I. Introduction It has been 40 years since Clark and Lyons first proposed the concept of biosensors in 1962. Biosensors have received significant attention and widespread application in fermentation processes, environmental monitoring, food engineering, clinical medicine, military and military medicine. In the first 15 years, biosensors were mainly based on enzyme electrodes. However, due to the high cost and instability of enzymes, the application of sensors using enzymes as sensitive materials was limited. In recent years, the continuous development of microbial immobilization technology has led to the development of microbial electrodes. Microbial electrodes use living microorganisms as molecular recognition elements, offering unique advantages over enzyme electrodes. They overcome weaknesses such as high cost, difficult extraction, and instability. Furthermore, they can simultaneously utilize coenzymes within microorganisms to handle complex reactions. Currently, the application of fiber optic biosensors is becoming increasingly widespread. Moreover, with the development of polymerase chain reaction (PCR) technology, DNA biosensors using PCR are becoming increasingly common. II. Research Status and Main Application Areas 1. Fermentation Industry: Among various biosensors, microbial sensors are most suitable for determination in the fermentation industry. Because there are often substances that interfere with enzymes during fermentation, and the fermentation broth is often not clear and transparent, it is not suitable for determination by methods such as spectroscopy. However, the application of microbial sensors is very likely to eliminate interference and is not limited by the turbidity of the fermentation broth. At the same time, since the fermentation industry is a large-scale production, the low cost and simple equipment of microbial sensors give them a great advantage. (1). Determination of raw materials and metabolites Microbial sensors can be used to determine raw materials such as molasses and acetic acid, and metabolites such as cephalosporins, glutamic acid, formic acid, methane, alcohols, penicillin, and lactic acid. The principle of measurement is basically to use a suitable microbial electrode and an oxygen electrode. The assimilation of microorganisms consumes oxygen, and the amount of oxygen reduction is measured by measuring the change in the current of the oxygen electrode, thereby achieving the purpose of measuring the concentration of the substrate. Among various raw materials, the determination of glucose is particularly important for process control. The glucose concentration can be estimated by using the metabolism of Psoudomonas fluorescens to consume glucose and detecting it through an oxygen electrode. Compared with glucose enzyme electrodes, the results of this type of microbial electrode are similar, but the microbial electrode has high sensitivity, good repeatability and practicality, and does not require the use of expensive glucose enzymes. When acetic acid is used as a carbon source for microbial culture, the acetic acid content above a certain concentration will inhibit the growth of microorganisms, so online measurement is required. The concentration of acetic acid can be measured by a microbial sensor composed of immobilized yeast (Trichosporon brassicae), a gas-permeable membrane and an oxygen electrode. In addition, there are microbial sensors for measuring glutamate by combining E. coli with a carbon dioxide gas-sensitive electrode, and microbial sensors composed of bacterial-collagen membrane reactors and combined glass electrodes can be used to measure cephalosporins in fermentation broth, etc. (2). Determination of total number of microbial cells In fermentation control, a simple and continuous method for directly measuring the number of cells has always been needed. It has been found that bacteria can be directly oxidized and generate current on the anode surface. This electrochemical system has been applied to the determination of cell number, and the results are the same as those of the traditional plaque counting method for measuring cell number. (3). Identification of metabolic test Traditional identification of microbial metabolic types is based on the growth of microorganisms on a certain culture medium. These experimental methods require a long culture time and specialized techniques. The assimilation of substrates by microorganisms can be measured by their respiratory activity. The respiratory activity of microorganisms can be directly measured using an oxygen electrode. Therefore, microbial sensors can be used to measure the metabolic characteristics of microorganisms. This system has been used for simple identification of microorganisms, selection of microbial culture media, determination of microbial enzyme activity, estimation of biodegradable substances in wastewater, selection of microorganisms for wastewater treatment, assimilation test of activated sludge, determination of biodegradable products, selection of microbial preservation methods, etc. 2. Environmental monitoring (1). Determination of biochemical oxygen demand The determination of biochemical oxygen demand (BOD) is the most commonly used indicator for monitoring the organic pollution status of water bodies. Conventional BOD determination requires a 5-day culture period, is complicated to operate, has poor repeatability, is time-consuming and labor-intensive, and has great interference. It is not suitable for on-site monitoring. Therefore, there is an urgent need for a new method that is simple to operate, fast and accurate, highly automated, and widely applicable. Currently, researchers have isolated two new yeast strains, SPT1 and SPT2, and immobilized them on glassy carbon electrodes to construct a microbial sensor for measuring BOD, with repeatability within ±10%. When this sensor was used to measure BOD in pulp mill wastewater, the minimum measured value reached 2 mg/L in 5 minutes. Another new microbial sensor uses a yeast strain resistant to high osmotic pressure as the sensitive material, functioning normally under high osmotic pressure. Furthermore, the strain can be stored dry for a long time and regains its activity upon immersion, providing a rapid and convenient method for measuring BOD in seawater. In addition to microbial sensors, a fiber optic biosensor has been developed for measuring lower BOD values ​​in river water. This sensor has a reaction time of 15 minutes and optimal operating conditions of 30°C and pH=7. This sensor system is almost unaffected by chloride ions (within the range of 1000 mg/L) and is unaffected by heavy metals (Fe3+, Cu2+, Mn2+, Cr3+, Zn2+). This sensor has been applied to the determination of BOD in river water and has obtained good results. There is now a BOD biosensor that has been phototreated (i.e., TiO2 is used as a semiconductor and irradiated with a 6 W lamp for about 4 minutes), which greatly improves its sensitivity and is very suitable for measuring low BOD in river water [5]. At the same time, a compact optical biosensor has been developed for simultaneously measuring the BOD values ​​of multiple samples. It uses three pairs of light-emitting diodes and silicon photodiodes. Pseudomonas fluorescens is fixed at the bottom of the reactor with photo-crosslinked resin. This measurement method is both rapid and simple, and can be used for six weeks at 4°C. It has been used in the process of industrial wastewater treatment. Commonly used important pollutant indicators include the concentration of ammonia, nitrite, sulfide, phosphate, carcinogens and mutagens, heavy metal ions, phenolic compounds, surfactants and other substances. At present, a variety of biosensors for measuring various pollutants have been developed and put into practical application. Microbial sensors for measuring ammonia and nitrate are mostly composed of nitrifying bacteria isolated from wastewater treatment devices and oxygen electrodes. Currently, a microbial sensor can measure nitrates and nitrites (NOx-) under both dark and light conditions. Its measurement in a saline environment makes it unaffected by other types of nitrogen oxides. It has been used to measure NOx- in estuaries with good results. Sulfide determination uses a microbial sensor made from obligate, autotrophic, aerobic *Thiobacillus thiooxidans* isolated and screened from acidic soils near pyrite mines. At pH 2.5 and 31°C, it was measured over 200 times per week with unchanged activity, but activity decreased by 20% after two weeks. The sensor has a lifespan of 7 days, and the equipment is simple, low-cost, and easy to operate. Currently, a photomicrobial electrode is also used to measure sulfide content, using *Chromatium* SP bacteria connected to a hydrogen electrode. Recently, scientists isolated a fluorescent bacterium in polluted areas. This bacterium contains a fluorescent gene and can produce fluorescent protein in response to pollution sources, thus emitting fluorescence. This gene can be introduced into suitable bacteria through genetic engineering to create a microbial sensor for environmental monitoring. Luciferase has now been introduced into *E. coli* to detect toxic arsenic compounds. Significant progress has been made in determining the concentrations of phenols and surfactants in water. Currently, nine Gram-negative bacteria have been isolated from soils in the West Siberian oil basin, using phenols as their sole carbon and energy source. These bacteria can enhance the sensitivity of the sensor's receptor portion. Its detection limit for phenols is 5 × 10⁻⁹ mol. The optimal operating conditions for this sensor are: pH 7.4, 35°C, and a continuous operating time of 30 hours. Another current-type biosensor for measuring surfactant concentrations, made from *Pseudomonas rathonis*, uses microbial cells immobilized on gels (agar, agarose, and calcium alginate) and polyethylene glycol membranes. It can maintain the activity and growth capacity of microbial cells in the gel over long distances using GF/A chromatographic paper or cross-linking induced by glutamate in the gel. This sensor can quickly recover the activity of the sensitive element after measurement. Another type of current-type biosensor, used to determine organophosphate pesticides, employs artificial enzymes. Utilizing organophosphate pesticide hydrolases, the measurement limit for nitrophenol and diethylphenol is 100 × 10⁻⁹ mol, requiring only 4 minutes at 40°C. A newly developed phosphate biosensor uses pyruvate oxidase G, combined with an automated CL-FIA desktop computer system, capable of detecting (32–96) 10⁻⁹ mol of phosphate, operating for over two weeks at 25°C with high repeatability. Recently, a novel microbial sensor has emerged, using bacterial cells as the biological component to determine the content of nonylphenol etoxylate (NP-80E) in surface water. Using a current-type oxygen electrode as the sensor, microbial cells are immobilized on a dialysis membrane on the oxygen electrode. The measurement principle is based on measuring the respiratory activity of *Trichosporum grablata* cells. This biosensor has a reaction time of 15–20 minutes and a lifespan of 7–10 days (for continuous measurement). Within the concentration range of 0.5–6.0 mg/L, the electrical signal exhibits a linear relationship with the NP-80E concentration, making it well-suited for detecting molecular surfactants in polluted surface water. In addition, the determination of heavy metal ion concentrations in wastewater is also crucial. A complete monitoring and analysis system for the bioavailability determination of heavy metal ions based on immobilized microorganisms and bioluminescence measurement technology has been successfully designed. An operon from Vibrio fischeri bacteria is introduced into Alcaligenes eutrophus (AE1239) under the control of a copper-inducible promoter. The bacteria emit light induction by copper ions, with the degree of luminescence proportional to the ion concentration. Embedding microorganisms and optical fibers together in a polymer matrix yields a biosensor with high sensitivity, good selectivity, a wide measurement range, and strong storage stability. Currently, this microbial sensor can achieve a minimum measurement concentration of 1 × 10⁻⁹ mol. There is also a current-type microbial sensor specifically designed for measuring copper ions. It uses recombinant strains of *Saccharomyces cerevisiae* as biological elements, which are fusions of the copper-inducible promoter from the *Saccharomyces cerevisiae* CUP1 gene and the *Escherichia coli* lacZ gene. Its working principle is that the CUP1 promoter is first induced by Cu2+, and then lactose is used as a substrate for measurement. If Cu2+ is present in the solution, these recombinant bacteria can utilize lactose as a carbon source, which leads to an alteration in the oxygen demand of these aerobic cells. This biosensor can measure CuSO4 solutions in a concentration range of (0.5–2) × 10⁻³ mol. Currently, various metal ion-inducible promoters have been transferred into *E. coli*, causing *E. coli* to exhibit luminescent reactions in solutions containing various metal ions. The concentration of heavy metal ions can be determined based on the intensity of this luminescence, with a measurement range from nanomolar to micromolar, requiring 60–100 min. A biosensor for measuring zinc concentration in wastewater has also been successfully developed. Utilizing the alkalophilic bacterium *Alcaligenes cutrophus*, it has been used to measure the concentration and bioavailability of zinc in wastewater, with satisfactory results. An algae sensor for estimating pollution levels in estuarine outflows consists of a cyanobacterium (*Cyanobacterium Spirlina subsalsa*) and a gas-sensitive electrode. It estimates changes in water toxicity caused by the presence of environmental pollutants by monitoring the degree of inhibition of photosynthesis. Using standard natural water as a medium, different concentrations of three major pollutants (heavy metals, herbicides, and carbamate pesticides) were measured, and their toxic reactions were detected in all cases, with high repeatability and reproducibility. Recently, due to the rapid development of polymerase chain reaction (PCR) technology and its widespread application in environmental monitoring, many scientists have begun to combine it with biosensor technology. One DNA piezoelectric biosensor using PCR technology can measure a specific bacterial toxin. Biotinylated probes are immobilized on a quartz crystal with a platinum surface containing streptozotoxin antibiotics. Cyclic measurements can be performed on the same crystal surface using 1 × 10⁻⁶ mol of hydrochloric acid. The same hybridization reaction is performed on DNA samples extracted from bacteria, followed by PCR amplification. The product is a specific gene fragment from the genus *Aeromonas hydrophila*. This piezoelectric biosensor can identify the presence of this gene in samples, making it possible to detect various *Aeromonas* species carrying this pathogen in water samples. Another type of channel biosensor can detect toxic substances such as *Pterygotaenia solani* neurotoxin produced by phytoplankton and jellyfish. It is currently possible to measure trace amounts of PSP toxin within a single planktonic cell. DNA sensors are also rapidly being applied. A miniaturized DNA biosensor converts DNA recognition signals into electrical signals for measuring cryptosporin and other waterborne infectious agents in water samples. This sensor focuses on improving nucleic acid recognition and enhancing its specificity and sensitivity, and seeks new methods to convert hybridization signals into useful signals. Current research focuses on integrating the recognition and conversion devices. Microalgin is a bacterial hepatotoxin produced in algal blooms caused by cyanobacteria. A biosensor with an immobilized surface cell plasmid genome has been developed to measure the concentration of microalgin in water, with a direct measurement range of 50–1000 × 10⁻⁶ g/L. A concept for a multiplex biosensor based on enzyme inhibition analysis for measuring toxic substances has also been proposed. This multiplex biosensor utilizes two transducers—a pH-sensitive electronic transistor and a thermosensitive thin-film electrode—and three enzymes—urease, acetylcholinesterase, and butyrylcholinesterase. The performance of this biosensor has been tested and shows good results. Besides fermentation industries and environmental monitoring, biosensors are also widely used in food engineering, clinical medicine, military and military medicine, primarily for measuring glucose, acetic acid, lactic acid, lactose, uric acid, urea, antibiotics, various amino acids such as glutamate, and various carcinogens and mutagens. III. Discussion and Outlook Harold H. Weetal of the United States pointed out that the commercialization of biosensors requires the following conditions: sufficient sensitivity and accuracy, ease of operation, low price, ease of mass production, and quality monitoring during the production process. Among these, low price determines the sensor's competitiveness in the market. Among various biosensors, microbial sensors have the greatest advantages in low cost, ease of operation, and simple equipment, making their market prospects enormous and attractive. In comparison, enzyme biosensors are relatively expensive. However, microbial sensors also have their own drawbacks, the main one being insufficient selectivity, caused by the presence of multiple enzymes in microbial cells. There are reports of adding specific inhibitors to address the selectivity problem of microbial electrodes. In addition, microbial immobilization methods need further improvement. Firstly, cell viability must be ensured as much as possible; secondly, the cell-base membrane must be firmly bound to prevent cell loss. Furthermore, the long-term preservation of microbial membranes also needs further improvement; otherwise, large-scale commercialization will be difficult. In summary, commonly used microbial electrodes and enzyme electrodes each have their advantages in various applications. If stable, highly active, and low-cost free enzymes are readily available, enzyme electrodes are the most ideal for users. Conversely, microbial electrodes are a more ideal choice when biocatalysis involves complex pathways, requires coenzymes, or the desired enzymes are difficult to separate or unstable. Other forms of biosensors are also rapidly developing and their applications are becoming increasingly widespread. With further improvements in immobilization technology and a deeper understanding of organisms, biosensors will undoubtedly open up a new world in the market.