactosidase α-complementation. In this reporter system, the β-galactosidase activity resulting from the assembly of a reporter lacZα and an existing α-acceptor in advance serves as a measure of transcriptional activity in vivo. To validate the potential of the lacZα-derived reporter system, a series of artificial operons were constructed, and the moderately strong lac promoter, ara promoter, and weak pbr promoter were chosen as the detection promoters. The response profiles of lacZα was similar to that of wild type lacZ in artificial lac operons. Due to its small size and efficient expression profile, the detection sensitivity of a lacZα-derived reporter system was significantly higher than that of the traditional full-length β-galactosidase and the fluorescent protein mCherry reporter system in artificial ara operons. As expected, the response sensitivity of the lacZα-derived reporter system was also demonstrated to be significantly higher than that of the β-galactosidase and mCherry reporter systems in lead-sensitive artificial pbr operons. The lacZα-derived reporter system may prove to be a valuable tool for detecting promoter activity, especially low-level transcription in vivo.