Microbial response time

Microbial sensors offer a number of assets, namely (a) they are less sensitive to inhibition by solutes and more tolerant to suboptimal pH and temperature values than are enzyme electrodes b) they have longer lifetimes than enzymes and (c) they are less expensive than enzyme electrodes as they require no active enzyme to be isolated. On the other hand, they lag behind enzyme electrodes in a few other respects thus, (a) some have longer response times than their enzyme counterparts b) baseline restoration after measurement typically takes longer and (c) cells contain many enzymes and due care must be exercised to ensure adequate selectivity e.g. by optimizing the storage conditions or using specific enzyme reactions) —some mutant microorganisms lack certain enzymes. 

Table 3.2 lists the most salient microbial sensors reported to date, together with the type of immobilized microorganism and measurement used, and the response time and dynamic range achieved in each instance. As can be seen, most of these biosensors rely on amperometric measurements. Some of them are described in detail below. 

Biochemical oxygen demand (BOD) is one of the most widely determined parameters in managing organic pollution. The conventional BOD test includes a 5-day incubation period, so a more expeditious and reproducible method for assessment of this parameter is required. Trichosporon cutaneum, a microorganism formerly used in waste water treatment, has also been employed to construct a BOD biosensor. The dynamic system where the sensor was implemented consisted of a 0.1 M phosphate buffer at pH 7 saturated with dissolved oxygen which was transferred to a flow-cell at a rate of 1 mL/min. When the current reached a steady-state value, a sample was injected into the flow-cell at 0.2 mL/min. The steady-state current was found to be dependent on the BOD of the sample solution. After the sample was flushed from the flow-cell, the current of the microbial sensor gradually returned to its initial level. The response time of microbial sensors depends on the nature of the sample solution concerned. A linear relationship was foimd between the current difference (i.e. that between the initial and final steady-state currents) and the 5-day BOD assay of the standard solution up to 60 mg/L. The minimum measurable BOD was 3 mg/L. The current was reproducible within 6% of the relative error when a BOD of 40 mg/L was used over 10 experiments [128]. 

In principle, there are two possible ways to measure this effect. First, there is the end-point measurement (steady-state mode), where the difference is calculated between the initial current of the endogenous respiration and the resulting current of the altered respiration, which is influenced by the tested substances. Second, by kinetic measurement the decrease or the acceleration, respectively, of the respiration with time is calculated from the first derivative of the currenttime curve. The first procedure has been most frequently used in microbial sensors. These biosensors with a relatively high concentration of biomass have a longer response time than that of enzyme sensors. Response times of comparable magnitude to those of enzyme sensors are reached only with kinetically controlled sensors. 

The ability of the chemolithoautotrophic bacteria Thiobacillus ferrooxidans to oxidize Fe has already been utilized for construction of a microbial sensor for the determination of iron [101]. The limit of determination of this biosensor is 60 pmol 1" with a response time ranging from 30 s to 5 min, depending on the Fe +-concentration in the sample. 

The monitoring of cyanide with microbial sensor is possible in two ways. The first principle is based on the inhibition of respiration of Saccharomyces cervisiae by cyanide [102, 103]. This sensor showed a linear response in the range 0-15 pmol 1 by a response time of approximately 10 min and a stability of 9 days. Another method for the determination of cyanide is enabled by the use of cyanide-degrading microorganisms such as Pseudomonas fluorescens [1041. This bacterium specifically oxidizes cyanide by consuming oxygen ... 

Ihn GS, Park KH, Pek UH, Moo Jeong (1992) Microbial sensor of biochemical oxygen demand using Hansenula anomala. Bull Korean Chem Soc 13 145-148 Sohn M-J, Hong D (1993) Comprehension of the response time in a microbial BOD sensor (II). Bull Korean Chem Soc 14 666-668... 

The extent to which DOM abundance and composition can be considered as end products of microbial activity rather than as drivers depends on the relationship between source dynamics and community response time. At one extreme, steady inputs, stable environmental conditions, and long residence time promote a dynamic equilibrium state in which DOM characteristics can be viewed as the product of source-activity interaction. At the other... 

Alternatively and in contrast with the previous approaches targeting toxicity or specific parameters, the transduction of microbial metabolism can also be applied for the detection of bacteria. However, this sensing principle shows poor selectivity and long response times. This type of biosensor can only be used for well-defined samples because of the possible presence of enzymes from sources other than the tackled species [96]. 

Water sorption, viable cell count of Rhizopus japonicum, relative mobile protons and deuterons and T2 relaxation times for freeze-dried xanthan and locust bean gum gels. (Source From Vittadini, E., Dickinson, L.C., and Chinachoti, P. NMR water mobility in xanthan and locust bean gum mixtures possible explanation of microbial response. Curb. Poly., 49,261,2002. With permission.)... 

A microbial FET for the determination of alcohol has been constructed by Tamiya et al. (1988). The cell membrane of Gluconobacter suboxydans, which converts ethanol to acetic acid, was attached in calcium alginate to the gate of a pH-FET and covered by a nitrocellulose layer. The differential output versus a membrane-free reference gate was linearly related to the logarithm of the ethanol concentration up to 20 mg/1. The sensor responded to propanol and butanol with similar sensitivity, but not to methanol. The response time was 10 min. Below 30°C the sensor was stable for 40 h. 

Planet, and, microbial responses with the sucroae esters. In addition to the above we have conducted experiments on the effects of the sucrose esters on plants and microorganisms and we report these data for the first time. However it should be reiterated that the sucrose esters were composed of acetic (C2) 2- and 3-methylbutyric (C5) and 3-methylvaleric (C6) acids and only the glucose molecule was ester if ied the fructose portion remained in the free hydroxyl state. Also the sucrose esters were extremely difficult to separate so that mixtures were initially obtained and tested. We shall see in the wheat coleoptiles bioassay that as the number of 3-methylvalerate residues decreases in the glucose moiety so the level of biological activity decreases at the 10 4 M level though in all cases there is 100Z inhibition at 10 3 m (Figure 7). 

When the electrode was removed from the sample solution and placed in a solution free from glucose, the output of the microbial electrode gradually increased and returned to initial level within 15 min at 30°C. (The current means the steady-state current hereinafter.) The response time of the microbial electrode was longer than that of the enzyme electrode. This may be caused by the time lag of the bacterial respiration. However, employment of a rate assay improved the response time and the... 

In comparison to enzyme-based biosensors, microbial biosensors show lower analyte selectivity, slightly slower response times, but often much better stability. Microbial biosensor determination of analytes such as amino acids, alcohol, and lactate show sensor stability over several days, whilst enzyme-based sensors may have operational lives of only 2-24 h. 

Biosensors based on enzymes have high sensitivity and selectivity. A variety of microbial biosensors have also been developed. However, it still remains a great challenge to develop a rapid, inexpensive but sensitive method for real samples. Compared to enzymatic biosensors, development of a highly satisfactory microbial biosensor is still hampered because they suffer from long response time, low sensitivity, and poor selectivity. The trends for the development of biosensors lie in miniaturization of the devices, nanotechnology, and biotechnology. Disposable screen-printed sensors have been developed for industrial wastes or natural water. Metal nanoparticles can enhance the electron transfer between redox center in proteins and electrode surface and show promise for detection... 

This series of reactions increases the response time of the microbial sensor compared with that of the enzyme sensor for arginine, which only requires the presence of the first enzyme. 

Cholesterol is de mined using an ampoometric microbial electrode [235]. In contrast, glucose is determined using a potentiometric bacterial electrode, employing the reduction of lipoic acid in the presence of Escherichia coli [236]. The latter is based on bacterial growth, and the response time is very long, between 1 and 2 hours. 

Although dynamic responses of microbial systems are poorly understood, models with some basic features and some empirical features have been found to correlate with actual data fairly well. Real fermentations take days to run, but many variables can be tried in a few minutes using computer simulation. Optimization of fermentation with models and reaf-time dynamic control is in its early infancy however, bases for such work are advancing steadily. The foundations for all such studies are accurate material Balances. 

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